\" measurement of dosimetric parameters and dose verification in stereotactic

$: \" Measurement of Dosimetric Parameters and Dose Verification in Stereotactic$
CERTIFICATE

This is to certify that the dissertation entitled

“Measurement of Dosimetric Parameters and Dose Verification in Stereotactic

Radiosurgery (SRS)”

is the bonafide record of research work done by

Reduan Bin Abdullah

during the period April to July 2013

under our supervision.

Supervisor, Co-supervisor,

Professor Dr. Ahmad Bin Haji Zakaria Encik Nik Ruzman Idris

Dean of School of Health Sciences Senior Medical Physicist

Universiti Sains Malaysia Hospital Universiti Sains Malaysia

16150 Kubang Kerian, Kota Bharu 16150 Kubang Kerian, Kota Bharu

Kelantan, Malaysia. Kelantan, Malaysia

i

ACKNOWLEDGEMENT

In the name of ALLAH, the most gracious, the most merciful

Alhamdulillah, thanks to Allah for His guide and willing, give me opportunity to

complete this research work finished in time smoothly.

Firstly,. My greatest appreciation goes to my supervisor Professor Ahmad Bin

Haji Zakaria for his support, guidance and knowledge not as a lecturer but to me as a

motivator during my work and writing the dissertation report. Special thanks are also

dedicated to Mr. Nik Ruzman Nik Idris, my co-supervisor and also as a senior medical

physicist at Department of Nuclear Medicine, Radiotherapy and Oncology, HUSM for

his cooperation, ideas and support during my work. I have learned so much and earned

knowledge and guidance from him during conducting this study. His helps and

supports gave me confidence and guts to continue finishing my research.

I am also like to thank Dr. Ahmad Lutfi Yusof, Head Department of

Department of Nuclear Medicine, Radiotherapy and Oncology, HUSM to give me

permission to use the facilities in the department, Mrs. Mazurawati Mohamed as a

Dosimetris of the department for helping me in collecting the data. Not forgetting, Dr

Azhar Abdul Rahman, the coordinator of Msc. of Radiation Science, School of

Physics, Universiti Sains Malaysia to give support to carry out the project and also all

lectures of Msc. Radiation Science Programme, School of Physics, staffs of Medical

Radiation Laboratory, School of Health Sciences and staffs of Department of Nuclear

Medicine, Radiotherapy and Oncology, HUSM for their great commitment and

support.

ii

Last but definitely not the least, I would like to especially thank my parents,

my wife and my children whom give me full support physically and morally to me to

finish this project.

iii

ABSTRACT

The purpose of this study was to measure the dosimetric parameters for small photon

beams to be used as input data for treatment planning computer system (TPS) and to

verify dose calculated by TPS in Stereotactic Radiosurgery (SRS) procedure. The

beam data required were Percentage Depth Dose (PDD), Off-axis Ratio (OAR), and

Scatter Factor or Relative Output Factor. Small beams of 5mm to 45 mm diameter

circular cone collimators used in SRS were utilized for beam data measurements. Two

type of detectors were used which are pinpoint 3D ionization chamber (0.016cc) and

EDR2 film dosimetry. The ionization chamber and EDR2 film give slightly similar

results on PDD curve but for OAR measurement, the film gives more accurate results

especially in penumbra length compared to ionization chamber due to characteristic of

the film which has higher spatial resolution. For second part of this study, we reported

the important quality assurance (QA) procedures before SRS treatment that influenced

the dose delivery. These QA procedures consist of measurements on the accuracy in

target localization and room laser’s alignment. The dose calculated to be delivered for

treatment was verified using pinpoint ionization 3D chamber and TLD 100H. The

mean deviation of measured dose using TLD 100H compared to calculated dose was

3.37%. Beside that, pinpoint ionization 3D chamber give more accurate results of

dose compared to TLD 100H. The measured dose using pinpoint ionization 3D

chamber are good agreement with calculated dose by TPS with deviation of

2.17%.The results are acceptable such as recommended by International Commission

on Radiation Units and Measurements (ICRU) Report No. 50 that dose delivered to

the target volume must be within ±5% error.

iv

TAJUK: PENGUKURAN PARAMETER DOSIMETRI DAN PENGESAHAN

DOSE PADA RADIOSURGERI STEREOTAKTIK

ABSTRAK

Tujuan kajian ini adalah untuk mengukur parameter dosimetri untuk alur foton kecil

untuk digunakan sebagai data input untuk sistem perancangan rawatan berkomputer

(TPS) dan untuk mengesahkan dos dikira dengan TPS dalam prosedur Stereotactic

Radiosurgeri (SRS). Data yang diperlukan adalah Peratus Dalam Dos (PDD), Nisbah

Off-paksi (OAR), dan Faktor Sebaran atau Faktor Output Relatif. Alur kecil 5mm

hingga 45 mm collimator kon digunakan dalam SRS telah digunakan untuk ukuran

data input. Dua jenis pengesan yang digunakan iaitu kebuk pengionan 3D pinpoint

(0.016cc) dan filem dosimetry EDR2. Kebuk pengionan dan filem EDR2 memberikan

hasil yang sedikit sama pada keluk Peratus Dalam Dos (PDD) tetapi untuk ukuran

OAR, filem ini memberikan hasil yang lebih tepat terutamanya bagi pengukuran

panjang penumbra berbanding kebuk pengionan kerana ciri-ciri filem yang

mempunyai resolusi ruang yang lebih tinggi.Untuk bahagian kedua kajian ini, kami

melaporkan jaminan mutu (QA) prosedur kualiti yang penting sebelum rawatan SRS

yang mempengaruhi penghantaran dos. Prosedur QA ini terdiri daripada ukuran

ketepatan dalam penempatan sasaran dan penjajaran laser bilik. Dos yang dikira akan

dihantar untuk rawatan telah disahkan dengan menggunakan kebuk pengionan 3D

pinpoint dan TLD 100H. Sisihan min dos diukur menggunakan TLD berbanding dos

dikira adalah 3.37%. Selain itu, kebuk pengionan 3D pinpoint memberikan hasil yang

lebih tepat dos berbanding TLD 100H. Dos yang diukur menggunakan kebuk v

pengionan 3D pinpoint memberikan keputusn yang lebih baik dengan dos dikira

dengan TPS dengan sisihan kurang daripada 2.17%. Keputusan yang diterima seperti

yang disyorkan oleh Suruhanjaya Antarabangsa Mengenai Unit Sinaran dan

Pengukuran (ICRU), berdasarkan Laporan No 50 bahawa dos dihantar kepada jumlah

sasaran mesti berada dalam kesilapan ± 5%.

vi

TABLE OF CONTENTS

CERTIFICATE i

ACKNOWLEDGEMENT ii

ABSTRACT iv

ABSTRAK v

TABLE OF CONTENTS vi

LIST OF TABLES viii

LIST OF FIGURES x

CHAPTER I INTRODUCTION 1

1.1 Stereotactic Radiosurgery (SRS) treatment process 1

1.2 Beam data needed for new TPS 5

1.3 Dose calculation algorithm 6

1.4 Dose verification 9

1.5 Aim and objectives of the study 10

CHAPTER II LITERATURE REVIEW 11

CHAPTER III MATERIALS AND METHODS 13

3.1 Materials 13

3.2 Methods 32

3.2.1 Beam Data Measurements 32

3.2.2 Quality Assurance (QA) procedures before SRS treatment

42

3.2.3 Dose verification 46

CHAPTER IV RESULTS 51

4.1 Beam data Measurements 51

4.2 Quality Assurance (QA) procedures before SRS treatment

69


vii

CHAPTER V DISCUSSION 82

5.1 Beam data Measurements 82

5.2 Quality Assurance (QA) procedures before SRS treatment

90


CHAPTER VI CONCLUSION 99

REFERENCES 102

APPENDIX 107

A Absolute Linac Output Calibration 107

B Radiation Isocenter Alignment 111

C Dose Verification Plan 115

viii

LIST OF TABLES

Table No.

Page

3.1 Technical specifications of PTW Pinpoint 3D ionization

chamber

22

4.1 Results of R100, R80, R50, D100 and D200 for different cone

diameters

53

4.2 Results of R100, R80, R50, D100 and D200 for different cone

diameters compare to 100 x100 mm2 open field measured

using ionization chamber

55

4.3 Results of R100, R80, D100 and D200 for different cone

diameters compare to 100 x100 mm2 open field measured

using EDR2 Film

57

4.4 Results of CAX, Field Size, Penumbra, Flatness and

Symmetry for different cone diameters measured using

ionization chamber

60


Symmetry for different cone diameters compare to 100 x100

mm2 open field

62


Symmetry for different cone diameters measured using EDR2

Film

64

4.7 Results of measured field sizes analyzed using Ray 3.0 Film

Dosimetry Software

66

4.8 Relative dose at off-axis distances for various circular cone

diameters measured using the film and the ionization chamber

67

4.9 Results of measured coordinates of the top centers of each

object compared with known coordinates specification

70

4.10 Results of localization errors of CT using Brainlab CT

Localizer

71

4.11 Results of localization errors of CT using Radionic CT

Localizer

72

ix

4.12 Results of CT localization error determined by 3 different

observers

73

4.13 Results of isocenter deviation on irradiated film analyzed with

conventional technique

75

4.14 Results of isocenter deviation on irradiated film analyzed with

Vidar scanner and Film Dosimetry software

75

4.15 Results on the shift distance of Target Positioner on irradiated

film

77

4.16 Results of overall target setup accuracy 78

4.17 Results calculated dose by TPS and measured doses using

pinpoint 3D ionization chamber

80

4.18 Results calculated dose by TPS and measured doses using

TLD 100H

81

8.1 Results of nominal output at calibration depth (1.5cm) 109

8.2 Results of gantry isocenter shift analyzed using Ray 3.0 Film

Dosimetry Software

112

8.3 Results of couch isocenter shift analyzed using Ray 3.0 Film

Dosimetry Software

114

x

LIST OF FIGURES

Figure No.

Page

1.1 Geometry of a circular cone dose calculation 6

3.1 Siemens Primus Linear Accelerator, SN : 3347 13

3.2 The various diameter of Radionic Circular Collimator 15

3.3 T41022 PTW MP3 computerized water tank 16

3.4 CNMC Model 74-320 mini water phantom 17

3.5 TN30013-2 PTW Farmer Ionization Chamber, SN : 5782 20

3.6 TN31016 PTW Pinpoint 3D Ionization Chamber,

SN: 441

21

3.7 Ready pack Kodak X-OMAT V film 23

3.8 PTW Unidose E Electrometer 25

3.9 PTW TANDEM Dual Channel Electrometer 26

3.10 Vidar VXR-16 Dosimetry PRO Advantage Film Scanner 27

3.11 Konica Film processor model SRA 101A in the dark

room

28

3.12 Head phantom with four different geometrical structures 29

3.13 CRS IMRT Thorax phantom with chamber insert 30

3.14 Solid water phantom slabs with TLD insert slab irradiated

under circular cone beams

31

3.15 PTW TBA control unit 33

3.16 Schematic diagram representing the experimental set up

for PDI measurement using ionization chamber.

34


for Off-axis Ratio (OAR) measurement using ionization

chamber

38

3.18 Experimental set up for Off-axis Ratio (OAR)

measurement using ionization chamber.

38


for scatter output factor measurement using ionization

chamber

41

xi

3.20 Experimental set up for Scatter Factor measurement using

ionization chamber

42

3.21 Radiosurgery Head phantom setup on the CT scanner and

the image of the study

45

3.22 IMRT Thorax phantom setup on the treatment couch 47

3.23 Verification of dose on the phantom using Phantom

Mapping software

49

3.24 Set-up geometry showing TLDs position, total depth of

16cm phantom used same as dimension of IMRT Thorax

phantom

50

4.1 PDD versus depth for various circular cones using

ionization chamber

52

4.2 PDD versus depth for 3 circular cones compare to 10 x10

cm2 field size.

54

4.3 PDD versus depth for 6 circular cones field size compare

to 10x10 cm2 open field

56

4.4 Off-axis Ratio for all circular cones diameters 58

4.5 Beam profiles for various circular cones plotting by

MEPHYSTO mc2 analyzing beam software.

59

4.6 Figure 4.6: Beam profiles for 3 circular cones compare to

100 x100 mm2 open field

61

4.7 Beam profile for 6 circular cones diameters measured

using EDR2 Film

63

4.8 Field size for 45 mm cone diameter, SID = 105 cm

measured using EDR2 film and analyzed by Ray 3.0 Film

Dosimetry software

65

4.9 Scatter factor for circular cone diameters with different

size of jaws opening

68

4.10 Image of treatment plan which center of target assigned at

the tip of cones geometry inside the Radionic human head

phantom using i-Plan TPS

69

4.11 Verification film of isocenter shift with respect to the 74

xii

room’s laser system

4.12 The schematic of analyzed verification films for

determining isocenter shifts

76

4.13 Verification film of isocenter shift with respect to the

target coordinates

77

4.14 Charge measured using pinpoint 3D ionization chamber

versus dose delivered

79

8.1 Star shot film of gantry isocenter shift analyzed using Ray

3.0 Film Dosimetry Software

112

8.2 Star shot film of couch isocenter shift analyzed using Ray

3.0 Film Dosimetry Software

114

xiii

CHAPTER I

INTRODUCTION

1.1 STEREOTACTIC RADIOSURGERY (SRS) AND TREATMENT PROCESS

Stereotactic radiosurgery (SRS) is generally defined as a single-fraction high-dose

radiation therapy treatment delivered using rigid immobilization to optimize precision

(Russel et al. 1995). This treatment is used in order to treat inaccessible intracranial target

in a single treatment without invasive surgery. In the early day, it was established to treat

functional disorder, then after several years it had been proven success to treat non-

malignant such as Arteriovenous Malformation (AVM), benign tumors such as acoustic

neuromas. Recently, it was used to treat wide variety of benign and malignant.

A magnetic resonance imaging (MRI) scan will be conducted before radiosurgery.

No frame is required for the scanning. The scans are from foramen magnum through

vertex. The scan images will be transferred through Local Area Network (LAN) to the

treatment planning system in the radiotherapy department. On the day of radiosurgery,

physician injected a local anaesthetic at four points and the stereostatic ring to patient’s

head using four saphire tipped pins. The ring or frame is called as Brown-Roberts-Wells

(BRW) frame.

1

Next, patient is taken to radiology department for a CT scan. The CT- localizer is

attached to BRW frame or head ring. The frame is attached to the frame holder that

attached to the couch of CT scanner and screwed to it. So that the patient head and the

frame extend beyond the couch. A series of CT images are taken with 2.4 mm slices

thickness. For AVM patient, patient was taken to angiogram suite in the same department

for cerebral angiogram procedure. The radiologist will do the procedure to localize the

AVM inside the brain by applying the contrast through the venous from limb up to brain.

After that, the angiogram localizer then is attached to the BRW frame again and then the

AP and Lateral radiograph images were taken. The radiograph films were taken to be

scanned for reference to localize the target volume in the CT image for SRS treatment

planning.

After the CT scan procedure, the patient was brought back to the ward. The

patient stays in a comfortable room to relax with family and friends, under the

supervision of nursing staff. In the meantime, the physicist transferred the CT data to

treatment planning computer (TPS) through LAN with DICOM format. By using the

automatic detection, identification of localizer markers only took a few seconds. From

that, the physicist started to contour the external body on the CT images. Then, the

neurosurgeon took over to draw the tumor boundary for all CT slices where the lesion is

present and visible. The simultaneous display of multiple slices enables neurosurgeon to

better understand the precise special location of the lesion and to better identify it with

minimal interactive effort. The neurosurgeon additionally marks critical structures, in this

case specifically the optic chiasm, brainstem and the optic nerves. If needed, the MRI and

CT scan images were fused to get better visualization of tumour volume. The fusion of

the images process took just a few second after neurosurgeon identified at least three

landmarks on the CT and MRI images. Fusion is accomplished using the intensity match

method and generally acceptable values must be less than 1.5mm deviation. Accuracy of

the fusion is checked by sliding the MR image over the CT image in different planes and

different slice positions.

