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CHAPTER 4

PRE TREATMENT PATIENT SPECIFIC QUALITY

ASSURANCE OF RAPIDARC PLANS

4.1 INTRODUCTION

Advanced treatment techniques use optimized radiation beam

intensities to conform dose distribution to the irregular shaped planning target

volume (PTV). This is especially useful when the PTV has a concave shape or

if organs at risk (OAR) lie very close to the PTV. When IMRT is delivered by

means of a multileaf collimator (MLC), its success strongly depends on the

behaviour of the leaves, namely their precise positioning and, in the case of

dynamic treatments, the accuracy of the leaf speed control. The dose

distribution is very sensitive to a number of factors, among which are the leaf

end design, the transmission through the leaves and between two adjacent

leaves, and the gap between two opposed leaves. Unlike conformal

homogeneous beams, intensity-modulated beams may have areas of high dose

gradient anywhere in the field and not at the boundaries so that a correct

modelling of the beam penumbra by the treatment planning system (TPS) is

especially important. Thus, intensity modulated radiation delivery requires

strict quality assurance (QA) hardware tests for MLC and specific patient

treatment plan validation procedures. The latter generally involve mapping

the plan fields onto a phantom that has been computed tomography scanned,

creating a so called hybrid plan, and comparing the results with measurements

made on that phantom. It is assumed that the validity of the results for the

phantom can be extrapolated to the patient.

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Ionization chambers are the preferred dosimeters for measuring

absorbed dose in intensity modulated radiation therapy deliveries. Although

other methods such as one-dimensional or two-dimensional arrays of

detectors, film, and electronic portal imaging devices, among others, have

proved increasingly useful, they are more suited to the measurement of

relative dose distributions. Dose measurements with ionization chambers

(ICs) reflect the average dose value over their volumes. Since the RapidArc

delivery involves complex treatment delivery procedures like variable dose

rate and variable gantry speed, the patient specific quality assurance for

individual patient should be done. In this paper we present our methodology

for RapidArc pre-treatment patient specific QA procedure for hundred

different cases using semiflex ionization chamber.

4.2 MATERIALS AND METHOD

4.2.1 Pre Treatment Rapid Arc Quality Assurance

Hundred different RapidArc plans conforming to the clinical

standards were selected for the pre treatment patient specific quality

assurances study using semiflex ionization chambers (0.125 cm3). A treatment

plan is considered clinically acceptable if the dose within the PTV is uniform

to within predefined limits, typically ± 5%, and the doses to OARs are lower

than those which would cause unacceptable normal tissue complications.

Verification plan was created for each treatment plan with the chamber-

phantom combinations CT scanned (Figure 4.1). All plans were aimed to

deliver 200 cGy at the isocentre. All the verification plans has were done

using the Eclipse planning system version 8.6 using the AAA algorithm. All

measurements have been done using Varian 2100 C/D linear accelerator. The

semiflex ionization chamber was inserted inside the Octavius CT phantom

(Octavius Phantom without compensating cavity) in such a way that the

central axis of radiation beam will be coincided with the centre of the

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sensitive volume of the chamber. Experimental setup is shown in Figure 4.2.

The source to detector effective point of measurement is kept at 100cm and

the source to surface distance (SSD) at 84cm. The ionization chamber was

connected to the array interface. From the array interface data has been

transferred to the PTW tansoft software using the RS 232 cable.

Figure 4.1 Verification plan window in eclipse treatment planning

system for the pre treatment patient quality assurance using

semiflex ionization chamber and octavius CT phantom

The semiflex chambers are air vented waterproof thimble chambers

used for measuring high-energy photon and electron radiation. The chamber

used in this study has a sensitive volume of 0.125 cm3, with a flat energy

response within a wide energy range. The nominal useful energy range is

from 30 kV to 50 MV photons and 6 MeV to 50 MeV electrons. The wall

material is graphite with a protective acrylic cover. The guard rings are

designed up to the measuring volume. The cylindrically shaped chamber has

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an inner diameter of 5.5 mm. The 0.125 cm3 chamber is ideal for 3D

dosimetry, since the measuring volume is approximately spherical resulting in

a flat angular response over an angle of ± 160° and a uniform spatial

resolution along all three axes. The specification of the ionization chambers,

phantoms, and software used for the study is specified in Table 4.1. Before

the measurements the chambers were cross calibrated with a secondary

standard at our clinic for the photon beam. All the results were cross

compared with fluence measurements using 2D seven29 array combined with

Octavius LINAC phantom. The gamma analysis was done using the PTW

verisoft software (version 3.2).

