small-field dosimetry: its importance in...
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Indra J. Das/ Small Fields (Page-1)
IJDas (1)AAPM-2013
Small-Field Dosimetry: Its Importance In Clinic
Indra J. Das, PhD, FAAPM, FACR, FASTRODepartment of Radiation OncologyIndiana University School of Medicine &IU Health Proton Therapy CenterIndianapolis, IN, USA IJDas (2)
AAPM-2013
Wrong detector used for BrainLab cone calibration
Misadministration Media Coverage
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Misadministration Media Coverage
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TG-155: Small Fields and Non-Equilibrium ConditionPhoton Beam DosimetryIndra J. Das (Chair)Indiana University School of Medicine, Indianapolis, IN 46202Paolo Francescon (Co-chair)Ospedale Di Vicenza, Viale Rodolfi, Vicenza 36100, ItalyAnders AhnesjöUppsala University & Nucletron Scandinavia AB, 751 47 Uppsala, SwedenMaria M. AspradakisDepartment of Radiation Oncology, Kantonsspital Lucerne, Lucerne SwitzerlandChee-Wai ChengMidwest Proton Radiotherapy Institute, Bloomington, IN, George X. DingVanderbilt University Medical Center, Nashville, TN 37232 Geoffrey S. IbbottRadiological Physics Center, MD Anderson Cancer Center, Houston, TX 77030Mark OldhamDuke University Medical Center, Durham, NC 27710M. Saiful HuqUniversity of Pittsburgh Cancer Institute, Pittsburgh, PA 15232Chester S. ReftUniversity of Chicago, Chicago, IL 60637Otto A SauerUniversity of Wuerzburg, Wuerzburg, Germany
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Treatment Fields
Magna-FieldsTraditional Fields
Advance Therapy FieldsSRS/SRTGamma KnifeCyber-KnifeTomotherapyIMRT
40x40 cm2 4x4 cm2
4x4 cm2 0.3x0.3 cm2
200x200 cm2
Small Field
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What is a Small Field?
Lack of charged particle Dependent on the range of secondary
electrons
Photon energy
Collimator setting that obstructs the source size
Detector is comparable to the field size
Indra J. Das/ Small Fields (Page-2)
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Source Size
Jaffray et al, Med Phys 20, 1417-1427 (1993)
90%, 70%, 50%, 30%, 10% iso-intensity line
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Calculation of Fluence Map Elements
Direct beam source size Open beam off axis distribution
Fluence modulationStep&shot
Dynamic
Wedges
Head scatter sources
flattening filter
collimators
wedges
Monitor back scatter
Collimator leakage, includingMLC interleaf leakage
shape of MLC leaf ends
MLC positioningBeam spectra
Spectral changes
Electron contamination
Processes to include
Components Critical for Small Fields
Courtesy Anders Ahnesjö
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Das et al, Med. Phys. 35, 206-215, 2008
Definition of Small Fields
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For each element, find the contributions from the relevant sources
Calculation of Fluence Map Elements
Raytrace to calculate attenuationor use approximate penumbra shape
POI-eye-view of the source!
Courtesy Anders Ahnesjö
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1.5 1.0 0.5 0.0 0.5 1.0 1.50.0
0.2
0.4
0.6
0.8
1.0
x �cm�
2.0x2.0 cm2
Source sizes
cm
---- 0.01---- 0.35---- 0.50
6.5 6.0 5.5 5.0 4.5 4.0 3.50.0
0.2
0.4
0.6
0.8
1.0
x �cm�
Ene
rgy
Flue
nce
Sc
norm
aliz
ed
Energy Fluence Penumbra/Output
1.5 1.0 0.5 0.0 0.5 1.0 1.50.0
0.2
0.4
0.6
0.8
1.0
x �cm�
1.0x1.0 cm2
1.5 1.0 0.5 0.0 0.5 1.0 1.50.0
0.2
0.4
0.6
0.8
1.0
x �cm�0.5x0.5 cm2
Large field
Nyholm et al Radiother Oncol 78, 347, 2006 and Phys Med Biol 51,6245, 2006 IJDas (12)AAPM-2013
Potential inconsistency dose calculations ↔ delivery
Profile and Leaf Tip Positions & Location
isocenter plane
Source plane
xxHVL xw
xcm
cm
xHVL - xw
Delivery: uses of lookup table or parameterized correctionTPS: - “ - or direct calculation of 50% transmission
Courtesy Anders Ahnesjö
Indra J. Das/ Small Fields (Page-3)
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Scott et al, Med Phys, 36, 3132, 2009
Dose and Penumbra with Spot Size
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Dosimetry
Absolute Dose
Relative Depth Dose
o [D(r,d)/D(r,dm)]
TMR
Profiles
Output, Scp (total scatter factor)o [D(r)/D(ref)]
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403530252015100.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
1.02
WEHU-ArizonaTemple UU PennErlanger Serago et al.Fan et al.Zhu et al.