2

After about 30 minutes, the neurosurgeon task over to physicist who will continue

the treatment planning. The first step is to position an isocenter with five to seven arc

planes in such a way as to cover the complete tumour volume. It turns out that the rather

complex shape of the lesion requires use of a second isocenter. This step provides full

coverage of the lesion but at the same time increase dose to the certain organ at risk so

that some adjustment must be done to make sure uniform dose to the target or lesion and

organ at risk must get dose less than the tolerance dose. The adjustment can be done by

removing arc angles and avoiding the hotspots area.

Then the oncologist will call to look at the dose-volume histogram, the acceptable

isodose cover the lesion volume, minimal dose to the organ at risk and maximum dose

inside the lesion volume. After all the factors were satisfied, the plan will be approved

and then with discussion with neurosurgeon, the oncologist will prescribe the dose to be

delivered to the lesion volume. The radiation dose was prescribed to 80-95% isodose line

to cover the lesion. The radiation dose varied from 12 to 24 Gy for a single fraction

radiosurgery (Zamzuri et al. 2006). The physicist prints out the result of the planning. The

plan printouts consist of all treatment parameters, collimator size, isodose to target

volume and organ at risk, dose volume histogram, arc plane angle, monitor unit per angle

and projections of target volume.

The physicist and the team then proceed with the records to the LINAC room to

perform Quality Assurance (QA) procedures before SRS treatment. The Radionic

couchmout was attached to the treatment table. The gantry was turned to 180 degree,

then; the Radionic collimator mount was attached and screwed to the tray slot on the

gantry head. After that, the plan’s collimator size was slotted into the collimator mount.

Then, the film adaptor was attached to the collimator mount. In order to control and

document the mechanical precision of the Linear Accelerator, the mechanical isocenter

standard (MIS; Radionics Inc.) stand is screwed on the floor straightly below the gantry

head. A small crosshair metal device is attached to the stand. The wall laser is lined up to

3

the crosshair of the device at the LINAC isocenter. After that, the device is replaced by a

plexiglass insert tipped with a tungsten sphere.

The maximum deviation of the center of the light shadow cast by the sphere to the

center of circle is less than 0.2 mm for all exposures. Besides that, a verification film is

placed in the collimator beam, using film holder attached to the collimator mount. For

each of various pre-selected gantry and table angles, the physicist then radiates different

areas of film through the tungsten sphere which is positioned in the center of collimator

beamline cross-section. The films then were developed and the physicist analyzed the

films. The test proves that at multiple relevant gantry and table angles, the beam precisely

passes through the laser marked LINAC isocenter.

After QA procedures finished, the patient was brought into the Linac room and

positioned on the treatment table. The headring is secured to the adaptor of the

couchmount and the Target Positioner is mounted to the headring. The treatment table is

fine positioned and adjusted in order to line up the target cross with the wall mounted

laser crosshair. The micro adjustments of the couchmount are used for fined-tuning the

alignment. After this is complete, the Target Positioner is removed from the headring.

The treatment team then proceeds out of the treatment room and further watches the

patient via the video control system. The treatment team reassured the patient that the

whole treatment process would involve minimal discomfort and patient is advised to not

move the head during the treatment process. Meanwhile, after entering the pre-planned

gantry and table angles, The LINAC operator then initiates the treatment. After half an

hour of treatment time, the clinical team re-enter the LINAC room and unmount the

headring from the couchmount. Next in order to remove the headring from the patient, the

neurosurgeon then unscrews the pins which fixated the headring and patient and the SRS

treatment delivery process finished.

4

1.2 BEAM DATA NEEDED FOR NEW TPS

For installing a new SRS treatment planning computer, a comprehensive set of new beam

data of the LINAC machine must be measured and then to be entered into the new

treatment planning software. In order to get a comprehensive set of beam data, specific

measurements must be completed before performing stereotactic treatments with very

small field sizes. There are three basic beam parameters to be measured for dose

calculation: tissue phantom ratios, relative output factors and single beam profiles.

Although dose calculation can be carried out using only a limited amount of beam

data measurements, the way in which this beam data is collected presents challenges to

the physicist. In particular, narrow radiosurgery beams with sharp dose gradient require

detectors and chambers with high spatial resolution. For this reason, the smallest detector

available should be used when performing small field dosimetry (Alfonso et al. 2008 and

Sauer et al. 2007).

For central axis measurement such as depth dose, tissue phantom ratio, scatter

output factor, the detector dimensions should be significantly smaller than the field sizes.

Special care is required when selecting and handling the required dosimetry equipment.

For small field sizes, it is particularly important to correctly align the water phantom and

the detector movement direction in relation to the beam axis and the beam center. Even if

the detector size is suitable for the small fields to be measured, accurate sensitivity

correction which is related to the energy dependency of the detector signal must be

applied in accordance with the specifications provided by manufacturer of the detector.

5

1.3 DOSE CALCULATION ALGORITHM

In general, the main assumption of stereotactic dose calculation models is that secondary

scatter can be assumed to be limited significance. Several authors have proposed and

investigated this hypothesis (Rice et al. 1987, Luxton et al. 1991, Hartman et al. 1995)

and dose calculations have tended to be a function of only three basic beam parameters:

tissue phantom ratios, relative output factors and single beam profiles. Scatter (phantom

scatter) is considered to be implicit to these measurements and does not vary significantly

with depth in a medium. Here the dose algorithm and calculation are described as they are

applied and implemented within the circular arc treatment modality.

Figure 1.1: Geometry of a circular cone dose calculation

6

The dose D at an arbitary point P in a plane through the isocenter of a fixed

photon beam is calculated using the following equation:

D(c,d,r,R) = MU .M . TPR (d,c). OAR (c, r’) . St (c). (SID/R)2

where:

c = Collimator diameter at isocenter, measured perpendicularly to beam

central axis (mm)

d = Depth of point in tissue (mm)

r = Radial distance from the central axis of the point of interest (mm)

R = Distance of point from source (mm)

MU = Monitor units of the fixed beam

M = Calibrated output of Linac (Gy/MU)

TPR (d,c) = Tissue phantom ratio for collimator diameter c at tissue depth d

OAR (c, r’) = Off-axis ratio for collimator diameter c at a radial distance r’ from the

central axis at isocenter level, where: r’ = r. SID / R

St (c) = Total scatter factor (often referred to as relative output factor for

collimator of diameter c)

Other factor which is taken into account for dose calculation is the number of arc, where

an arc it is simulated by a finite number of fixed beams. Then, Uniform Monitor Unit

distribution also calculates by obtaining a uniform monitor unit value per degree of

distribution. All these calculations assuming the tissue as a density equivalent to water, or

it can be equivalent depth, taking into account any tissue density inhomogenities.

There also several limitation of Circular Cone Algorithm. When a radiosurgery

dose is calculated, the change in the curvature of the skin surface across the field of each

fixed beam segment of an arc is not considered but errors introduced with this problem

are very small (Technical Reference Guide Rev. 1.6, Brainlab Physics Document, 2012).

7

The circular cone dose algorithm uses tabulated measured values for the dose calculation.

So that, the usage of these algorithm outside the range of measured values is not

recommended. As example, when using the circular cone algorithm in dose calculation

for depth different from the depth at which the off-axis ratios have been measured, the

results may be inaccurate due to calculated penumbra width may be different from the

penumbra in real dose delivered. The extrapolated values do not represent reality with the

same accuracy as dose algorithm generally does. When using the circular cone algorithm

in dose calculation near inhomogenous areas such as lung or bone tissue or close to the

tissue border (both within range of a few centimeters), the calculated dose can deviate

from real dose delivered by more than 10%.

8

1.4 DOSE VERIFICATION

Particular attention may be paid to tests for TPSs that deal with specialized techniques

such as stereotactic radiosurgery. Absolute dose verification entails a rigorous

comparison between measured dose using detectors and dose produced by TPS based on

the input data (measured beam data).To determine the best dosimetry system for

measurements at the small focal point of beams, several different detectors were

investigated. These included an ionization chamber, LiF thermoluminiscent dosimeter

chips and films.

All of these detectors were calibrated in a phantom against an ionization chamber

whose calibration is traceable to Secondary Standard Dosimetry Laboratory (SSDL).

Ionization chamber measurement of these small beams and steep dose gradients often

suffer from lack of lateral equilibrium and chamber volume effect. Due to the general

uncertainty and difficulty in these measurements, measured data should be verified using

two or more dosimetric methods (AAPM Report No.47). It is important for practicing

medical physicist to be able to quantify errors involved in the use of particular TPS in

clinical use at his or her department. In this study we chose to verify the absolute dose

from measurement with different detectors compared to calculated dose from SRS

treatment planning system before it can be clinically used.

9

1.5 AIM AND OBJECTIVES OF THE STUDY

AIM

The aim of the study was to measure the dosimetric parameters for small photon beams to

be used as input data for treatment planning computer system (TPS) calculation and to

verify the dose calculated by TPS in Stereotactic Radiosurgery (SRS) procedure.

OBJECTIVES

The objectives of this study were:

1. To measure the dosimetric parameters such as Percentage of Depth Dose (PDD),

Off–axis Ratio and Scatter Factors for various fields used in SRS treatment as

input data for treatment planning computer system (TPS) calculation

2. To perform the important Quality Assurance (QA) procedures before SRS

treatment

3. To verify the accuracy of the dose calculated by TPS by measuring the dose

delivered using pinpoint 3D ionization chamber and TLD 100H.

10

CHAPTER II

LITERATURE REVIEW

There were two crucial components in SRS in order to achieve the aims of treatment

planning. The first component is to deliver precise and uniform dose to the target lesion

with multiple noncoplanar beams. The second factor is the accuracy in positioning of

patient by introducing the stereotactic apparatus. Result from these two factors, SRS

provide uniform dose and sharp dose falloff out of small target lesion and at the same

time produce minimum damage to the surrounding critical organs (usually brain tissue)

by application of a collimated and confined beam directed toward the target (Faiz M.

Khan, 2011).

The application of very small beam make SRS is significantly different from

conventional radiotherapy. In order to understand the characteristic of the small beam

dosimetry is very challenging. The small field dosimetry imposes slightly different

criteria on the measurement devices that may be consider for being used compare to those

used for larger field sizes. One difficulty is by selecting the right size of ionization

chamber’s volume whether it can perturb the beam by volume averaging over their

relatively large active volume. Then, it produces significant dose measurement errors

when measuring PDD, Off Axis Ratio, Beam profile, Scatter Output Factor and absolute

dose for small fields (Serago et al. 1992).

11

The geometric position of the isocenter during rotations is usually assumed to be

inside a virtual spherical volume. Minimizing the isocenter movement could improve the

accuracy of SRS treatment and this issue has been considered with special intention. The

AAPM Task Group Report 142 recommends up to ± 1 mm deviation between the

radiation and mechanical isocenter is acceptable for SRS treatments. This

recommendation made because the result of study has been reported that 2 mm

positioning error in spine SRS could lead to more than 5% loss of tumour coverage and

more than 35% increase in dose delivery to the healthy tissues. With an accuracy of 1 mm

of SRS treatment, the error in dose delivery was reduced to less than 2%. It must note that

errors in SRS treatment delivery are nonrecoverable, since the treatment is delivered in

one session. The isocenter verification process is compulsory to be performed before each

SRS treatment (Pejman Rowshanfarzad et al. 2011).

One of the most important features of radiation treatment of radiation treatment

planning is dose calculation. Errors in dose calculation by treatment planning computers

are one of component in the overall uncertainty in dose delivery to the patient. Many

authors have made recommendations about the accuracy required of treatment planning

computers, differentiating between calculations at central axis and off-axis points. Error

in dose calculation by treatment planning computers are known to arise when calculation

algorithm do not account for electron disequilibrium near interfaces between tissues of

different density (L. Mc Dermott et al. 2004).

12

CHAPTER III

MATERIALS AND METHODS

3.1 MATERIALS 3.1.1 Linear Accelerator

Figure 3.1: Siemens Primus Linear Accelerator, SN: 3347 The comprehensive set of beam data measurements were carried out on radiation beam

generated by linear accelerator of Siemens Primus that available in Nuclear Medicine,

Radiotherapy and Oncology department of HUSM. The linac was installed since 2001

and has being used for external radiation therapy treatment and various researches for

twelve years already. The linac was being used to deliver conventional external beam

radiation therapy with several types of high energy photon and electron.

13

Electrons been accelerated by high frequency electromagnetic wave to treat

superficial tumor or , it can be made to strike a tungsten target for producing x-rays, for

deep-seated tumor. There are two energy of photon available for the linac which is 6MV

and 10MV; while the electron beams available are 6, 9, 12, 15, 18 and 21 MeV. Total of

58 leaves of Multi Leaves Collimator (MLC) responsible in beam shaping in X jaw

direction. This linac also equipped with miniature multileaf collimator (mMLC) for

treating Fractionated Stereotactic Radiotherapy (SRT) and cones to facilitate SRS

procedure. With appropriate technical modifications and enhanced treatment planning

capabilities, this linac can also be used for other advanced radiotherapy procedure like

Intensity Modulated Radiation Therapy (IMRT).

14

3.1.2 Circular Collimator

Figure 3.2: The various diameter of Radionic Circular Collimator

Circular collimator (cone) is stereotactic hardware in traditional SRS that defines a

circular aperture for the beam. The cones are inserted to the collimator housing with

retainer cap to keep it in place then secured to the base plate at gantry head. The housing

basically could be mounted to and removed from the Siemens Primus linac with precise

and reproducible alignment without difficulties. They are all conically molded for sharp

penumbra and steel-jacketed for precise alignment.

A set of 13 circular collimators of Radionics Inc., USA from 5 mm to 45 mm in

diameter at isocentre distance with 2.5 mm steps were available for the delivery of SRS

treatment. The accessories were mated with the linear accelerator and the irradiation was

carried out with a 6 MV photon beam with a fixed jaws opening of 5x 5cm2. High energy

radiation from linac is collimated to fine beams using this tertiary collimation system then

being focused to the small target precisely.

15

3.1.3 Motorized Water Tank

Figure 3.3: T41022 PTW MP3 computerized water tank In this work, measurements were made in water using medium-sized PTW MP3 water

tank. Measurement was done inside the water because it is nearly tissue equivalent and

has uniform composition regardless of its origin. Besides being universally available, it

closely approximates the radiation absorption and scattering properties of muscle and

other soft tissues and more than 70 percent of human body contain by water.

The tank should extend at least 50mm beyond all four sides of the measurement

field size at the depth of measurement. It should also extend to at least 50mm beyond the

maximum depth of measurement (AAPM TG-106, 2000). The wall of the tank is made up

from 20mm thick of acrylic, and the scanning range is of 50 x 50 x 40.8 cm³. The water

tank movement could be controlled by a removable control pendant that being connected

to TBA control unit with TFT display and menu-controlled interface for a manual control

of the water tank.

16

Another component is ScanLIFT, which is a high-precision electro-mechanical

carriage on wheels, with 500mm range of movement to allow for height adjustment of the

MP3-M water tank. Then, set of detector holders assist in installing and positioning the

ionization chambers and solid state detectors in PTW water phantoms quickly. The

detector position in PTW water phantom also could be automatically control by TBA

control unit via RS232 interface.

3.1.4 Mini Water Phantom

Figure 3.4: CNMC Model 74-320 mini water phantom

CNMC Model 74-320 water phantom with dimension of 30x30x40 cm3 was being used in

this study. The water tanks were constructed of 3/8 in acrylic and provided with side-

mounted handles, a drain and a ball valve. This phantom presented with convenient

manual ion chamber depth positioning.The ion chamber holder teflon block slides on two

stainless steel rods and is driven by a stainless steel lead screw.

With that, the chamber position inside the phantom could be adjusted up and

down the water tank manually by turning the handle. Various ionization chamber could

be hold by the chamber holder block, which has a 7 mm hole with 3 set screws. The

chamber holder that is supplied with the phantom can accommodate cylindrical ion

17

chambers of various diameters, from 3 mm to 16 mm. Plane-parallel ion chamber holders

are also available as an option.

3.1.5 Beam Analysis Software

Mephysto mc2 version 1.7 was used for collecting and analyzing the beam measurement

data. This computer software basically controlled the PTW MP3 3D radiation field

analyzer system. It is a modular, workflow-oriented software platform for beam data

acquisition and analysis using a PTW water phantom system. It well-equipped with

intuitive navigator user interface and a planning module option for Radiotherapy

Treatment Planning System (RTPS). It also can be used for absolute dosimetry option for

absolute dose measurements with the TANDEM electrometer or other PTW electrometers.