Table 4.1 Specifications of the ionization chambers, phantoms and

software used in the study

Ionization chamber PTW Semiflex Chamber

(31010-1571)

PTW 2D-Array 729

(T10024)

Type Semiflex Chamber vented cubic ion chamber Array

Electrometer/software PTW - TanSoft V 1.2 PTW- Matrix Scan (S080050)

Active volume 0.125 cm3 0.125 cm

3 (single ion chamber)

Polarizing voltage 400V 400V

Wall material PMMA + Graphite Graphite

Phantom used PTW Octavius CT phantom PTW Octavius LINAC phantom

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Figure 4.2 Set-up for the pre-treatment quality assurance tests for

rapid arc treatment delivery. Semiflex ionization chamber

inserted inside the Octavius CT phantom. SSD was kept at

84 cm

4.2.2 Gamma Analysis

Gamma analysis was done to compare two 2D dose maps.

Generally the dose distribution comparison is sub divided into regions of high

and low dose gradients. In low gradient regions, the doses are compared

directly with an acceptance tolerance placed on the difference between the

two dose maps. In high dose gradient regions, a small spatial error, in either

of the dose maps or small miss alignment results in large dose difference

between the dose maps. Dose difference in the high dose gradient regions

may therefore be relatively unimportant and the concept of DTA distribution

is used to determine the acceptability of agreement. The DTA is the distance

between a dose point in a dose map and the nearest point in the other dose

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map that exhibits the same dose. The dose difference and DTA evaluations

complement each other when used as determinants of agreement accuracy

between the dose maps. The simultaneous use of DTA and a percent dose

difference (DD) is proposed by Daniel A Low et al (1998). These parameters

can help evaluate the agreement of the two distributions in terms of

misalignment and difference, respectively. So in this study the various gamma

index constraints which are a combination of particular DTA value with

specific dose difference tolerance value were used.

4.3 RESULTS

Figures 4.3, 4.4, and 4.5 shows the percentage variation of absolute

dose between planned and measured for head and neck, thoracic and abdomen

cases respectively. The statistical analysis for absolute dose measured with

semiflex ionization chamber is shown in Table 4.2. Positive absolute mean

dose variation of 0.56 % was observed with thorax cases with a standard

deviation (SD) of ± 1.13 between the plans with a range of -1.78% to 2.70%.

Negative percentage dose errors were found with head and neck and abdomen

cases, with a mean variation of -0.43 % (SD ± 1.50), (range -3.25 % to 2.85

%) and - 0.35 % (SD ± 1.48), (range -3.10 % to 2.65 %) for head and neck

and abdomen cases respectively. Positive variations in measured absolute

doses were observed with thoracic cases. Most of the cases selected in this

category for planning were oesophagus. Also negative variations in mean

dose were observed with head and neck and abdomen cases, where the MLC

movement is stringent and more number of critical organs was involved. The

results were compared with the fluence measurement using the 2D array

seven29.

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Figure 4.3 Percentage variation for planned and measured isocentre

absolute dose using semiflex ionization chamber for head

and neck cases

Figure 4.4 Percentage variation for planned and measured isocentre

absolute dose using semiflex ionization chamber for thoracic

cases

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Figure 4.5 Percentage variation for planned and measured isocentre

absolute dose using semiflex ionization chamber for

abdomen cases

Table 4.2 Statistical analysis for absolute dose measured with semiflex

ionization chamber

Statistical analysis for semiflex

ionization chamber

Head and neck Thorax Abdomen

maximum negative deviation (%) -3.25 -1.78 -3.10

maximum positive deviation (%) 2.85 2.70 2.65

mean deviation (%) -0.43 0.56 -0.35

standard deviation 1.50 1.13 1.48

sample variance 2.26 1.28 2.19

count 40 25 35

negative 25 8 23

positive 15 17 12

negative to positive ratio 1.67 0.47 1.92

confidence level 0.47 0.35 0.46

Kurtosis (for all 100 patients) -0.48

Skewness (for all 100 patients) 0.22

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Ionization chamber and 2D array relative dose measurement results

were highly comparable. Relative dose measurements with 2D array agreed

well with the TPS calculated dose for all the cases. The measured and

calculated fluence maps were both normalized to the same point and analysis

was done. Evaluation was done using PTW verisoft software by keeping the

passing criteria as 3mm DTA, 3% DD, for 95% of the evaluated dose points.