Total scatter factor from different institutions
Cone Diameter (mm)
Con
e F
acto
r (S
t)
Das et al, J Radiosurgery, 3, 177-186, 2000
Small Field Dosimetry Problem
Institutional variability in 6 MV Radionics SRS dosimetry
12% diff
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403530252015100.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
Pinpoint (par)
Pinpoint (per)
0.125ion(par)
0.125ion(per)
Diamond
TLD
MC(1mm)
MC(5mm)
CEA (Film)Kodak(Film)
Total scatter factor with various detectors
Cone Diameter (mm)
Co
ne F
acto
r (S
t)
Das et al, J Radiosurgery, 3, 177-186, 2000
Dosimetric Variation with Detectors
14%
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0.5 1.0 1.5 2.0 2.5 3.00.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Out
put F
acto
r
Side of Square Field (cm)
Dose to Water For Small Fields: Volume Averaging
PTW 31014 (Pinpoint)PTW 31010 (Semiflex)PTW 30006 (Farmer)
Monte CarloDiamondDiodeLAC+filmFilm
From Roberto Capote, IAEAIJDas (18)
AAPM-2013
30282624222018161412100
10
20
30
40
50
60
70
80
90
100
110
Film
Ion (0.125 cc)Ion (parallel-plate)Ion (Pinpoint)Diamomd
Dose profiles for a 40 mm cone with different detectors
Distance (mm)
No
rmal
ized
Do
se (
%)
201816141210864200
10
20
30
40
50
60
70
80
90
100
110
Diamond
Ion (Parallel Plate)
Film
Ion (Pinpoint )
Dose profile of a 12.5 mm cone
Distance (mm)
Nor
mal
ized
Dos
e (%
)
Hedrian, Hoban & Beddoe, PMB, 41, 93-110, 1996Das et al, J Radiosurgery, 3, 177-186, 2000
Profiles with different detectors
Indra J. Das/ Small Fields (Page-4)
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Ratio of Readings?
enD
enD
enD
ax
0
( ) ( )( ) ( )
( )mh
ab
d h hD E h d h
d h
( ) ( )rr m
ref ref aref ref
D r Q r W S
D Q e
m
m
a
Q W SD
m e
1 2
( )
re fQF F
rC
QC
Q(E,r) = Qr Pion Prepl Pwall Pcec Ppcf,
( ) ( )rr m
ref ref aref ref
D r Q r W S
D Q e
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AAPM-2013
Radiation Measurements
Charged particle equilibrium or electronic equilibrium Range of secondary electrons Medium (tissue, lung, bone)
Photon energy and spectrum Change in spectrum
Field size Off axis points like beamlets in IMRT
Changes en/ and S/ Detector size
Volume Signal to noise ratio
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CPE & Electron Range
Electron range= dmax in forward direction
Electron range in lateral direction
Nearly energy independent
Nearly equal to penumbra (8-10 mm)
Field size needed for CPE Lateral range
16-20 mm
CPE, Charged Particle Equilibrium
dmax
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Alfonso, et al. Med Phys 35, 5179-5186 (2008)
IAEA/AAPM proposed pathway
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Relative Dosimetry
rel
relf
Qf
Q
fQ
fQff
QQ ading
Output
MD
MDk msrclin
msrclin )(Re
)(
/
/clin
msr
clin
msr
clin
clin
msr
clin
,w,w
,w,w,,
refmsr
msr
msr
msr
msr
msr
,,,,,
ffQQQoQQoDW
fQ
fQw kkNMD
msrclin
msrclinmsr
msr
clin
clin
msr
msr
msr
msr
clin
clin
clin
clin
msr
msr
clin
clinmsrclin
msrclin
,,
,w
,w
/
/ ffQQf
Q
fQ
fQ
fQ
fQ
fQ
fQ
fQff
QQ kM
M
MD
MD
M
M
msrfmsrairw
fclinfclinairwffQQ PS
PSk msrclin
msrclin
)(
)(
,
,,,
IJDas (24)AAPM-2013
Why So Much of Fuss?