Beam data acquisition can be made easy and fast with multilple queue drop and drag task

list that available in the software.

The software also can automatically plotted ionization depth curves and beam

profile graph by obtaining the data from the measurement. The ionization depth curves

can be converted into PDD according to all established international protocol. From the

graph, it can show the result of depth for maximum dose for PDD graph. It also can show

the PDD table for all the field sizes that have been measured inside a graph with different

colors to identify different field sizes. Other than that, a table generator for table creation

and PDD conversion to Tissue Phantom Ratio (TPR) curves is available.

Beside the PDD graph, the result of symmetry, flatness and penumbra for beam

profile also can be showed. The result based on the standard that we selected such as

AAPM Task Group 51 and IAEA Technical Document 398. All raw data is save as

Microsoft Excel files then it will be easier to be imported and exported with copy and

paste function.

18

3.1.6 DETECTORS

3.1.6.1 Ionization Chamber

Ionization chamber (IC) is one of the detectors that widely used for detection of ionizing

radiation by measuring the number of ion pairs created within the gas caused by

ionization process. The usage of ionization chamber is a standard protocol for measuring

ionization process in the radiation therapy field. It is direct measurement dosimeter

compare to dosimetry film and thermoluminiscent detector (TLD).

The inert gas such as argon is used as medium inside the sensitive volume. The

ionization process occurs when the photons interact with the gas inside the chamber and

produced ion pair which is positive and negative charges. The electric field created by the

potential difference between the anode and cathode causes the negative charge (electron)

of each ion pair to move toward the anode while the positively charged gas atom or

molecule is drawn to the cathode. The movement of the ions to the collecting electrodes

results in an electronic pulse. Since these pulses are usually too small to be detected, the

most common approach is to measure the ion chamber’s current which is produced by

many radiation interactions in the detector and is more easily measured than the

individual pulses. There were several types of ionization chamber used throughout this

study which are PTW Farmer, PTW Pinpoint 3D, and PTW Semiflex ionization

chamber.

19

http://en.wikipedia.org/wiki/Ion

3.1.6.1.1 PTW Farmer Ionization Chamber

Figure 3.5: TN30013-2 PTW Farmer Ionization Chamber, SN: 5782

A calibrated cylindrical ionization chamber with a cavity volume of at least 0.125 cm3

but not more than 0.6 cm3 is required. The effective point of the measurement shall be

determined based on valid international dosimetry standard as example IAEA TRS 398:

2000 and the corresponding recommendation of the detector provider (Technical

Reference Guide Rev. 1.6, Brainlab Physics Document, 2012).

Farmer type ionization chamber type: 30013 was basically a vented cylindrical

ionization chamber that ruggedly constructed with very sensitive volume of 0.6 cm3. The

absorbed dose to water for the chamber is 5.362 cGy/nC calibrated under Co-60 beam.

The guard ring of the chamber was designed fully guarded up to the measuring volume.

The wall material is graphite with a protective acrylic cover, and the electrode is made of

Aluminum. It is suitable for measuring high-energy photon and electron radiation in air or

in phantom material. It is waterproof which means no protective sleeve required while

being used inside the water.

20

This chamber includes a 3.3 ft (1 m) cable, BNC Triax connector, and a PMMA

buildup cap. The electron energy range is from 10 MeV to 45 MeV while the nominal

photon energy ranges from 50kV to 50MV. Since this chamber is delicate construction,

extreme care is needed when handling it. An acrylic build-up cap for in-air measurement

in 60Co beams is included with the chamber, as well as a calibration certificate by

Secondary Standard Dosimetry Laboratory (SSDL), Agency Nuclear Malaysia.

3.1.6.1.2 PTW Pinpoint 3D Ionization Chamber

Figure 3.6: TN31016 PTW Pinpoint 3D Ionization Chamber, SN: 441 PTW Pinpoint 3D Chamber Type 31016 is a fully guarded chamber, waterproof and

suitable for measurement in air, solid state phantom and water as well. It was designed

specifically for characterization of linac radiation beam. Due to its superior spatial

resolution, it is ideal for measurement of small field as encountered in Intensity

Modulated Radiation Therapy (IMRT), Intra Operative Radiation Therapy (IORT) and

Stereotactic Radiotherapy (SRT) beam.

21

A very small detector for high resolution profile measurements and dosimetry of

small field is required (Technical Reference Guide Rev. 1.6, Brainlab Physics Document).

This Pinpoint chamber is ideally suited to this purpose with inner diameter of only 2.9mm

and sensitive volume of 0.016cm3. After being calibrated, this chamber can be used to

measure depth dose and absolute dose. This chamber shows a flat angular response, since

the measuring volume is approximately spherical. It consists of 28mm long aluminum

stem for mounting and a 1.3m cable with choice of connector. The nominal energy range

is up to 6MV for photon and from 6MeV to 50MeV for electron respectively.

Table 3.1: Technical specifications of PTW Pinpoint 3D ionization chamber

Active volume 0.016cm3

Area density 89mg/cm3

Sensitivity 0.04nC/cGy

Leakage current <4 x 10-15 A

Wall thickness 0.66mm

Wall material Acrylic (PMMA) and graphite

Electrode material Al 99.98 R

Electrode diameter 0.3mm

Electrode length 1.6mm

22

3.1.6.2 FILMS

3.1.6.2.1 X-OMAT Therapy Verification, Kodax

Figure 3.7: Ready pack Kodak X-OMAT V film A 25.4 cm x 30.5 cm ready pack film was cut into 4 pieces with same size inside the dark

room. Then the side of small film was closed with black PVC wire tape to provide a light-

tight condition on the cutting side of the film. The film was placed midway between the

collimator beam and tungsten ball, using film holder attached to the collimator mount.

For each of various pre-selected gantry and table angles, the physicist then

radiates different areas of the same film through the tungsten sphere which is positioned

in the center of collimator beam line cross-section. 70 MU was used to expose the film

for each position angle. The films then were developed and the physicist analyzed the

films and concluded that the maximum deviation of the center of the light shadow cast by

the sphere to the center of circle is less than 0.2 mm for all exposures.

23

3.1.6.2.2 Extended Dose Range 2 (EDR2) Ready-pack Film, Kodax

Dosimetry film used in this study was EDR2 film. It is a ready-pack film for monitoring

and evaluating the dose at the therapy energies. The film is relatively insensitive film, but

it can detect wide range of exposure and doses with a linear response. The film response

is up to 7 Gy. The dimension of 25.4 cm x 30.5 cm of film was used. It can be cut into

half in the dark room but must be sealed with black PVC wire tape to provide a light-tight

condition on the cutting side of the film. The same batch was used for calibration and the

PDD and profile measurements. After the film was irradiated, it was processed by using

film processor.

3.1.6.3 Thermoluminiscent Detectors (TLDs)

Besides ionization chambers and films, thermoluminescent detectors were also used in

this study. The TLDs used in this study were lithium flouride doped with magnesium,

copper and phosphorus. This type of TLD also called as TLD 100H. TLD 100H is

suitable in environmental, personal and extremity dose monitoring. TLD 100H used in

this study is the chip type with dimension of 3.2 mm x 3.2 mm x 0.09 mm. Due to its

smaller in size, it is suitable to simulate point dose detector in our study.

The density of lithium fluoride which is the main element for this type of TLD is

2.64 g/cm3 and it has an effective atomic number of 8.4 which is close to that of soft

tissue (7.4), and therefore it may be considered nearly tissue equivalent with an energy

response that is uniform down to energies of about 200 keV. Before the TLD 100h chips

could be used for dosimetry purpose, they had to be annealed in the oven at 240⁰C for 10

minutes. Then, they must be cooled at room temperature for a minimum 1 hour before

irradiation procedure. 24 hour after irradiation, the response of the TLDs was read out

using Harshaw TLD reader.

24

3.1.7 ELECTROMETER/DOSEMETER

3.1.7.1 PTW Unidose E Electrometer

Figure 3.8: PTW Unidose E Electrometer

UNIDOS E electrometer is a product of PTW which being used in this study. It is

basically suitable for universal use in both radiation therapy and diagnostic radiology.

This digital electrometer is compact, economical, lightweight and user-friendly. It

complies with the several standards such as IPEM guidelines on dosimetry transfer

instruments as a secondary standard dosemeter or electrometer, thus make it being used

worldwide.

PTW ionization chambers and solid-state detectors can be connected to this device.

It can measure integrated dose or charge and dose rate or current simultaneously. The

values of dose and dose rate are measured in Gy, R, Gy/min, R/min or Gy/m. While, the

electrical values charge and current are measured in Coulumb and Ampere. The values

can be possibly store in a chamber library. This device includes automatic leakage

compensation and the high voltage between the ion chamber electrodes is checked

automatically. It utilized RS-232 interface for device control and data output. UNIDOS E

25

indeed a device with high degree of accuracy and excellent resolution and available in

features both mains and battery operation.

3.1.7.2 TANDEM Dual Channel Electrometer

Figure 3.9: PTW TANDEM Dual Channel Electrometer

The TANDEM electrometer is a dual channel therapy dosemeter in accordance with IEC

60731 (field class) with resolution of 10 femtoAmpere(fA). It can be operated by a

Personnel Computer (PC) as an absolute therapy dosemeter. This type of electrometer is

suitable for absolute dosimetry when using with analyzing software and for relative

dosimetry with TBA systems. Ion chambers and solid state detector could be connected

to it. The TANDEM is being able to do very fast scanning measurements in motorized

water phantom. It has very short time constant of 10 ms and variable voltage supply, thus

makes it possible to set minimum measuring intervals of 10 ms.

The chamber voltage for both channels is individually programmable in 50 V

increments up to 400 V with reversible polarity. It features auto-range and offset

compensation and was calibrated in electrical current (A). TANDEM is also designed to

perform radiation field measurements referenced to an ionization chamber. In conjunction

with a TBA therapy beam analyzer, MEPHYSTO software controls TANDEM for fast

26

and accurate beam data acquisition. A trigger input synchronizes measurements with

external signals.

3.1.8 Film Dosimetry Scanner and film dosimetry software

Figure 3.10: Vidar VXR-16 Dosimetry PRO Advantage Film Scanner

The scanner that has being used throughout this project was Dosimetry PRO Advantage

film scanner that available in Medical Radiation laboratory of School of Health Sciences.

This scanner able to digitize Kodak Extended Dose Range Film (EDR2), Kodak X-

OMAT Verification film, and also gafchromic type film such as External Beam Therapy

(EBT) film (International Specialty Product (ISP)). The scanner able to scan multiple

images at one time and saves files to a PC. It also delivers outstanding geometric

accuracy, consistency and reliability.

For analyzing the films, Ray 3.0 Film Dosimetry software was used. The film

software can make calibration curve by plotting pixel values versus dose in centiGray

(cGy). It also can plot the PDD curve and beam profile. Beside that, it can use to analyze

QA film such as star shot for measuring shift on gantry and table isocenter. So that, with

the existence of dosimetry and QA analysis software, it is reliable for measuring

consistency and precision of treatment plan from treatment planning computer system.

This film dosimetry scanner and software offers ease of use, rugged and reliable

performance.

27

3.1.11 X-ray Film Processor

Figure 3.11: Konica Film processor model SRA 101A in the dark room

The film processor used to process Kodak X-OMAT therapy Verification film is Medical

Film Processor, model SRA 101A (Konica Minolta Medical And Graphic INC.). It is

semi automatic film processor and must be utilize inside the dark room. It takes half an

hour for the system to warm up and reach the operational temperature is about 34°c. The

fixer and developer chemical are checked whether it has been oxidized to oxygen gases

from surrounding or not.

In order to make sure the chemical is in good condition, the chemical that remain

in their replenishing container situated at the bottom of the processor must be replaced at

least once in two months whether the processor has been used or not. If the processor is

frequently used then the chemical must be frequently replaced. The rollers inside the

processor must be clean when replacing the chemical. Before inserting the test film, it is

recommended to insert the unused film to make the processor system is ready and to

remove the dirt on the roller.

28

3.1.12 Radiosurgery Head Phantom

Figure 3.12: Radiosurgery head phantom with four different geometrical structures

We used a phantom supplied by previous TPS vendor (Radionic Inc.) for purpose

evaluating the accuracy of stereotaxy of new TPS (i-Plan). The head phantom is an

octagon shape object which is attached to an oval plastic to its base. Within the octagon

shape is a material, which has the same electron density as the human bone and mimic as

the real human skull. Inside the phantom there were located four structures which are

cone, cylinder, sphere and cube and placed on the Perspex platform.

Previously, the exact stereotactic coordinates for top center of each object are

known which is provided by manufacturer then we can compare them with the calculated

coordinates from the planning system (Lu Wang et al, 2006).

29

3.1.13 CRS IMRT Thorax Phantom

Figure 3.13: CRS IMRT Thorax phantom with chamber insert

The model IMRT-2LFC thorax phantom is designed to address the complex issues

surrounding the commissioning and comparison of treatment planning systems including

for SRS. This phantom provides a simple, reliable method for verification of individual

patient’s treatment plans and delivery.

The phantom is elliptical in shape and represents an average human torso in

proportion, density and structure. The size of phantom is 30cm x 30cm x 20cm. The

phantom is manufactured from materials that faithfully water equivalent within 1% from

50 KeV to 25 KeV. We not used the whole phantom but we used only the inner side of

phantom which is round in shape and suitable for arc therapy QA verification and can suit

with BRW frame. At the centre of the phantom there is an insert for pint point 3D

chamber.

30

3.1.14 Solid Water Phantom

The solid water phantom slabs are made of acrylic material or water equivalent RW3

material. The solid water phantom is designed for use with photon radiation in range from

70 kV up to 50MV and for electron radiation from 1 MeV up to 50 MeV. Due to fact that

human body or tissues contains of 70% of water, the best representative of human body

when interacting with ionizing radiation is water.

The phantoms are used for monitor calibration and quality assurance

measurements. Depth dose measurements are made by varying the measuring depth. To

provide backscatter, slabs are placed below the radiation detector. The slab phantom

consist of several plate 1 mm thick, several plates of 2 mm thick, several plate of 10mm

thick, several plate of 20 mm thick, 1 plate of 30 mm thick and 1 plate of 50mm thick.

The size of the complete phantoms is 30 cm x 30 cm x 30 cm with varied slab thickness.

This combination makes it possible to vary the measuring depth in increments of 1 mm.

Figure 3.14: Solid water phantom slabs with TLD insert slab irradiated under circular cone beams

Radionic Circular cone collimator

TLD chips inside the phantom slabs

31

3.2 METHODS

3.2.1 BEAM DATA MEASUREMENTS

3.2.1.1 PERCENTAGE DEPTH DOSE (PDD) MEASUREMENTS

3.2.1.1(a) PDD measurement using ionization chamber

PDD basically establish the depth variation along the central axis of the beam.

Measurement was being done for 6MV photon energy generated by linear accelerator

Siemens Primus using PTW Pinpoint 3D ionization chamber. Circular cone with 45mm

diameter in size was slotted into the cylindrical housing that being mounted to the head of

linear accelerator. The retainer cap is screwed to keep it in place. The gantry and

collimator position was being set to zero degree angles. The MLC jaws opening were

being set to 50 mm x 50 mm. It is important to verify by inspection that the area outside

the shielded central part of the conical collimator is completely covered by the jaws

(Technical Reference Guide Rev. 1.6, Brainlab Physics Document). Motorized PTW MP3

Water tank, together with the lift was placed under the linear accelerator head and aligned

with room’s laser and filled with distilled water.

PTW Pinpoint 3D chamber was inserted to the chamber holder and placed

perpendicular to radiation beam. The field chamber was positioned so that the centre of

its active volume was parallel to centre of beam axis. The water phantom software used

to measure depth from 0 to the desired depth which is 300 mm depth inside the water. A

step size of 1mm for 0 to 50mm depth and a step size of 5 mm for greater depths were

used. These parameters could be set with water phantom software.