Figure 4.6 shows the gamma analysis results for the hundred different

RapidArc plans. The maximum percentage value failed in gamma analysis

was found to be 4.95, 4.75, and 4.88 for head and neck, thorax, and abdomen

cases respectively. The results indicate that the measured values agree well with

that of the calculated by the treatment planning system in the treated volume

region. In all the cases analysed the percentage dose points failed the gamma

criteria is less than 5% (Table 4.3). Also the higher variation observed may be

because of the complex MLC movements with small effective opening which

will be more challenging to deliver the radiation. Also the larger deviations were

observed particularly in the low dose regions outside the treatment volume. So

by properly defining the region of interest (planning target volume) for the

gamma analysis, it is possible to increase the pass percentage.

Figure 4.6 Gamma analysis for 3mm DTA, 3% DD for all the hundred

patients

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Table 4.3 Gamma analysis results for hundred cases using PTW

verisoft software

Gamma analysis (% dose points failed)

Head and

neck Thorax Abdomen

minimum 2.35 2.12 2.25

maximum 4.95 4.75 4.88

mean 3.38 3.16 3.33

(SD) 0.82 0.67 0.80

4.4 DISCUSSION

The differences between the planned and measured doses can be

attributed to three different sources of error: dosimeter, delivery system, and

dose calculation system. However, if the dosimeter is properly chosen,

commissioned, and maintained, errors related to calculation, delivery, or a

combination of the two can be reduced. The selection of ionization chambers

for the dose verification process for RapidArc involves issues particular to the

dosimetry of dynamically delivered small fields, varying dose rates, and the

summation of multiple low MU segments. Detector characteristics, such as

energy dependence, the size of the collecting volume, charge leakage, design,

and materials are important considerations. In addition, volumetric modulated

arc therapy dosimetry conditions are radically different from the open field

conditions under which the chambers are calibrated, and may invalidate the

use of such calibration factors in these radiation beams. Dose discrepancies

between the TPS and measurements also arise due to inaccurate beam and

component modeling, the dose calculation algorithm, and beam delivery.

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During RapidArc delivery, the detector is often located either

outside of the field or in penumbra regions, resulting in volume averaging

which is especially important over gradient regions. To overcome this

problem suitable volume chambers are suggested for RapidArc absolute dose

measurements. Semiflex ionization chamber with 0.125 cm3 proved to be

ideal for RapidArc absolute dosimetry. Also in gamma analysis the

percentage of passing points depends on choice of normalization point. The

requirements for dosimetric accuracy in gamma analysis are highest in

regions of low dose gradient and that the requirements for geometric accuracy

are highest in regions of high dose gradient. There are therefore two criteria

for accuracy. Dose points situated in the penumbra region were found to be

responsible for low passing criteria. Study published by Danielle Fraser et al

(2009) suggests that the degree of underestimation was the greatest for the

smallest volume chamber for intensity modulated type of radiation delivery.

Ion chamber based detector arrays are known to have insignificant

energy and dose-rate dependence for megavoltage photon beams, but required

a large sensitive volume, with diameter of the order of 5mm for each

chamber, to gain signal and will therefore exhibit a volume averaging effect

in steep dose gradient regions. Therefore the 2D array was used for fluence

measurement rather than the absolute point dose measurement in the present

study. For point dose measurements, the semiflex ionization chamber was

kept inside the Octavius CT phantom without the compensating cavity.

A trend for measured dose values to be lower than the calculated

ones was observed mainly with head and neck cases. This observation may be

explained by the leaves shielding the ICs from the primary beam during a

fraction of the irradiation. Others potential cause for under response may be

the ionization chamber leakage. Leakage is more marked for small volume

ionization chambers, because chamber sensitivity is proportional to volume.

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In practice, absolute dose discrepancy between a treatment planning system

and measurement can be clinically accounted by applying TPS correction

factor that adjusts the analytical algorithm to better match systematically

different intensity modulated measurements. Alternatively, treatment plan

specific correction factors can be applied to ionization chamber to account for

fluence perturbation effects in RapidArc delivery. In this study the average

discrepancy between TPS calculated and measured remains within ICRU

Report 24 recommendations for clinical accuracy of ± 5%. Also it is important

to normalize the dose distribution in the high dose region and exclude the area

receiving a dose less than a certain minimum dose for the RapidArc QA

analysis.

4.5 CONCLUSION

On the basis of the studies performed, it can be concluded that the

semiflex ionization chamber having a volume of 0.125cm3 can be used

efficiently for performing the pre-treatment quality assurance of RapidArc

plans for all the sites. The results provide an overall accuracy when compared

to fluence measurement done using 2D array seven29.