Reference (ref) conditions cannot be achieved for most SRS devices (cyberknife, gammaknife, tomotherapy etc)
Machine Specific reference (msr) needs to be linked to ref Ratio of reading (PDD, TMR, Output etc) is not the same
as ratio of dose
2
1
2
1
M
M
D
D
msrclin
msrclin
ffQQk
M
M
D
D ,,
2
1
2
1
Indra J. Das/ Small Fields (Page-5)
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6 MV; Central Axis
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
0 1 2 3 4 5 6 7 8 9 10 11
Field Size (cm)
Rel
ativ
e d
ose
at d
max
Scanditronix-SFDScanditronix-PFDExradin-A16PTW-PinpointPTW-0.125ccPTW-0.3cc
PTW-0.6ccPTW-MarkusWellhofer-IC4
15 MV; Central Axis
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
0 1 2 3 4 5 6 7 8 9 10 11
Field Size (cm)
Rel
ativ
e d
ose
at d
max
Scanditronix-SFD
Scanditronix-PFDExradin-A16
PTW-PinpointPTW-0.125cc
PTW-0.3ccPTW-0.6cc
PTW-MarkusWellhofer-IC4
Field Size Limit for Accurate Dose Measurements with Available Detectors
Das et al, TG-106, Med Phys, 35, 4186, 2008
IJDas (26)AAPM-2013
Detector Manufacturer Type volumeSFD Scanditronix Photon diode 1.7x10-5cm3
PFD Scanditronix Photon diode 1.9x10-4cm3
Exradin A-16 Standard Imaging Ion chamber 0.007cm3
Wellhofer-IC4 Scanditronix Ion chamber 0.40 cm3
Pinpoint PTW Ion chamber 0.015cm3
0.125cc PTW Ion chamber 0.125cm3
0.3cc PTW Ion chamber 0.3 cm3
0.6cc PTW Ion chamber 0.6 cm3
Diamond PTW Diamond 0.003cm3
Markus PTW Parallel plate 0.055cm3
Edge Detector Sun Nuclear Diode 10-5cm3
Other
Detectors
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Radiation Detectors
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PTW Choice for small Fields
www.PTW.de
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Sensitivity vs Volume of Detectors
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00
Volume (cm3)
Rel
ativ
e se
nsi
tivi
ty
PFD
SFD
A-16 PinPointMarkus
IC4
0.125cc
0.3cc
0.6 cc
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Understand Detector, Connector & Cable
Srivastava et al, SU-GG-T-270, 2010
Indra J. Das/ Small Fields (Page-6)
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Ion Chamber Issues in Small Fields Volume averaging Poor signal: signal to noise ratio Pion: Ion recombination
300 volts may not be needed May be in proportional counter mode or gas
multiplication Two voltage method may not be applicable
Voltage
Cur
rent
Large volume Ion chamber
Small volume Ion chamber
100 200 300 400 500 6000
IJDas (32)AAPM-2013
1.E-5
1.E-4
1.E-3
1.E-2
1.E-1
1.E+0
1.E+1
0.0 1.0 2.0 3.0 4.0 5.0 6.0
E (MeV)
Ph
oto
n F
lue
nc
e
0.5 cm, primary
5 cm, primary
0.5 cm, scatter
5 cm, scatter
WATER, at dmax (a)
Verhaegen, Das, Palmans, PMB, 43, 2755-2768, 1998
1.E-4
1.E-3
1.E-2
1.E-1
1.E+0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
E (MeV)
Ph
oto
n F
lue
nc
e
0.5 cm
5 cmAIR, at exit (c)
6 MV
1.50
1.55
1.60
1.65
1.70
1.75
1.80
1.85
1.90
1.95
2.00
0 1 2 3 4 5 6
Diameter of Cone (cm)
Ave
rag
e E
ner
gy
(M
eV
)
Spectra & Effective Energy from SRS Cones (0.5-5 cm)
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Sanchez-Deblado et al, Phy. Med. Biol., 48, 2081, 2003
Radiological Parameters
IJDas (35)AAPM-2013 F. Araki, Med. Phys. 33, 2955-2963 (2006).