32

The experimental set up for PDD measurement is shown in Figure 16 below. The

chamber was connected to the MP3 dual-channel electrometer for automatic data

collection. The pointer was turned on to measure source-to-surface (SSD) was 100 cm

and continuous Linac irradiation was activated. The measurement was being done in

continuous mode with the PTW Pinpoint 3D ionization chamber moved along the vertical

from bottom (depth 300mm) to the surface of water phantom (depth 0mm) automatically

controlled by TBA control unit.

The charge was collected by a PTW-TANDEM dual-channel electrometer and

PTW Mephysto mc2 software was used to analyze the readings and plotted the PDD. The

percentage of charges were obtained by normalizing the charges of any depth to

maximum charge at a depth which is called dmax . The circular cone of 45mm diameter

then replaced with other circular cones that available subsequently and the procedure was

repeated for each diameter.

Figure 3.15: PTW TBA control unit

33

Figure 3.16: Schematic diagram representing the experimental set up for PDI measurement using ionization chamber. 3.2.1.1(b) PDD measurement using EDR2 Dosimetry Film

The response of EDR2 film was calibrated before it was used PDD and profile

measurements. The cut EDR2 was used to carry out the calibration. Same batch of film

was used throughout the studies to avoid response of a particular type of film due to

manufacturing process.

For 6 MV photon beam, the output of the beam is calibrated to 1cGy/MU

delivered at depth of maximum dose (dmax) which is 1.5 cm depth inside the water with

reference field size 10cm x 10cm and SSD 100 cm. Thus, the EDR2 film was placed in

the solid water phantom at 1.5 cm depth with SSD 100cm. Different MUs were used to

irradiate the film slips using opening field size 10 cm x10 cm.

34

The developed film slips were scanned by using a film scanner. 16 bits of data

depth and resolution 300 was set as the scanning setting. The scanner pixel values of the

films were read by selecting a ROI of 6cm x 6 cm at the center of the beam image. A

graph of averaged pixel value of versus dose was plotted by using Ray Version 3.0 film

dosimetry software. Then, the graph of calibration was used for film analysis throughout

the study for measurement PDD and beam profile.

For PDD measurement, a ready-pack EDR2 film was positioned at top of 10 cm

thickness of solid water phantom. The solid water phantom slabs were positioned

vertically on the treatment couch. The film was aligned to be at the centre of the solid

water phantom slab and the center of the film adjusted straightly to the central axis of the

cone. Cello tape was applied on the each side of the film to maintain the film position.

Then, another 10cm thickness of solid water phantom slabs was sandwiched on the film.

So that, the overall thickness of the solid water phantom to 20 cm.

45 mm circular cone diameter was slot in into the circular collimator with the

gantry angle at zero position. The secondary jaws were set at 6cm x 6cm. SSD was set at

100 cm and the irradiation was made with 300 MU. Then, another film was used with the

same set up for measuring PDD for other cone diameter. The other measurement was

done for selected cone which were 32.5 mm, 25 mm, 15 mm, 10 mm and 5 mm.

After 6 irradiations of the films were done, the EDR2 film was processed using

film processor inside the dark room. Then the films were scanned by the Vidar film

scanner and analyzed with the film dosimetry software. The films were analyzed based on

the calibration graph that has been done previously. To obtain the PDD curve, the

darkness on the film was normalized at the point of maximum response on the film along

the central axis of the beam. Then a line was drawn along the central axis of the beam on

35

the film and the software plotted the PDD against depth automatically. The data and

images for all irradiated film were stored inside the computer.

3.2.1.2 OFF-AXIS RATIO MEASUREMENT

The off-axis ratio function is determined by measuring the cross profile intersecting the

central axis of beam in a direction perpendicular to the central axis beam. The off-axis

ratio values depend on the diameter of the circular cone collimator c, the distance from

central axis r, the depth in tissue d and the source to surface distance SSD. The off-axis

ratio function OAR is defined as:

OAR (c,r,d,SSD) = D (c,r,d,SSD)/ D (c,0,d,SSD)

It is ratio of dose measured at radial distance r relative to dose at the central axis

for a collimator diameter c. Measurement are made at isocenter level, at a constant depth

d in the phantom. The depth usually selected larger than dmax (d is recommended to be at

75 mm that is SSD = 925 mm). Thus formula of OAR might be simplified to:

OAR (c,r,d) = D (c,r,d,SSDd)/ D (c,0,d,SSDd)

3.2.1.2(a) Circular Cone Off-axis and beam profile using ionization chamber

Beam profile established from the off-axis data that being measured perpendicularly to

the beam central axis. It is normally made at several depth of interest. Measurement was

being done for 6MV photon energy generated by linear accelerator Siemens Primus using

PTW Pinpoint 3D Ionization chamber as field chamber.

36

The experimental set up for beam profile measurement using ionization chamber

basically quite similar with the PDI measurement set up. Motorized PTW MP3 water tank

together with the lift was placed under the linear accelerator head properly and filled with

distilled water. Collimator of 45mm diameter was secured to the gantry head and the

gantry been set to zero angle. No reference chamber was placed midway between the

collimator and the field chamber because the cones size was small and the existence of

reference chamber effect the measurement data. While, PTW Pinpoint 3D ionization

chamber as a field chamber was inserted to the chamber holder and placed perpendicular

to radiation beam. The chamber was being positioned so that the centre of its active

volume was parallel to centre of beam axis. The field chamber was connected to the MP3

dual-channel electrometer for automatic data collection. The irradiation geometry was

source to chamber distance (SCD) is 100 cm with measurement depth at 7.5 cm inside the

water for OAR determination.

During the measurement, the symmetrical axis of PTW Pinpoint 3D ionization

chamber was perpendicular to the beam axis at measurement depth. The chamber was

moved perpendicularly away from its central axis. The charges were collected by a PTW-

TANDEM dual-channel electrometer and PTW Mephysto mc2 software was used to

analyze the readings. Following this procedure, Off Axis as a function of distance away

from the central beam axis were finally plotted using the software. The charges were

normalized to charge at central axis. The circular cone of 45mm diameter then replaced

with other circular cones that available subsequently and the procedure was repeated for

each circular cone size.

37

Figure 3.17: Schematic diagram representing the experimental set up for Off-axis Ratio (OAR) measurement using ionization chamber

Figure 3.18: Experimental set up for Off-axis Ratio (OAR) measurement using ionization chamber.

Scanning direction

Pintpoint 3D Ionization Chamber position

38

3.2.1.2(b) Off-axis Ratio and beam profile using EDR2 Dosimetry Film

Beam profiles are important to ensure that the radiation beam is uniform throughout the

selected field size in this study the field sizes are referred to the cone diameter. By using

the EDR2 film for measuring profile, characteristics of the beam can be determined such

as length of penumbra, percentage of the flatness and symmetry.

For beam profile measurement, a ready-pack EDR2 film was positioned at top of

15 cm thickness of solid water phantom. It was aligned to be at the centre of the solid

water phantom slab and the center of the film adjusted straightly to the central axis of the

cone. Cello tape was applied on the each side of the film to maintain the film position.

Then, another 5cm thickness of solid water phantom slabs was placed on top the film. So

that, the overall thickness of the solid water phantom to 20 cm and the film positioned at

5 cm depth inside the solid water phantom. 45 mm circular cone diameter was slot in into

the circular collimator with the gantry angle at zero position. The secondary jaws were set

at 6cm x 6cm. SSD was set at 100 cm and the irradiation was made with 300 MU. Then,

another film was used with the same set up for measuring beam profile for other cone

diameter. The other measurement was done for selected cone which were 32.5 mm, 25

mm, 15 mm, 10 mm and 5 mm.

After 6 irradiations of the films were done, the EDR2 film was processed using

film processor inside the dark room. Then the films were scanned by the Vidar film

scanner and analyzed with the film dosimetry software. The films were analyzed based on

the calibration graph that has been done previously. To obtain the profile curve, the

darkness on the film was normalized at the center axis the film. Then a line was drawn

along the central axis of the beam on the film and the software plotted the profile against

distance automatically. The data and images for all irradiated film were stored as TIFF

file inside the computer.

39

3.2.1.3 SCATTER FACTOR MEASUREMENT

The total scatter factor St is sometimes referred to as the relative output factor. It is

defined as the ratio of dose measured for a treatment planning program specific setup and

a linac calibration specific setup. The total scatter factor St is defined as:

St (c) = D (c, 0, dmax, SSDdmax) / D (100x 100 mm2, 0, dcal, SSDcal) It is the ratio of dose at isocenter in depth dmax on central axis for a collimator diameter c

relative to the dose measured at reference condition in standard 100x 100 mm2 square

calibration field. The scatter factors accounts for the attenuation of the beams that is due

to the collimating effect of collimator in comparison to open field irradiation.

In this work, CNMC Model 74-320 water phantom was used for measurement of

scatter factor for all circular cones collimator that available. The mini water tank was

filled up with distilled water and then placed on the treatment couch of linear accelerator.

The field size of the beam defined by Radionic circular cone of 45mm diameter inserted

to the housing which being mounted to the head of linear accelerator. The retainer cap is

screwed to keep the cone in place.

Then, PTW Pinpoint 3D Chamber was inserted to the chamber holder and then

connected to the PTW Unidose E electrometer outside the treatment room directly. The

couch adjusted so that the center chamber’s active volume was parallel to centre of beam

axis. The gantry and collimator position was being set to zero degree while SSD was set

at 100 cm. Zero depth was set at phantom depth ruler for this position. Then, the chamber

was lowered 5 cm beneath water phantom surface which is the nominal depth of

maximum dose for photon beam of 6MV. The experimental set up is shown in Figure

3.19 and 3.20.

40

Few times of 100MU exposure was being done at first to get stable readings. After

that, the irradiation was proceeded three times to get the average value. The circular cone

of 45mm diameter then replaced with other circular cones that available subsequently and

the procedure was repeated for each diameter. The scatter factor for each circular cones

used were calculated by finding the ratio of charge obtained at 5 cm depth to the charge

obtained at similar depth for reference field size of 10 x10cm2.

Figure 3.19: Schematic diagram representing the experimental set up for scatter factor measurement using ionization chamber

41

Figure 3.20: Experimental set up for Scatter Factor measurement using pinpoint 3D ionization chamber. 3.2.2 QUALITY ASSURANCE (QA) PROCEDURE BEFORE SRS TREATMENT

3.2.2.1 Accuracy in target localization with respect to CT images

BRW frame was screwed to head phantom. The Brainlab CT- localizer is attached to

BRW frame or head ring. The frame is attached to the frame holder that attached to the

couch of GE Light Speed Qx/i CT scanner and screwed to it. So that, the phantom and the

frame were extended beyond the CT couch. We used a CT scan of 512 x 512 matrix and

2.4 mm slices thickness used for scanning to increase the image resolution. After the

scanning finished, the same procedure was repeated by using the Radionic CT localizer is

attached to BRW frame.

Pinpoint 3D ionization chamber positioned at 5cm depth inside the water.

42

Then, the CT images were transferred to i-Plan (Brainlab) treatment planning

workstation through DICOM transfer. By using iPlan RT Image 4.1, we contoured the

skull as external contour and each of the four objects. After finished contouring, we used

autocenter function to position the isocenter to each of the structures. Then, by moving

the isocenter superiorly until it reached the top of each structure, we determined the

coordinates of the top centers of these objects.

Three physicists were evaluated the coordinates of four objects separately. The

results were recorded into the table. The coordinates determined on the TPS were

compared with the vendor-provided values. The same procedure was done to the images

with using Radionic CT localizer. The accuracy of localization can be evaluated using the

total localization errors. The formula of total localization error is:

r = (( ΔAP)2 + ( ΔLat)2 + (ΔVert.)2)1/2

3.2.2.2 Accuracy in target localization with respect to laser alignment

Apart from conventional fixed radiotherapy, it is very important to confirm the accuracy

of the isocenter, as SRS treatment uses the rotation of gantry and couch. To do this,

mechanical isocenter standard (MIS; Radionics Inc.), rectilinear phantom pointer (RLPP;

Radionics Inc.), and laser target localizer frame (LTLF; Radionics Inc.) were used. The

MIS aligns the wall and ceiling lasers; therefore, the accuracy of MIS was checked by

films verification before adjustment of these lasers. The tolerance of MIS was within

0.5mm.

The accuracy of laser alignment was verified using RLPP. A verification film was

irradiated by a 6 MV photon beam through a lead ball attached to RLPP for 0°, 90° and

270° gantry angle and the couch fixed at 0° angle. The Kodak X-Omat therapy

verification film was used for verification. The same procedure was repeated with the

43

gantry fixed at 0° but the couch was angled for 0°, 60° and 300°.The irradiated films then

process using semi-automatic Konica film processor inside the dark room.

This technique was introduced by Lutz, Winston and Maleki at Havard Medical

School in 1998 (P. Rowshanfarzad et al. 2011). This test is relatively simple and

compulsory to be conducted before every SRS procedure. This test still used radiographic

film as the detector. The general disadvantages in using films include the cost of films,

chemical and processor maintenance. The results are based on the manual evaluations and

visual inspection which makes them highly dependent on the skills of the observer. The

uncertainty result has been reported to be 0.3 to 0.4 mm (P. Rowshanfarzad et al. 2011).

In this study, we make some adjustment to the test by using the Ray 3.0 Film

Dosimetry software to analyze the irradiated films. The film was scanned using Vidar

scanner. The coaxial deviation of film images for circular beam shape and lead ball was

measured using Ray 3.0 film dosimetry software. From the results, the accuracy of laser

alignment can be verified quantitatively.

3.2.2.3 Accuracy in target localization with respect to target isocenter

The determined 3D target coordinates from treatment planning were placed accordingly.

LTLF was used to match the target coordinate to the isocenter of linear accelerator. A

verification film was irradiated by a 6MV photon beam for 0°, 90° and 270° gantry angle

through a lead ball attached to RLPP. The Kodak X-Omat therapy verification film was

used. The irradiated film then process using semi-automatic Konica film processor inside

the dark room.

44

The film was scanned using Vidar scanner. The coaxial deviation of film images

for circular beam shape and lead ball was measured using Ray 3.0 film dosimetry

software. From the results, the target isocenter accuracy based on machine iscocentric can

be verified quantitatively. Besides that, intercomparision was made using LTLF and

RLPP to confirm the target coordinates from human errors and device inaccuracy. A

small difference between the coordinates of RLPP and LTLF due to laser inaccuracy

around the isocenter was adjusted by changing the coordinates of LTLF.

Figure 3.21: Radiosurgery head phantom setup on the CT scanner and the image of the study

45

3.2.3 DOSE VERIFICATION

First, BRW frame was placed on the head phantom. The Brainlab CT- localizer is

attached to BRW frame or head ring. The frame is attached to the frame holder that

attached to the couch of GE Light Speed Qx/i CT scanner and screwed to it. So that, the

phantom and the frame were extended beyond the CT couch. We used a CT scan of 512 x

512 matrix and 2.4 mm slices thickness used for scanning to increase the image resolution.

The pinpoint 3D ionization chamber also inserted to the interchangeable rod insert at the

center of the phantom.

After scanning, the CT images were imported by i-Plan (Brainlab) planning

system with using RT image 4.1 from CT scanner workstation. The contouring the image

was done with skull of phantom, as external contour and each of the four objects. After

finished contouring, we used autocenter function to position the isocenter at tip of cone

geometry. Then, we plan a treatment plan by using a suitable circular cone size. With

using RT Dose 4.1 application, the treatment plan was created with only two fixed cone

beam angles were assigned to isocenter and TPS algorithm calculated the MU and dose at

the isocenter for each beam based on the beam data measured.

There was no place inside the head phantom to insert the ionization chamber. In

order to measure the dose, the ionization chamber must be positioned inside the head

phantom. Then, the IMRT Thorax phantom without BRW frame attached was scanned

with the same parameter as head phantom. The scanning was done with ionization

chamber inserted inside the phantom.

46

Figure 3.22: IMRT Thorax phantom setup on the treatment couch

After that, the thorax phantom images were contoured the same as the head

phantom but this time the active volume of ionization chamber was assigned as target

volume in Phantom Mapping Software. The Phantom Mapping provided with iPlan RT

Dose allows us to verify whether our treatment plan meets our requirements. From that,

the treatment plan based on the head phantom was loaded into the verification plan

(thorax phantom). In this verification plan, the beam angle and the collimator size could

not be changed because it only based on the ordinary plan. Only isocenter position could

be assigned accordingly based on the point of interest that required. The MU and dose

calculation also changed accordingly based on the thorax phantom, calculated by circular

cone algorithm.

The IMRT Thorax phantom only has insert for ionization chamber and not for

TLD chips. It is impossible to put TLD chips inside the phantom. Then, as a substitute

solid water phantom with TLD insert was used. The assumption is made that dose

calculated base on the CT images of the IMRT Thorax phantom give same dose as in the

Pinpoint 3D Ionization chamber position inside the IMRT

47

solid water phantom due to both phantoms were made from same materials which is

PMMA.

Before irradiation procedure carried out to phantom, the QA procedures before

SRS treatment were done using head phantom (as mention in section 3.2.2) to make sure

accurateness of isocenter coordinates of the target. After that, the IMRT Thorax phantom

was positioned on the treatment couch accordingly based on room’s laser alignment. The

ionization chamber was connected to the electrometer outside the treatment room through

the connecting cable. The treatment delivered to the phantom according the treatment’s

plan. There were four irradiations done, firstly for 0° degree in gantry angle with low MU

and then for 90° degree in gantry angle with low MU and repeated for respective gantry

angle with higher MU. The charge for every arc angle was collected. Comparisons

between the calculated dose which is from i-Plan treatment plan dose distribution and the

measured dose from the ionization chamber were made.

48

Figure 3.23: Verification of dose on the phantom using Phantom Mapping software.

Then, the same procedure was repeated by replacing the ionization chamber with

TLDs. For the TLD irradiation, only for 0° degree in gantry angle of beam direction was

done because of limitation of collimator mount. The irradiation of TLD chips were done

at same depth based on treatment plan. 4 TLD chips were used for every irradiation and

the TLD chips were positioned totally inside the irradiation field.

After 24 hours of irradiation, the TLD chips were analyzed using TLD reader.

Inside the reader the process started with preheating at 135°C for 10 second followed by

acquiring process up to 240° C. The ROI was set from channel 51 up to channel 150 in

order to get accurate and constant readings. The results directly converted from charge to

absorbed dose by the reader. Previously, the TLD chips and reader was calibrated. The

49

TLD reader was assigned a RCF factor for batch of TLD 100H chips. The RCF was

0.4258 nC/µGy. Each chip was also assigned ECC values. The reproducibility of TLDs

was evaluated three exposures was found to be within 5%.

Figure 3.24: Set-up geometry showing TLDs position, total depth of 16cm phantom used same as dimension of IMRT Thorax phantom

50

CHAPTER IV

RESULTS

4.1 BEAM DATA MEASUREMENTS

4.1.1(a) Percentage Depth Dose (PDD) Measurement using Ionization Chamber

The PDDs measured in the water for 5mm to 45mm diameter field sizes are presented in

Figure 4.1. In general, we observed a shift to greater depth of dmax with increasing field

sizes, with slightly same depth of dmax for field size of 12.5 mm to 22.5 mm and generally

increased starting at field size of 25 mm to 45mm. Serago et al. 1992 and Verhaegen et al.

1998 also reported that similar shift of dmax to greater depth with increasing field size for

a 6MV accelerator. Such observation is opposite to traditional radiation therapy fields

where dmax decreases as field size increases. This is explained on the basis of phantom

scattering than collimator scattering.

51

0

20

40

60

80

100

120

0 50 100 150 200 250 300Depth (mm)

PD

D (

%)

5mm

7.5mm

10mm

12.5mm

15mm

17.5mm

20mm

22.5mm

25mm

27.5mm

30mm

32.5mm

45mm

Figure 4.1: PDD versus depth for various circular cones using ionization chamber

Table 4.1 shows the variation of PDD at 100 mm and 200 mm depth with diameter of

SRS circular cones. From this table it is observed that PDD increased with increasing

cone diameter. PDD values were increased due to higher production of scatter radiation at

larger field size compared to smaller field sizes of beams.

52

Table 4.1: Results of R100, R80, R50, D100 and D200 for different cone diameters Cirular Cone Diameter (mm)

R100 (mm) R80 (mm) R50 (mm) D100 (%) D200 (%)

5 12.99 46.44 117.29 56.15 29.88

7.5 13.02 48.88 119.75 57.17 31.03

10 13.04 50.61 123.15 57.76 31.45

12.5 15.03 51.86 125.34 58.80 31.57

15 15.01 52.17 126.00 58.78 31.87

17.5 15.01 53.98 127.62 59.41 32.23

20 15.07 54.18 128.81 59.82 32.49

22.5 15.03 55.08 129.28 59.71 32.30

25 16.99 55.26 130.29 60.20 32.61

27.5 15.02 56.64 131.72 60.79 32.89

30 15.99 56.57 132.91 61.56 33.31

32.5 16.04 57.89 133.45 61.60 33.58

45 16.99 60.62 139.05 63.29 34.81

R100 = Depth where the dose 100% D100 = Dose at 100mm depth R80 = Depth where the dose 80% D200 = Dose at 200mm depth R50 = Depth where the dose 50% In Figure 4.2, it shows that the depth dose of three cone diameter field size compare to

100x100 mm2 field size. The obvious differences between the three circular beams were

at their build up region of PDD curves. From 3 mm to 11 mm depth inside water, the

differences of PDD values were up to 6.25 % except for 5mm field size. While from 10

mm to 300 mm depth, the PDD values differences were less than 6.52 %. The figure also

showed that the standard 100x100 mm2 field size gave highest PDD value at surface

53

while 45 mm circular beam gave the lowest PDD value. At the surface of water, the PDD

value for 100x100 mm2 field size slightly higher than PDD value for 45 mm circular field

size. The characteristic of PDD curve of 100x100 mm2 field size especially the gradient

was not as steepest as PDD curve of small field size.

0

20

40

60

80

100

120

0 18 36 54 72 90 108 126 144 162 180 198 216 234 252 270 288

Depth (mm)

PD

D (

%)

5mm20mm45mm100mm

Figure 4.2: PDD versus depth for 3 circular cones compare to 100x100 mm2 field size.

Table 4.2 shows the variation of PDD value at 100 mm and 200 mm depth with diameter

of SRS circular cones compare to 100x100 mm2 field size. From this table it is observed

that PDD value at depth 100 mm and 200mm increased with increasing field sizes. PDD

values were increased due to higher production of scatter radiation at larger field size

compared to smaller field sizes of beams.

54

Table 4.2: Results of R100, R80, R50, D100 and D200 for different cone diameters compare to 100x100 mm2 open field measured using ionization chamber Circular Cone Diameter (mm)

R100 (mm) R80 (mm) R50 (mm) D100 (%) D200 (%)

5 12.99 46.44 117.29 56.15 29.88

20 15.07 54.18 128.81 59.82 32.49

45 16.99 60.62 139.05 63.29 34.81

100 16.00 65.00 149.00 65.65 36.91

R100 = Depth where the dose 100% D100 = Dose at 100mm depth R80 = Depth where the dose 80% D200 = Dose at 200mm depth R50 = Depth where the dose 50%

4.1.1(b) Percentage Depth Dose (PDD) measurement using EDR2 Film

Percentage depth dose (PDD) value at surface measured using ionization chamber was in

range 38.7 % to 47.98 %. Figure 4.3 shows that for film measurement, the PDD values

were in range 72.81% to 96.55%. It seems that the film has overestimated the PDD values

compared to PDD values measured using ionization chamber. The higher PDD values at

surface is due to the dominantly contribute from the primary radiation and scattering

radiation from the air from the surrounding and the circular collimator. Beside that, it was

less or none contribution from scattered radiation when primary radiation interact with

water which is called phantom scatter. In general, the PDD values at surface detected by

the film decreased with increasing of the field sizes. This effect contradicted with PDD

measured using ionization chamber. In the figure, PDD values at surface decreased with

field sizes from 5mm to 17.5 mm but the slightly same from 20mm to 45 mm field sizes.

55

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140 160 180 200 220 240

Depth (mm)

Pdd

(%)

5 mm10 mm15 mm25 mm32.5 mm45 mm100 mm

Figure 4.3: PDD versus depth for 6 circular cones field size compare to 100x100 mm2 open field. In Table 4.3, it shows that the results of dose at 100 mm and 200 mm are contrast to

results using the ionization chamber (refer to Table 4.1). The PDD values at depth of 100

mm and 200 mm decreased with increasing cone diameter. It was due to further deep in

water, the response of the film to PDD values is not accurate. First, because of the

scattering radiation with the low energy cannot be detected by the films. Second, the film

used is not suitable for measuring low dose. The film used is suitable for extended dose

range measurements.

56

Table 4.3: Results of R100, R80, D100 and D200 for different cone diameters compare to 100x100 mm2 open field measured using EDR2 Film Circular Cone Diameter (mm)

R100 (mm) R80 (mm) D100 (%) D200 (%)

5 9 103 80.78 65.5

10 9 127 83.19 67.4

15 14 102 80.17 63.03

25 14 69 70.28 53.94

32.5 14 72 71.20 58.83

45 15 80 73.11 57.65

100 14 75 72.16 58.24

R100 = Depth where the dose 100% D100 = Dose at 100mm depth R80 = Depth where the dose 80% D200 = Dose at 200mm depth

57

4.1.2 Off-axis Ratio measurement of circular cone collimator 4.1.2(a) Off-axis Ratio using ionization chamber

Figure 4.4 shows the Off-axis Ratio for various circular cones beam measured at depth

75mm inside water. It is obvious from the curves in this figure that off-axis ratio of small

fields show strong field size dependence with rapidly decreasing as the field diameter

decreases. The steepest dose gradient can be seen for 5mm diameter field size. Generally,

the dose falls rapidly at the edge of the beams. Therefore, the tissue that situated near the

target area could be avoided from getting unnecessary dose.

0.0

20.0

40.0

60.0

80.0

100.0

120.0

0 10 20 30 40Distance (mm)

Perc

enta

ge (%

)

5mm

7.5mm

10mm

12.5mm

15mm

17.5mm

20mm

22.5mm

25mm

27.5mm

30mm

32.5mm

45mm

Figure 4.4: Off-axis Ratio for all circular cones diameters

58

Figure 4.5 shows the beam profile for all circular cones from 5 mm to 45 mm diameters

plotting by MEPHYSTO mc2 analyzing beam software. The ionization chamber used for

this measurement only measured charge 40% less for 5mm diameter field size compared

to 45 mm diameter field sizes. If the field size increased by 2.5mm, the charge collected

also increases by 100%.

Figure 4.5: Beam profiles for various circular cones plotting by MEPHYSTO mc2

analyzing beam software.

59

The results of beam characteristics for 13 circular cone diameters measured using

ionization chamber shown in Table 4.4 which analyzed by MEPHYSTO mc2 analyzing

beam software. The outer diameter of 4.3mm of ionization chamber is still bigger in

measuring penumbras in very small radiation fields. The results of penumbra length less

than 4 mm for all 13 circular cone diameters measured using this pinpoint 3D ionization

chamber.

Table 4.4: Results of CAX, Field Size, Penumbra, Flatness and Symmetry for different cone diameters measured using ionization chamber Circular Cone Diameter (mm)

CAX Deviation (mm)

Field Size (cm)

Penumbra Left (mm)

Penumbra Right (mm)

Dmax (%)

Dmin (%)

Flatness (%)

Symmetry (%)

5 -0.07 0.501 2.56 2.52 100.10 63.38 22.46 0.25

7.5 0.03 0.696 2.79 2.80 100.04 66.19 20.36 0.16

10 0.06 0.931 2.97 3.01 100.34 70.38 17.55 0.08

12.5 0.01 1.192 3.13 3.16 100.03 74.00 14.96 0.16

15 0.06 1.431 3.29 3.20 100.03 77.71 12.61 0.03

17.5 0.12 1.3674 3.39 3.39 100.26 79.74 11.40 0.23

20 -0.02 1.931 3.41 3.41 100.29 82.58 9.68 0.21

22.5 0.07 2.161 3.53 3.46 100.26 83.88 8.89 0.14

25 -0.01 2.430 3.51 3.51 100.27 85.69 7.84 0.20

27.5 -0.01 2.714 3.49 3.60 100.20 87.30 6.88 0.20

30 -0.02 2.960 3.56 3.58 100.33 88.14 6.47 0.17

32.5 0.02 3.219 3.57 3.57 100.89 89.68 5.88 0.37

45 0.01 4.418 3.85 3.77 100.89 92.29 4.45 0.27

60

Figure 4.6 shows the beam profiles for 3 circular cones compare to 100x100 mm2 open

field at depth 75mm inside water. It is obvious from the curves in this figure that off-axis

ratio of small fields show strong field size dependence with rapidly decreasing as the field

diameter decreases. The steepest dose gradient can be seen for 5mm diameter field size

but the effect not be seen clearly 100x100 mm2 field size.

0

20

40

60

80

100

120

-59.46 -48.65 -37.84 -27.03 -16.22 -5.41 3.24 14.05 24.86 35.68 46.49 57.3

Distance (mm)

Per

cen

tag

e d

ose

(%

)

5mm20mm45mm100mm

Figure 4.6: Beam profiles for 3 circular cones compare to 100 x100 mm2 open field

61

It can be clearly seen according to results in Table 4.5 that penumbra strongly dependence

to the field size, penumbra length increases with increasing field size. The penumbra

length of standard 100 x100 mm2 field size increases 53.3% compared to penumbra

length of 45mm diameter of circular cone

Table 4.5: Results of CAX, Field Size, Penumbra, Flatness and Symmetry for different cone diameters compare to 100x100 mm2open fields Circular Cone Diameter (mm)

CAX Deviation (mm)

Field Size (cm)

Penumbra Left (mm)

Penumbra Right (mm)

Dmax (%)

Dmin (%)

Flatness (%)

Symmetry (%)

5 -0.07 0.501 2.56 2.52 100.10 63.38 22.46 0.25

20 -0.02 1.931 3.41 3.41 100.29 82.58 9.68 0.21

45 0.01 4.418 3.85 3.77 100.89 92.29 4.45 0.27

100 -0.81 10.04 7.08 7.07 100.50 96.99 1.78 0.38

62

4.1.2(b) Off-axis Ratio using EDR2 Film

Figure 4.7 shows the beam profile of 5mm to 45mm diameters of circular cones measured

using the EDR2 film. Due to the higher spatial resolution of the film, the result of the

penumbra, flatness and symmetry have been evaluated more precisely scanned using the

Vidar scanner and analyzed with Ray 3.0 Film Dosimetry software.

0

20

40

60

80

100

120

-49

-42

-35

-28

-21

-14 -7 0 7 14 21 28 35 42 49

Distance (mm)

Rel

ativ

e D

ose

(%

) 5mm10mm15mm25mm32.5mm45mm

Figure 4.7: Beam profile for 6 circular cones diameters measured using EDR2 Film

63

Based on Table 4.6, the highest symmetry is for 15mm diameter field size which is 1.66%.

The result of symmetry measured using film not so good as result of measurement using

ionization chamber (refer Table 4.4). Beside that, the film measurement gave the highest

result of penumbra length which was 3.0 mm for 45mm circular cone. It was 78.7% less

compared to results of penumbra length measured using ionization chamber.

The difference condition of measurement using film compared to measurement

using ionization chamber was the measurement medium used. The film measurements

was made at 50mm depth of solid water phantom (water equivalent slab phantom), which

has slightly different attenuation properties compared to water. The result of penumbra

length of 45mm field size appears to be somewhat always larger compared to the other

smaller field sizes.

Table 4.6: Results of CAX, Field Size, Penumbra, Flatness and Symmetry for different cone diameters measured using EDR2 Film Cone Diameter

(mm) Flatness (%) Symmetry (%) Penumbra (mm)

Left (-ve) side Right (+ve) side

5 50.14 0.13 1.75 1.50

10 12.89 0.85 2.25 1.75

15 10.34 1.66 2.50 2.40

25 9.40 0.11 3.00 2.75

32.5 8.82 0.54 2.50 2.50

45 7.42 0.42 3.00 3.00

64

Figure 4.8 shows how the field size measured at Full Width Half Maximum or diameter

length at 50% of relative dose on the film analyzed with the Ray 3.0 Film Dosimetry

software. By using isodose analyze tab, the isodose for the cone diameter has been

evaluated. It showed that the isodose well round in shape based on the darkness on the

film produced by the circular cone beams.