Cyber Knife Dosimetry
0.5%
IJDas (36)AAPM-2013
Francescon et al, Med. Phys. 25(4), 503, 1998
MC
MC
MC
MC
MC
MC
Indra J. Das/ Small Fields (Page-7)
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Correction Factors
Francescon, et al Med Phys 35, 504, 2008
Correction Factor depends on:Field sizeSource size (FWHM)Detector type
IJDas (38)AAPM-2013Kawachi et al, Med Phys, 35, 4591-4598, 2008
Errors in Measured Reading
IJDas (39)AAPM-2013
0.90
0.92
0.94
0.96
0.98
1.00
1.02
1.04
1.06
1.08
1.10
1.12
1.14
1.16
0 5 10 15 20 25 30 35
Field size (mm)
Cor
rect
ion
fact
or
Siemens; PTW diode 60012
Elekta; PTW diode 60012
Siemens; Exradin A16
Elekta; Exradin A16
Siemens; Sun Nuclear Dedge
"Elekta; Sun Nuclear Dedge"
Siemens; PTW Pinpoint 31014
Elekta; PTW Pinpoint 31014
Siemens; PTW microLion
Elekta; PTW microLion
Francescon et al Med Phys,38, 6513, 2011
Published data on
IJDas (40)AAPM-2013 Scott et al, Phys Med Biol 57 4461–4476, 2012
Detector Density Effect
IJDas (41)AAPM-2013 Chung et al , Med Phys, 37, 2404-2413, 2010
Correction Factor vs Ion Chambers
IJDas (42)AAPM-2013 Med. Phys. 38 (12), 6992, 2011
of Linear Accelerators (Varian)
Indra J. Das/ Small Fields (Page-8)
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Radiotherapy and Oncology 100 (2011) 429–435
Med. Phys. 38 (12), 6513-6527, 2011
of Linear Accelerators
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Cyber Knife
Pantelis et al, Med Phy. 37, 2369-2379, 2010
IJDas (45)AAPM-2013 Sterpin et al, Med Phys, 39, 4066, 2012
Tomotherapy kmsr Reference Dosimetry
Reference: 5x10 cm2, 85 cm SSD, 10 cm
IJDas (46)AAPM-2013
kQ is not Constant in Small Field
Kawachi et al, Med Phys, 35, 4591-4598, 2008
IJDas (47)AAPM-2013 Sham et al, Med Phys, 35, 3317-3330, 2008
Depth Dose & Source Size
IJDas (48)AAPM-2013
Profile & Source Size
Sham et al, Med Phys, 35, 3317-3330, 2008
Indra J. Das/ Small Fields (Page-9)
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Effect of Inhomogeneity
Range of secondary electrons Simple scaling based on density
M. K. Woo, and J. R. Cunningham, "The valididty of density scaling method in primary electron transport for photon and electron beams," Med. Phys. 17, 187-194 (1990).
Perturbations of the detector T. Mauceri, and K. R. Kase, "Effects of ionization chamber
construction on dose measurements in heterogeneity," Medical Physics 14, 653-656 (1987).
R. K. Rice, J. L. Hansen, L. M. Chin, B. J. Mijnheer, and B. E. Bjarngard, "The influence of ionization chamber and phantom design on the measurement of lung dose in photon beam," Medical Physics 15, 884-890 (1988).
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Electron range & inhomogeneity
IJDas (51)AAPM-2013
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Rel
ativ
e D
ose
Depth (cm)
0.5 cm, 0.25 cm3 lungHomogeneous
Mon te Carlo
EPL
TMR Ratio
Power Law TMR
Convolut ion
CCC Water
Jones & Das, Med. Phys. 30, 296, 2003 Jones & Das, Med. Phys. 32, 766, 2005
Inhomogeneity Corrections
IJDas (52)AAPM-2013
TG-155 Recommendation Dosimetric measurements should be carried out with more than one detector system. Small volume detector should be used that has minimum energy, dose and dose rate
dependence as discussed in TG-120 and Report No103 should be used. Stereotactic diodes or electron diodes are recommended for field sizes < 1x1cm2 Micro chambers are best suited for dosimetric measurements for field sizes > 1x1
cm2 however, signal to noise as well as polarity effect should be evaluated. The quality of electrometer and triaxial cable as well as any connector and cables
need to be of high quality. Stereotactic diode with micron size sensitive volume should be the detector of choice
for measurements in beams in radiosurgery. The energy spectrum does vary in small fields such as SRS, and IMRT but these
changes result in insignificant variations in stopping power ratios when compared to those of the reference field used in dosimetry codes of practice.
The treatment planning system performance should be carefully validated when used for the treatment planning incorporating small fields. Although pencil beam and convolution/superposition dose engines are expected to perform well in small field treatment geometries and in almost homogeneous media, dose engines based on the Monte Carlo method are the most accurate method for modelling dose from small fields in heterogeneous media. The calculation grid size should be significantly smaller (~1/10) compared to the field size.
Small field dosimetry should have an independent audit by a different physicist either internal or external like Radiological Physics Center verification.
IJDas (53)AAPM-2013
Summary
Small volume detector should be used that has minimum energy, dose and dose rate dependence.
Micro-ion chambers are best suited for small field dosimetry; however, signal to noise should be evaluated.
Stereotactic diode are ideally suited for radiosurgery beams.
If field size is small compared to detector measurements should be performed at a greater source to surface distance with proper correction.
IJDas (54)AAPM-2013
-Summary
Energy spectrum does vary in small fields such as SRS, and IMRT, however, its impact is not significant.
Stopping power ratio in small fields for most ion chambers is relatively same as the reference field.
Spot check and verification of smaller fields should be carried out with at least another independent method (TLD, film, MC, etc).
Stay tuned to newer data and IAEA and AAPM TG guidelines.
Indra J. Das/ Small Fields (Page-10)
IJDas (55)AAPM-2013
Thanks