Figure 4.8: Field size for 45 mm cone diameter, SID = 105 cm measured using EDR2 film and analyzed by Ray 3.0 Film Dosimetry software.

65

According the results in Table 4.7, the largest deviation for the field size measured was

for 5mm diameter field size. The result of deviation decreases with increasing field size.

We can see the film turns darker as the field size is larger due to the intensity of radiation

increases with increasing field size.

Table 4.7: Results of measured field sizes analyzed using Ray 3.0 Film Dosimetry Software

Cone Diameter

(mm)

Actual Field Size at SID (mm) Measured Field Size at SID (mm)

Deviation (%)

5 5.25 6.9 -31.43

10 10.5 11.3 -7.62

15 15.75 16.4 -4.13

25 26.25 26.9 -2.48

32.5 34.13 35.5 -4.01

45 47.25 47.9 -1.38

In the Table 4.8, it can be seen that the values measured with ionization chamber are

smaller than those measured with films, particularly in the dose fall off region for 10mm

field sizes to 32.5mm field size except for 45mm field size; this is result of steep gradient.

Beside that at the peripherals region, the values measured with ionization chamber are

higher than those measured with films for all field sizes. This is because the film used is

extended dose film and they not sensitive to response on the very low dose radiation in

the peripheral region.

66

Table 4.8: Relative dose at off-axis distances for various circular cone diameters measured using the film and the ionization chamber Off Axis Distance

(mm)

10mm

(film)

(%)

10mm

(I.C)

(%)

15mm

(film)

(%)

15mm

(I.C)

(%)

32.5mm

(film)

(%)

32.5mm

(I.C)

(%)

45mm

(film)

(%)

45mm

(I.C)

(%)

0 100 100 100 100 100 100 100 100

2 98.09 94.93 98.05 98.71 101.30 100.30 99.68 100.30

4 88.97 64.52 92.54 86.41 101.51 99.86 98.42 100.33

6 37.75 23.44 82.89 53.34 102.42 99.60 99.24 99.94

8 9.97 7.52 43.64 18.07 102.00 98.90 100.12 99.81

10 4.39 3.74 12.45 7.16 102.35 96.82 98.98 99.13

12 2.67 2.33 5.00 4.05 100.94 93.12 98.17 99.02

16 1.46 1.54 1.89 2.21 88.70 51.81 93.54 95.64

18 1.37 1.36 1.46 1.94 75.66 19.00 91.96 91.92

20 1.19 1.21 1.00 1.79 26.54 8.72 67.47 82.03

25 1.27 0.83 0.89 1.29 2.89 3.40 6.45 13.26

30 0.51 0.59 - - 1.01 2.14 1.58 4.55

67

4.1.3 Scatter Factor Measurement Figure 4.9 shows the variation of scatter factor with field diameter of SRS cones

measured using pinpoint cylindrical 3D ionization chamber with different secondary jaws

opening. Generally, the filed sizes scatter factor is higher for 60mm x 60mm jaws

opening compared to 50mm x 50mm jaws opening. The scatter factor increased by

average 106% for all circular cone diameter field size except for 5mm diameter field size

for 60mm x 60 mm jaws opening compared to 50mm x 50mm jaws opening. However,

the ionization chamber may be too large for field sizes scatter factor measurement in the

radiation fields of collimator with 5mm and 7.5mm diameter.

0

0.2

0.4

0.6

0.8

1

1.2

5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 30 32.5 45

Cone Diameter (mm)

Scatt

er

Ou

tpu

t

6cmx6cm5cmx5cm

Figure 4.9: Scatter factor for circular cone diameters with different size of jaws opening.

68

4.2 QUALITY ASSURANCE (QA) PROCEDURE BEFORE SRS TREATMENT

4.2.1 Accuracy in target localization with respect to CT images

Figure 4.10 shows CT images that were transferred to i-Plan (Brainlab) treatment

planning workstation through DICOM transfer. By using iPlan RT Image 4.1, the

contoured of skull and each of the four objects can be seen in 3D view. The coordinates

of four objects separately were evaluated and the results were recorded into the table. The

coordinates determined on the TPS were compared with the vendor-provided values. The

same procedure was done to the images with using Radionic CT localizer.

Figure 4.10: Image of treatment plan which center of target assigned at the tip of cones geometry inside the Radionic human head phantom using i-Plan TPS

69

The coordinates of the top of each of the four objects determined by i-Plan RT Image 4.1

software are shown in the Table 4.9 with different CT localizers used. The known

specifications are also listed in the table. The differences between the two are presented in

the last three columns. The average differences and standard deviations of the differences,

which reflect the error bars in determining these coordinates, were derived by summing

all differences in the last three columns.

Table 4.9: Results of measured coordinates of the top centers of each object compared with known coordinates specification.

Objects

Phantom specifications iPlan Stereotactic coordinates (mm)

Differences (mm)

AP Lat Vert AP Lat Vert AP Lat Vert

Cylinder 0.0 0.0 30.0 0.3 0.6 30.8 0.3 0.6 0.8

Cube 20.0 -17.0 40.0 20.4 -17.6 41.3 0.4 0.6 1.3

Cone -35.0 -20.0 40.0 -34.8 -20.4 41.3 0.2 0.4 1.3

Sphere 25.0 20.0 32.7 25.2 20.2 32.3 0.2 0.2 0.4

Average 0.28 0.45 0.95

Standard Deviation 0.10 0.19 0.44

70

Table 4.10 and 4.11 also show the total localization error which represent by Δr. The total

error which is measured on TPS using Brainlab CT localizer is 1.1 ± 0.4 mm while total

localization error for Radionic CT localizer is 2.5 ± 0.6 mm. The value of total

localization error for Radionic CT localizer is 227% more than value compare to Brainlab

CT localizer. Although i-Plan RT Image 4.1 software can recognize Radionic CT

localizer, the error of localization is 44% less accurate compared to Brainlab CT localizer.

Table 4.10: Results of localization errors of CT using Brainlab CT Localizer

No. of measurement

mm

Δ AP Δ Lat Δ Vert Δ r

P1

0.3 0.6 0.8 1.04

0.4 0.6 1.3 1.49

0.2 0.4 1.3 1.37

0.2 0.2 0.4 0.49

Average 0.3 0.5 1.0 1.1

Standard Deviation 0.1 0.2 0.4 0.4

Δ AP, Δ Lat, Δ Vert. represent the components of errors in AP, lateral and vertical

directions respectively, Δ r represents the total localization error.

71

Table 4.11: Results of localization errors of CT using Radionic CT Localizer

No. of measurement

mm


P1

0.4 0.1 2.7 2.73

0.2 0.1 3.2 3.21

0.3 0.4 1.6 1.68

0.4 0.3 2.5 2.55

Average 0.3 0.2 2.5 2.5




Table 4.12 shows the results of CT localization error due to 3 different observers. The

highest error produced by P3 observer. This is due to the lack of knowledge with the new

TPS. The mean error is 1.7 ± 0.7 mm. These two components of error which are error due

to image resolution produced by CT images which is strongly depend on the slice

thickness used and also by systemic due to localization point measured by the observer.

These 2 factors that contributed to the total localization errors must be reduced to ensure

the accuracy of target position of iPlan TPS.

72

Table 4.12: Results of CT localization error determined by 3 different observers

No. of measurement

mm


P1

0.3 0.6 0.8 1.04

0.4 0.6 1.3 1.49

0.2 0.4 1.3 1.37

0.2 0.2 0.4 0.49

P2

0.2 0.2 1.8 1.82

0.6 0.2 2.0 2.10

0 0.6 0.5 0.78

0.1 0.4 2.0 2.04

P3

0.2 0.2 2.1 2.12

0 0.1 2.0 2.00

0.1 0.6 1.6 2.93

0.3 1.2 2.0 2.35

Average 0.2 0.4 1.5 1.7




73

4.2.2 Accuracy in target localization with respect to laser (The Winston –Lutz Test)

The radiographic exposures show the tungsten sphere near the center of circular port for

all three exposures (refer Figure 4.11). The displacement of the tungsten sphere center

from the port was measured on all three exposures and the results analyzed using

conventional technique was displayed in Table 4.13. The maximum deviation at isocenter

was at gantry angle 0° was 0.56 mm which 0.06 mm exceeded the tolerance value. In

overall, the average deviation of isocenter with respect to laser was 0.37 mm. In general,

the result in Table 4.14 which analyzed using Vidar scanner and Film Dosimetry software

gave a quite similar result except the average deviation was 0.43 mm. The results are still

in line with tolerance suggested by MOH based on Manual for Quality Assurances

Programmed (QAP) which is 1.5 mm.

Figure 4.11: Verification film of isocenter shift with respect to the room’s laser system

Source to Isocenter Distance (SID) = 100 cm

Isocenter to Film Distance (IFD) = 8 cm

MF (de-magnification factor ), SID/ (SID+IFD) = 0.93

74

Table 4.13: Results of isocenter deviation on irradiated film analyzed with conventional technique

Overall isocenter deviation (linac + Collimator mount)

Gantry 90 0 270

Collimator 0 0 0

Distance (mm) 0.2 0.6 0.4

Magnification Factor (MF) 0.93

Deviation at Isocenter =

Distance *MF (mm)

0.19

0.56

0.37

*Good result ≤ 0.5mm

Maximum Deviation = 0.56 mm Average Deviation = 0.37 mm

Table 4.14: Results of isocenter deviation on irradiated film analyzed with Vidar scanner

and Film Dosimetry software

Overall isocenter deviation (linac + Collimator mount)

Gantry 90 0 270

Collimator 0 0 0

Distance (mm) 0.4 0.6 0.4

Magnification Factor (MF) 0.93

Deviation at Isocenter =

Distance *MF (mm)

0.37

0.56

0.37

Maximum Deviation = 0.56 mm Average Deviation = 0.43 mm

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4.2.3 Accuracy in target localization with respect to target isocenter

As shown in Table 4.15 and Table 4.16, average displacement error was 0.88 ± 0.8 mm,

mainly due to gantry sag and localization error for the target localizer. The values are still

in line with the accuracies suggested by AAPM Report No. 54, 1995, which is 1.0 mm. In

general, the target isocentric deviation in the vertical direction is much greater than one in

the AP or lateral direction because of gantry sag to compensate for the gantry weight

(David R. Choi et. al, 2001). In our case, vertical direction for 0° in gantry angle gave the

highest deviation (1.90 mm). In this study we do not study the effect of weight because

we used phantom as a subject. In the real treatment, the patient’s weight could also

contribute to the displacement error.

*HF0 = Head feet direction at 0°gantry angle, LR = Left right direction

Figure 4.12: The schematic of analyzed verification films for determining isocenter shifts.

76

Figure 4.13: Verification film of isocenter shift with respect to the target coordinates

Source to Isocenter Distance (SID) = 100 cm

Isocenter to Film Distance (IFD) = 5 cm

MF (de-magnification factor), SID/ (SID+IFD) = 0.952

Table 4.15: Results on the shift distance of Target Positioner on irradiated film

Overall isocenter deviation (linac + Collimator mount + Target Positioner)

Parameter measured Distance on Film (mm)

Magnification Factor (MF)

Distance on Isocenter (mm)

HF0 2

0.952

1.90

HF90 1.2 1.14

LR 0.1 0.10

AP 0.4 0.38

Average 0.88 ± 0.8

77

Based on the Winston Lutz Test:

Longitudinal mechanical deviation of Linac (DLO) = 0.56 mm

Lateral mechanical deviation of Linac (DLA) = 0.56 mm

Vertical mechanical deviation of Linac (DVE) = 0.56 mm

CT slice thickness used for the scan (STH) = 1.0 mm

Table 4.16: Results of overall target setup accuracy Results

HF0*MF≤ STH+DLO+0.5mm 1.90 ≤ 2.06

HF90*MF≤ STH+DLO+0.5mm 1.14 ≤ 2.06

LR * MF ≤ DLA + 1.0mm 0.10 ≤ 2.56

AP * MF ≤ DVE + 1.0mm 0.38 ≤ 2.56

78

4.3 DOSE VERIFICATION Figure 4.14 shows the charge increases linearly with increasing dose. The quantity of

charge measured depends on the size of active volume. The active volume for this

chamber is only 0.016cc. From the gradient of graph the sensitivity of the pinpoint

ionization chamber was 3.95E-3nC/cGy.

0

0.5

1

1.5

2

2.5

50 100 150 200 250 300 350 400 450 500

Dose (cGy)

Ch

arg

e (

nC

)

Figure 4.14: Charge measured using pinpoint 3D ionization chamber versus dose delivered.

79

In Table 4.17 shows results of difference between calculated dose by TPS and dose

measured using ionization chamber in range 0.52% to 2.56%.. The difference (R)

between the calculated dose (Dc) and mean measured dose (D’m) was calculated using

formula:

R = [(D’m - Dc)/ Dc] x 100%

The pinpoint 3D ionization chamber slightly overestimated the dose compared to

calculate dose by TPS. The absorbed dose to water was converted to absorbed dose to gas

(medium inside the active volume of ionization chamber) and finally converted back to

absorbed dose to water inside the medium of measurement or phantom material (PMMA).

All appropriate dose corrections were applied to get accurate results.

Table 4.17: Results of calculated dose by TPS and measured dose using pinpoint 3D ionization chamber

No. of

Arc

Beam Direction Calculated Dose at Isocenter (cGy)

Measured Mean Dose at Isocenter

(cGy)

Deviation (%)

Gantry Couch

Start Stop

1 240 300 0 341 342.79 0.52

2 320 10 90 331 342.53 3.48

3 320 10 45 336 344.59 2.56

Average 336 343.30 2.17

KTP = 1.0198

Attenuation coefficient wall (PMMA) to water = 0.961 (Attix)

Absorbed Dose to water (ND,W,Q0) =252.96cGy/nC

Average gas/air to wall medium (PMMA) stopping power = 0.944 (Attix)

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(Average water to gas stopping power)λ = 1.093 (Attix)

Prepl for 6MV photon = 0.987 (AAPM TG 21)

P’repl for inner diameter of chamber (mm) for 6 MV = 0.996 (Attix)

Dwat = M * ND,W,Q0 ………………. (1)

Dgas = Dwat * ((µen/ ρ) gas to PMMA * (L/ρ) gas to PMMA)/ Prepl …………………. (2)

(Dwat)λ = (Dgas)λ* P’repl * (Average water to gas stopping power)λ ………………… (3)

As comparison, results in Table 4.18 showed the TLD also have a good agreement of

calculated and measured dose. The mean calculated dose was 275.5 cGy, while the

measured dose was 264.09 cGy. TLD’s measured dose results still underestimated

compared to calculated dose but the deviation or discrepancy were less than 4% and

considered to be within experimental error.

Table 4.18: Results of calculated dose by TPS and measured dose using TLD 100H No. of

Arc

Beam Direction Calculated Dose at Isocenter (cGy)

Measured Mean Dose at Isocenter

(cGy)

Deviation (%)

Gantry Couch

Start Stop

1 0 0 0 85 83.08 -2.26

2 0 0 0 466 445.1 -4.48

Average 275.5 264.09 -3.37

81

CHAPTER V

DISCUSSIONS

5.1 BEAM DATA MEASUREMENTS

5.1.1 PDD measurement

Depth dose distribution in small field sizes such as used in SRS should be measured with

the detectors of size as small as possible compared to the field size, in this study we used

the pinpoint 3D ionization chamber of 0.016cm3 active volume and outer diameter is

4.3mm. According to Faiz M. Khan (2011), for measurement of central axis depth dose or

PDD, an essential criterion is that the sensitive volume of the detector should lie within

uniform electron fluence generated by photons within ± 5%.

Depth dose show maximum dose at a particular depth that depends on collimator

size. However, the algorithm used for dose calculation requires all data to be normalized

to the same depth. In this section, this depth is referred to as dnorm. Usually, the

normalization depth is chosen to be average depth of peak dose. For 6MV photon beam,

dnorm is 15mm. In this study, the set up of a detector accurately and precisely at the

effective point of measurement which is respect to the center of active volume is the

crucial part. Even a slight misalignment of the detector location and direction of motion

with central ray could result in the detector being well out of the center of field. This is

different compared to the larger field sizes measurements.

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Conventional detectors such as the Farmer–type ionization chamber used for dose

calibration or smaller thimble-style ionization chambers used to scan the radiation

distribution may not be appropriate for small field dosimetry. These ionization chambers

have dimensions which exceed the diameter of the smaller circular beams. However, even

for those radiation beams which are marginally larger in size than the actual ionization

chamber, lateral electronic equilibrium may not be established throughout the active

volume of the chamber. For this reason, film, TLD or small diodes seem to be appropriate

choices of radiation detectors to determine relative dose for small field sizes.

The major difference PDDs measured using the ionization chamber and film

dosimetry was at the surface dose (dose at 0cm). The ionization chamber not accurately

measured the percentage dose at the surface. It is because the different density between

air and water at the point of measurement and lack of effect charge particle equilibrium.

According to Faiz M Khan (1993), for a sufficiently small field size, one may assume that

the depth dose at point is effectively the result of the primary radiation, that is, the

photons which have transverse the overlying medium without interacting. The

contribution of the scattered photons to the depth dose in this case is negligibly small or

zero. But as the field size is increased, the contribution of the scattered dose is greater at

larger depths than at depth of dmax, the percent depth dose increases with increasing field

size.

Therefore from Figure 4.3, the gradient of PDDs curve are not steep as PDDs

curve measured using the ionization chamber (refer Figure 4.1). In order to produce the

accurate PDDs curve, the exposure to the EDR2 film must be increased instead of using

300MU to increase the response of the at the further depth inside the solid water phantom.

In our results it showed the inconsistence relative dose measured with film. Vivian et al.

1999 reported that an investigation into film calibration using beams where the central

axis was either perpendicular or parallel to the calibration films. It was found that film

83

sensitivity was higher for perpendicularly oriented beams. Therefore, film calibrated

perpendicularly oriented beams and irradiated with parallel beams will produce relative

dose or absolute dose values lower than expected. In our study it was happen obviously at

the tail of the PDDs curve.

Other factor that affecting the result is the different medium was used in

measuring PDDs. The film measurements was made inside 20cm of solid water phantom

(water equivalent slab phantom), which has slightly different attenuation properties

compared to water. Besides that, several potential errors according to film processing

condition which cannot be control such as changes of temperature for film processing

chemical, the stability of chemical and oxidation of chemical when exposed to the

surrounding inside the dark room. Other errors that may occur are when we working with

the films such as the interfilm emulsion may be differences and artifacts caused by air

pockets adjacent to films may changes the dosimetry properties of film itself.

The advantages of using EDR2 film are higher spatial resolution and permanent

record. The EDR2 no need the specific duration after processing to be analyzed. Besides

that, the PDD’s film also can be used for determine beam profile and isodose distribution

on the same film.

84

5.1.2 Off-axis Ratio and Beam Profile measurement

Beam profile for photon beam has three important characteristics which are penumbra,

flatness and symmetry. The flatness is the constancy of intensity across the beam. In the

small field size dosimetry, the flatness is not crucial because in small filed size the shape

of beam profile will be a peak shape because of very steep dose gradient mostly for 5mm

to 15mm diameter of cones.

For symmetry of the beam is important to ensure the intensity of the beam is

uniformly distributed. The symmetry is required to be ±1% to ±2% from one side of the

beam to the other. But for SRS, all the beams are circular in shape and assumed as small

then it is not crucial as penumbra measurements. The results showed that all 13 circular

cone diameters have symmetry less than 1% measured using ionization chamber. Beside

that, the asymmetry of the beams is reflecting to misalignment of laser since measurement

setup is based on the laser alignment. Depth and field sizes are both factors that affect to

beam symmetry and flatness. The deeper measurement was made inside medium then the

beam more uniform due to increase of phantom scatter.

While in geometric penumbra must be small enough for small field size

dosimetry which is range about 2mm to 3mm only (David Shepard, 2009). According to

Faiz M. Khan (2011), beam collimation in SRS functioning to reduce geometric

penumbra, a tertiary collimation system is used to bring the collimator diaphragm closer

to the surface. A larger penumbra will cause more scattering electron produce and

resulting on the irradiating healthy tissue surrounding near to the target volume. In the

open field size dosimetry, the flatness is required for clinically used to be ± 3% according

to Manual of Quality Assurance Programme (QAP) for radiotherapy services by Ministry

of Health, 2012.

85

For a cross beam profile measurement, the detector size is again important

because of the steep dose gradients at the field size edge. The dosimeter, in such case,

must have high spatial resolution to measure field penumbra accurately, which is

critically important in SRS. Ionization chamber is the most precise and the least energy-

dependence system but usually has a size limitation The size of ionization chamber used

for this purpose should not only be smaller than the beam diameter but should have

sufficient spatial resolution to describe the steep dose gradient in the penumbral region

accurately. The effect of lack of lateral electronic equilibrium and steep dose gradient

were noticed even when measurements were performed with a small volume (less than

0.02 cc) ionization chamber. There are disadvantageous of using the ionization chamber.

While, energy dependence, uniformity of emulsion thickness and processing

condition are the problems associated with films. Film has the best spatial resolution but

show energy dependence and greater dosimetric uncertainty (± 3%). We have found film,

because of its excellent spatial resolution; to be very convenient for measurement of small

radiation distributions especially for beam profiles.

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5.1.3 Scatter Factor measurement

The scatter factor for 50mm x 50mm jaws increases for 5mm diameter field size to 20mm

diameter field size. The steepest gradient observed at 5mm to 7.5mm diameter field size

which the value increased rapidly by 74.9%. Ideally, small beam dosimetry or

radiosurgery beams exhibit a sharp decrease in output with decreasing field size (David

Shepard, 2007). Then, the values saturated from 22.5mm to 32.5mm diameter field size

and slightly increased for 45mm diameter field size. The lowest scatter factor value was

for 5mm diameter field size which was 47.7% less than normalized value. Opposite that,

the highest value was for 45mm diameter field size which was 7.2% less than the

normalized value.

Radionic collimator housing is longer in length and makes the collimation of field

more nearer to the isocenter compared to other collimator housing such as Brainlab. The

distance from isocenter to the lower end of the mounted collimator (with gantry in 0

degree angle) is 230 mm. It allows more primary radiation and scattering radiation

coming from circular cone collimator reach the detector and reduce the scattering

radiation that produced by interaction with air because the distance from surface of

circular cone collimator to the isocenter has reduced. The scatter factor curve for 60mm x

60mm jaws opening is generally similar as 50mm x 50mm opening only for 32.5mm

diameter field size, the value is slightly decreased.

It is important to verify by inspection that area outside the central part of the

conical collimators is completely covered by the jaws. The jaws also ensure do not

overlap the circular collimator opening. This situation happen when there was MLC leaf

stuck and not calibrated properly. The advantage of using smaller opening jaws is to

avoid unintended leakage radiation.

87

It is obvious from the curves in this figure that scatter factor of small fields show

strong field size dependence with rapidly decreasing as the field diameter decreases. This

is due to decreasing primary dose for field sizes smaller than the lateral electron range

where lateral electronic equilibrium no longer exists. However the ionization chamber

may be too large for scatter output measurements in the radiation field of the collimator

with 5mm diameter.

Contributions to the scatter factor may be considered to arise from two groups of

particles. Direct particles arise from the machine head and indirect particle arise from

collimation devices such as collimator housing and circular cone. Scatter factor are

strongly dependence on the field size and the energy of photon beam. In this work, only

the effect of field size has been studied. If more particles pass through the collimator

opening and reach the point of interest (center of active volume of ionization chamber)

inside medium of water then lateral scatter equilibrium effect was said to exist which

arise from the collimate field size. As the diameter of field size of circular cone is bigger,

the direct photon (direct particle) still remained the same but contribution from the

scattered photon or indirect particle from the cone size is differ.

In the small field dosimetry, there was less contribution of scattering radiation

from the medium (water) and it is called as phantom scatter. Faiz M. Khan (1994) was

explained the effect of phantom scatter when the field size increased. The explanation is

general and agrees what was happen in the small field size dosimetry. The effect of field

size on the scatter factor due to phantom scatter alone is significant as long as distance

between the point of measurement and the edge of field is shorter than the range of the

laterally scatter electron produced. When the certain distance is reaches, there is no

further increased in the scatter factor caused by phantom scatter. When the field size is

reduced below that required for lateral scatter equilibrium, the scatter factor decrease

rapidly. The decreasing scatter factor for the small field size may be cause of large

88

number of direct particles and at the same time the indirect particle was reduced due to

collimator housing and circular cone in SRS procedure. Besides that, 50 mm x 50 mm

was applied for X and Y jaws also limit the production of scatter radiation by 1 %.

The scatter factors measured with the ionization chamber are generally smaller

than those obtained with other techniques, especially for smaller field sizes. This is due to

the lack of lateral electronic equilibrium which is more pronounced for smaller field sizes.

The other factor due to underestimate result is the imposition of the ionization chamber

during measurement may cause the entire or certain part of active volume is not enclosed

within the central uniform dose region. During the measurement, we totally depend on the

room’s laser to guide us to position the center of chamber at the right position. If the laser

was misaligned then it affects the correct positioning of the ionization chamber.

From the results also mean that, with small collimators treatment time can be

longer. This is due to the small value of scatter factor dominantly for 5mm diameter field

size which treatment time two times longer compared to 12.5 mm diameter field size. In

calculation of MU, the Scatter Factor is inversely proportional to the MU if the dose

prescribed is not changed. Then, if the scatter factor value is decrease then the MU will

increase and increase the treatment time because the dose rate for the machine is not

change for SRS treatment which is 300MU/min.

89

5.2 QUALITY ASSURANCE (QA) PROCEDURES BEFORE SRS TREATMENT

Unlike TPSs for non-stereotactic radiotherapy, the accuracy of a stereotactic treatment-

planning system has two components: the accuracy of dose calculation and the accuracy

of stereotaxy. The accuracy of stereotaxy depends on CT localization, laser alignment,

isocentric accuracy of linear accelerator and various human errors. It is found that the

differences of coordinates of top centers of each object compared to known specification

using Brainlab CT Localizer as shown in Table 4.10 determined by the treatment

planning system is 0.28 ± 0.10 mm, 0.45 ± 0.19 mm and 0.95 ± 0.46 mm in AP, lateral

and vertical directions, respectively when using 2.4 mm slice thickness. While by using

Radionic CT localizer (refer Table 4.11), the differences of coordinate is 0.33 ± 0.10 mm,

0.23 ± 0.15 mm and 2.50 ± 0.67 mm in AP, lateral and vertical directions, respectively

when using 2.4 mm slice thickness. From the result, the highest mean difference is 2.50 ±

0.67 mm for vertical direction using the Radionic CT localizers.

Among the components of localization errors of CT, the errors of vertical

direction were much greater than AP and lateral directional errors, mainly due to image

resolution. As the accuracy of CT localization is directly related to 3D image resolution.

David R. Choi et al, 2000 also reported that, in geometric phantom test, a 3 mm slice was

used and the resulting error was 1.2 ±0.5 mm. This value is comparable to results of the

other studies 0.91 ± 0.3 mm for 2 mm slice and 1.58 ± 0.5 mm for 4 mm slice. Our results

are slightly agreed with study done by David R. Choi et al. 2000, the resulting error was

0.91 ± 0.3 mm for 2 mm slice thickness used compare to our study which is 1.1 ± 0.4 mm

for 2.4 mm slice thickness. From this study, it is found that an increase in slice thickness

may affect in the inferior-superior (vertical) direction. The CT localization error increases

with increasing slice thickness used.

90

This error was mainly due to the error of vertical coordinate. In general, the

isocentric deviation in the vertical direction is much greater than ones in the AP or lateral

direction because of gantry sag to compensate for gantry weight. In the lateral direction,

the deviation 0.19 mm and 0.37 mm at 90° and 270° of gantry angle respectively. Based

on our result, it was agreed with previous study which using the same test (Winston-Lutz)

reported by Rowshanfarzad et al. 2011.

Mechanical isocenter and laser alignment can be shifted over time. The linac

machine was being used for 12 years and it possible to have slightly mechanical problem.

The deviation also could be occurred if the cone mount not be repositioned perfectly after

the service or maintenance. Besides that, the error occurred due to systemic error when

analyzed the irradiated film. The irradiated film must have a good contrast in order to

identify the center of collimator and center of tungsten sphere image accurately. In

conventional technique, the accurate of the result strongly depend upon the experience of

the physicist that analyzed the irradiated film. By using the Vidar scanner, image can be

clearly seen by manipulating the image with zooming technique but the result was

overestimated due to the higher spatial resolution of the film.

Couch-mounted systems strongly rely on a set of three wall-mounted lasers to

provide a frame of reference in the treatment room. The lesion is positioned at machine’s

isocenter by visually aligning these lasers with stereotactic coordinates that are scribed

onto the target positioner. Sanford et al. 1998 reported that, using these lasers as the sole

mechanism for positioning radiosurgery patients can result in greater than 1mm

systematic uncertainty.

91

The target coordinates from treatment planning are located at the isocenter of the

linear accelerator using LTLF. As the center of exposed beam is equal to the isocenter of

linear accelerator, we can estimate the deviation of target coordinates using a fiducial

marker of the target localizer. As the target coordinates from treatment planning are

defined relative to the BRW frame, if we assume that the frame is firmly attached to the

patient’s skull, we could detect human errors, the accuracy of LTLF, laser alignment and

circular cone’s alignment.

The accuracies in localized the target at the correct coordinates is very important in

SRS treatment to protect surrounding tissue especially brain tissue from getting

unnecessary dose. So the aim to achieve less than 1 mm displacement error depend on the

cooperation from every person that involved in delivering the treatment including therapy

radiographer, physicist, oncologist, neurosurgeon and also engineer.

92

5.3 DOSE VERIFICATION

A treatment plan was generated on the treatment planning platform used for SRS at our

center. The Treatment Planning System runs on HP Z800 Workstation and utilized i-Plan

RT Dose 4.1 software developed by Brainlab. The dose and MU calculation are generated

by the circular cone algorithm based on the beam data measured. The plan incorporated

the standard group of arcs to give uniform dose to the target volume.

In our study, the target volume is pinpoint 3D ionization active volume. The CT

study was utilized to calculate the target coordinates. By contouring the active volume on

the CT images, the result of target volume was 0.014cc compared to the 0.016cc which is

specified by manufacturer. The result is due to error of target localization on the CT

images and at the same time affected on calculating absolute dose to the point of interest

by circular cone algorithm.

A water-to-phantom material correction was introduced to compensate for

difference in electron density of water and the phantom material. The temperature-

pressure-corrected reading of the ionization chamber in nanocoulomb was converted by

the chamber calibration factor to cGy. The mean calculated dose was 336 cGy, while the

measured dose was 343.3 cGy which constitutes a dose deviation of 2.17% from the

calculated dose. No energy displacement and chamber wall correction factors were

considered.

The main errors are likely due to position of the ionization chamber within

straddling fluence segments of the beam (P.Basran et al. 2008). In is clear that placing the

ion chamber in a lower-dose gradient region would likely improve the agreement of

measured and calculated dose. However it is very hard to distinguish between high-dose

and low-dose region from the stereotactic plans because of the concentration of the

93

radiation’s intensity focused to small beam area. In the measurement setup, we are relying

on the accuracy of the room laser’s alignment. The phantom also position on the IMRT

extended couch which made from carbon fiber to avoid the attenuation effect of the beam

that will affect the charge collected by ionization chamber.

In small beam dosimetry, by using a small active volume of ionization chamber

lead to underestimate on the dose result especially if the entire chamber is not enclosed

within the central uniform dose region due to displacement error. The reason for an

underestimation of dose is lack of lateral electron equilibrium. Besides that, even though

an ionization chamber used are generally thought as small probably it is large enough so

that a non-uniform dose is averaged over the ionization chamber’s active volume. As a

result, the dose measured to be smaller than it is.

The pin point 3D ionization chamber was inserted inside the IMRT Thorax

phantom during CT scanning procedure and the treatment planning was done based on

the image of the phantom plus ionization chamber which inserted into the phantom. The

dose was calculated at the center of the active volume which constitute of gas. It is creates

slightly inhomogenous medium condition between gas and water equivalent materials

(PMMA). The circular cone algorithm has limitation in calculating dose inside

inhomogenous medium. For circular cone algorithm in dose calculation near

inhomogenous areas such as lung or bone tissue or close to the tissue border (both within

range of a few centimeters), the calculated dose can deviate from real dose delivered by

more than 10%. So that, the measurement condition must be avoid having very large

amount of air surrounding the detector.

94

The results show that the differences in measured dose and calculated dose by

TPS are in range -2.26% to -4.48% in TLD 100H measurement (refer Table 4.18) prior

take account attenuation correction for differences in attenuation characteristic between

Perspex and solid water phantom materials (PMMA). The TLD chips positioned at 7.2

cm inside the solid water phantom. The TLD chips were arranged on the solid water

phantom which is purposely made for TLD Chips calibration. That solid water phantom

was drilled to position TLDs inside the solid water phantom slab. A 0.6cm of Perspex

slab then put on the top of water phantom slab and screwed to it to remove air gap. Then

another rest of solid water phantom was sandwiched to the solid water phantom (for TLD

calibration) to make overall thickness of phantom to be 16cm (refer Figure 3.24).

Therefore, the incoming radiation particles interacted with the Perspex first before it was

reached to TLD. Some of the particles were attenuated by Perspex materials.

However, attenuation correction difference is very small but could not be ignored.

High energy of photon beam used (6 MV) interact with phantom medium producing

higher possibility of Compton scattering effect occurred. Compton scattering interaction

dominantly occurred in high energy range. The Compton scattering effect also depends

on the electron density of the medium. This could possibly mean that the Perspex slab

have difference of attenuation characteristics compared to solid water phantom

slab(PMMA materials) in which the dose was calculated by TPS based on CT image of

IMRT Thorax phantom which is also made from PMMA materials. Perspex has abilities

to attenuate the radiation beam slightly higher than PMMA but less than water because

Collision Stopping Power value at average energy of 6 MV photon is slightly higher for

Perspex compared to PMMA. This may explain why results of measurements

demonstrated a lower dose values compared to the calculated dose values.

95

There also many other contributing factors that affected the accuracy of the TLD

in measuring the dose. For high sensitivity TLD such as TLD 100H, the annealing

procedure is one of the factors affecting the Thermo Luminescent (TL) signal values. The

response of lithium fluoride (LiF) TLD is sensitive to thermal history. The TLD materials

were annealed before being used to empty the electron traps associated with defect

structures in the crystal lattice. The recommended thermal treatment for TLD 100H is

annealing at 240° C for 10 minutes. When the LiF is reused, it is important to repeat the

same thermal history to maintain the sensitivity of the TLD. But because of the annealing

duration is too short, the temperature inside the annealing oven may be fluctuated and that

will affect the performance and response of TLD 100H.

Another factor contributing to the error of the result is the handling and storage

procedures using TLD 100H. Many aspects of handling and handling of the TLD can

affect their TL sensitivity, stability, precision and minimum detectable of dose. For the

purpose of discussing possible uncertainties in stability and sensitivity associated with the

handling of TLD, it is assumed that the choice of a particular form TLD is based

primarily on dosimetric considerations. Within limits, the TL sensitivity of TLD is

directly proportional to the mass of active phosphor present. In each TLD chips, the mass

of active phosphor present is fixed during manufactured and must not change when used

in the measurements. So that, the TLD must be taken care not to scratch or abrade their

surfaces. Although TLD chips are fairly rugged, they should not be dropped onto a hard

surface as they may be fracture or damage. Then, the tweezers or plastic forceps must be

used when handling the TLD chips and not simply with bare fingers. The present of the

dust or carbon on our skin surface (fingers) may in contact with the surface of TLD chips.

When, the TLD is heated during annealing or readout procedures, the carbon will burn

and produce a new brownish layer onto the surface of the TLD chips and that may affect

the reading of the TL signal.

96

The TLD reader systems also affecting the readings of TL signal response of TLD

100H. The Time Temperature Profile (TTP), the photomultiplier tube (PMT) noise,

reference light reading, PMT temperature, bias voltage and the nitrogen gas flow rate

must be checked before readout procedure is carried out. The TTP is the temperature

profile to which the TL material is heated as function of time. Therefore the TTP set on

the reader system must be suitable with the type of TLD used. Suitable TTP setting is

important to get accurate TL reading and then the reading of TL is converted to the

meaningful dosimetric unit which is dose. PMT noise is reading taken without TLD on

the planchet and a part of quality assurance (QA) procedure of TLD reader to measure the

electronic noise in the system and to determine if there are any light leaks into the system.

The PMT noise reading should be less than 0.4 nC. If the reading of PMT noise is more

than that value, the planchet must be cleaned with alcohol to remove the contamination.

Reference light reading measure the light output in order to produce a constant light

output. If the reading is higher than reference reading, there are electronic problem with

the system. The nitrogen gas must be flowed through the planchet compartmeny to extend

planchet life and to eliminate the unwanted oxygen which can induce the Tl signals.

These entire factors must be in tolerance range and satisfy in order to get accurate TLD

response in terms of TL signal values that converted to the dose value.

Same as measurement using ionization chamber, the dose accuracy measured with

TLDs also depend to the accuracy of positioning of the TLD 100H chips, air gaps within

the phantom slab, accuracy of laser alignment and accuracy of target isocenter. TLD

100H is very sensitive to changes of surrounding temperature. The phantom must be

cleaned before inserting the TLD 100H chips in order to remove the dust on the phantom

surface. After irradiation, it must be kept in room’s temperature and far away from very

bright light and hot devices.

97

The volume of TLD is small enough, the density is about equivalent to tissue

make it suitable to use as detector for dose measurement inside the phantom. There are

also no obvious changes in the density inside the medium and no tendency of creating

inhomogeneous medium condition. So that, the TLD 100H suitable for measuring point

dose inside the phantom because of the density which is slightly same as water.

Meanwhile, it is not a direct measurement detector. Then, the entire factors that related

that can affecting the TL reading should take into account.

98

CHAPTER VI

CONCLUSION

Dosimetry measurement must be done with more carefully than the usual care for small

field sizes. Smaller the field, the error is greater. Significant variations of dosimetric

parameters observed between small fields with respect to reference 10 x10 cm2 field size

due to many factors such as the lack of lateral charged particle equilibrium, volume

averaging effect, the higher spatial resolution detector and exact or accurate ionization

chamber positioning.

In this study, it is suggested that for routine radiosurgery, PDD and scatter output

are measured with ionization chamber and OARs are measured with films. PDD, Off-axis

Ratio and Scatter Factor for 5mm diameter field size measured using ionization chamber

seem not accurate but in the practical views, we not used the 5mm field size to the treat

the lesion. Most common target diameters for the treatment of arterior-venous

malformations are found to be between 24 and 30 mm (AAPM Report 47, 1994).

All measurements (PDD, Off-axis Ratio, and Scatter Factor) covered the range of

field sizes and depths that will be used in subsequent treatment planning. The following

beam data (PDD, Off-axis Ratio and Scatter Factor) are needed as input for the circular

cone dose algorithm for radiosurgery with round stereotactic collimators. The beam data

was saved into Physics Administration in the TPS workstation. Based on these data, a

99

treatment plan was made and dose calculated at the target isocenter which is at center of

the active volume of ionization chamber. The mean dose calculated by the TPS based on

the beam data entered was 336 cGy at isocenter. The accuracy of dose calculation of

Brainlab i-Plan RT Dose 4.1 algorithms and MU calculation depend directly on the

accuracy and quality of measured beam data.

The accuracy of the dose verification mainly due to the accuracy of beam data

measurements including PDD, OAR, Scatter Output and Absolute Linac Output

Calibration because of the TPS computer calculated the dose based on the measurement

beam data. But in the same time, accuracy of SRS dose delivery also correlated to the

following: CT image localization, isocentric deviation of linear accelerator including

circular cones and laser alignment and also human errors related to target coordinates

setup. The overall accuracy measured in this study was 1.98 ± 1.2 mm, excluding the

human errors. The errors mainly due to gantry sag to compensate the gantry weight and

also mechanical problem after 12 year it has been used. Meanwhile the Radionic CT

localizer is not recommended to use as localizer when doing the CT procedure for SRS

patients because it will result in larger localization error

In measuring a point dose, the usage of small active volume of ionization chamber

gave slightly overestimate of measured dose value compared to the calculated value from

treatment planning computer. The errors in the results are due positioning error, volume

averaging effect and lack of lateral particle equilibrium. Some limitation of circular cone

algorithm in calculating dose for several occasion also result in higher discrepancy on

calculated dose and measured dose.

100

Meanwhile, TLD 100H give good agreement results of dose but slightly less

accurate compared to pinpoint ionization chamber (0.016cc). In this study, the results of

measured dose using TLD give a good agreement with calculated dose generated by TPS

with deviation of less than 4% while the deviation for measured dose measured using

ionization chamber is about 2%. The results are acceptable such as recommended by

ICRU that dose delivered to the target volume must be within ±5% error.

Instead of measuring the point dose, the isodose distribution could be verified to

ensure that the surrounding tissues and organs at risk receive dose within the expectation.

The further study must be carried out to verify the dose distribution with film and TLD.

101

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106

APPENDIX A

ABSOLUTE LINAC OUTPUT CALIBRATION (IAEA TRS 398:2000)

a) Determination of 6MV Photon Output

For measuring the output of LINAC, we follow the IAEA TRS-398 protocol. Farmer type

ionization chamber type: 30013 was used to measure the output. 25g/cm3 of distilled

water filled into the mini water. The water tank placed on the treatment couch and

straightly under the gantry of LINAC. Then the chamber was inserted to the chamber

holder attached with the water tank. To make sure the center of chamber’s sensitive

volume (reference point) was coincide with isocenter of the 10cm x10cm field size of the

beam, the couch was adjusted so that the light field’s cross hair met the black line on the

chamber’s cap.

We calibrated the output with source to surface distance (SSD) technique which is

100cm. Then the chamber was lowered down to the measurement depth which 5 cm from

the water surface. The chamber connected through the connecting cable with electrometer

outside the treatment room. Electrometer’s voltage was set +400V. Beam energy and

monitor unit were set to 6MV and 100MU respectively. Before the beam turned on the

electrometer was zeroing to remove the remnant electrical charge. The beam turned on

and made few exposures to warm up the electrometer and to make sure the electrometer’s

readings were stable. Two readings obtained each for full voltage positive polarity (+400),

full voltage negative polarity (-400), half voltage positive polarity (+200) and half voltage

negative polarity (-200). After finished, the pressure and temperature of the room was

recorded using the barometer and thermometer which were earlier left inside the room

during the measurement. All the data was recorded into the Microsoft Excel sheet.

107

b) Determination of 6MV Photon beam PDI

The measurement of PDI for 5cm depth was taken because we must normalize the output

value to the calibration depth which is at 1.5 cm depth. Then the chamber was positioned

at 1.5 cm depth and the readings of charge collected on the depth with the same parameter

as at 5cm depth. In order to increase the accuracy the readings was taken with

electrometer’s voltage set for +400V. The average charge at +400V for 5cm depth was

normalized to the average charge at +400V for 1.5cm for calculating the PDI at 5cm

depth. The data was recorded into the Microsoft Excel sheet.

c) Determination of TPR20,10 for 6MV Photon beam

This measurement was done to know the beam quality for the specified chamber that used

for the output measurement. The same chamber now moved to 20cm depth inside the

water. The couch adjusted for SSD 80 cm to maintain the Source Chamber Distance

(SCD) was 100cm. The same set up for energy, MU and field size was used as previous

measurement (output measurement). The electrometer voltage was set at +400V. The

beam was turned on to start the exposure and repeated for three times. The readings were

recorded.

The same measurement was repeated but the chamber was positioned at 10 cm

depth with SCD was 90cm. The same set up again for energy, MU and field size was used

as previous measurement. The electrometer voltage was set at +400V. Average charge

measurement at 20 cm depth was divided to average charge measurement at 10 cm depth.

The value was compared to table 6.III in the TRS 398; 2000 to get calculated value for

kQ. The value than was recorded into the Microsoft Excel sheet for calculating nominal

linac output.

108

Table 8.1: results of nominal output at calibration depth (1.5cm)

109

From the Table 8.1, the result of absorbed dose to water at Dmax is 1.012 cGy/MU. The

deviation is 1.2% higher. Meanwhile, the result still accepted based on tolerance value

stated inside the Manual for Quality Assurances Programmed (QAP) released by Ministry

of Health which is ± 2%. All correction factors as elaborated previously have been

applied in order to improve the accuracy of the result. The Absorbed dose to water is very

important for calculating MU as dose prescribed for every SRS treatment. This value also

called as dose rate. The result is required to be entered in Beam Profile Editor or Physics

Administration in the TPS. The following data was used by TPS algorithm in order to

calculate the absolute dose to the point of interest and MU/arc angle for SRS treatment.

110

APPENDIX B

RADIATION ISOCENTER ALIGNMENT

a) with respect to the gantry

The purpose of the test is to determine the proportionality of radiation isocenter with the

gantry central rotational axis. The intersection of the central of a series of beams, each

directed from a different gantry angle, should lie within a circle of specified diameter in a

plane containing these central rays (AAPM Report No. 47). This circle should also

include the mechanical isocenter. This is also generally demonstrated with a star shot film.

To produce the star shot film, firstly the collimator and gantry angle at zero degree.

A ready pack Kodak X-Omat therapy verification film was sandwiched into the blue

Styrofoam and positioned on the treatment couch so that the film part must be hang in the

air to make sure no attenuation from the any couch material midway between radiation

beam and the film. The centre of film aligned to be at center of rotational gantry axis by

using the room’s laser and the pointer. The blue card board that supporting the film was

remains stationary on the couch by applying the xylophone tape to each corner of it.

Then, the field size was set to 40cm width for X jaw and 1cm width for Y jaw.

Monitor unit was set 150 MU for every exposure. The nine exposures were done with 40

degree apart between exposures, starting at 180 degree angle with clockwise gantry

rotation. After finished the exposures, the film was remove from the cardboard and

processed by using film processor inside the dark room.

111

For analyzing the star shot film, the film was scanned with Vidar scanner. By

using the Ray 3.0 (film dosimetry software), the intersection of the central axis of

recorded radiation beams which cut all the centerlines or were tangential to them was

determined.

Figure 8.1: Star shot film of gantry isocenter shift analyzed using Ray 3.0 Film Dosimetry Software Table 8.2: Results of gantry isocenter shift analyzed using Ray 3.0 Film Dosimetry Software

Result on Star test Tolerence Pass/ Fail

1.3 mm 2mm diameter Pass

112

b) with respect to the table

The purpose of the test is to determine the proportionality of radiation isocenter with the

treatment table or couch central rotational axis. The intersection of the central of a series

of beams, each directed from a different couch angle, should lie within a circle of

specified diameter in a plane containing these central rays (AAPM Report No. 47). This

circle should also include the mechanical isocenter. This is also generally demonstrated

with a star shot film.

To produce the star shot film, firstly the collimator and gantry angle at zero degree.

A ready pack Kodak X-Omat therapy verification film was placed on the flat surface of

the treatment couch at 100 cm from the target. Then, the field size was set to 20 cm width

for X jaw and 1 cm width for Y jaw. Monitor unit was set 150 MU for every exposure.

Seven exposures were done with 40 degree apart between exposures, starting at 240

degree angle with anti-clockwise couch rotation. After finished the exposures, the film

was remove from couch surface and processed by using film processor inside the dark

room.

For analyzing the star shot film, the film was scanned with Vidar scanner. By

using the Ray 3.0 (film dosimetry software), the intersection of the central axis of

recorded radiation beams which cut all the centerlines or were tangential to them was

determined.

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Figure 8.2: Star shot film of couch isocenter shift analyzed using Ray 3.0 Film Dosimetry

Software

Table 8.3: Results of couch isocenter shift analyzed using Ray 3.0 Film Dosimetry Software

Result on Star test Tolerence Pass/ Fail

1.0 mm 2mm diameter Pass

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APPENDIX C

DOSE VERIFICATION PLAN

1.0 Plan 1

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Plan 2

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\" measurement of dosimetric parameters and dose verification in stereotactic

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