implementation of the iaea-aapm code of practice for the …€¦ · · 2018-05-07implementation...
TRANSCRIPT
M. Saiful Huq, PhD, FAAPM, FInstP
Dept. of Radiation Oncology, University of Pittsburgh Cancer Institute and UPMC Hillman Cancer Center, Pittsburgh, Pennsylvania, USA
Implementation of the IAEA-AAPM Code of Practice for the dosimetry of small
static fields used in external beam radiotherapy
Collaborators
• Yongqian Zhang, Ph.D.
• Min-sig Huang, Ph.D.
• Troy Teo, Ph.D.
• Kevin Fallon, M.S.
• Cihat Ozhasoglu, M.S.
• Ron Lalonde, Ph.D.
Contributors to IAEA TRS-483
Missing: Ahmed Meghzifene and Stan Vatnitsky
• The Code of Practice addresses the reference and
relative dosimetry of small static fields used for
external beam photon radiotherapy of energies with
nominal accelerating potential up to 10 MV. It does
not address other radiotherapy modalities such as
electron, proton and orthovoltage beams
TRS-483 CoP
• It provides a CoP for machine specific reference (msr)
dosimetry in a clinical high energy photon beams. It is
based on the use of a ionization chamber that has been
calibrated in terms of absorbed dose to water ND,w,Qo or
ND,w,Qmsr in a standard’s laboratory’s reference beam of
quality Qo or Qmsr.
• It also provides guidance for measurements of field
output factors and lateral beam profiles at the
measurement depth
TRS-483 CoP
• Radiation generators: 10 cm x 10 cm field can be set
• Follow TRS-398 CoP or AAPM TG-51 or equivalent protocol
• Radiation generators: 10 cm x 10 cm ref field cannot be
set
• Define machine specific reference (msr) field, fmsr
• Dimension of fmsr field
• Should be as close as possible to the conventional reference
field
Reference dosimetry: msr field
Machine type msr field CyberKnife 6 cm diameter fixed collimator
TomoTherapy 5 cm x 10 cm field
GammaKnife 1.6 cm or 1.8 cm diameter collimator helmet, all sources simultaneously out
BrainLab microMLC add on For example 9.8 cm x 9.8 cm or 9.6 cm x 10.4 cm
SRS cone add-ons The closest to a 10 cm x 10 cm equivalent square msr field achievable
• fmsr should extend at least a distance rLCPE beyond the
outer boundaries of the reference ionization chamber
FWHM ≥ 2rLCPE + d
msr fields for common radiotherapy machines
• How were the field sizes for the msr dosimetry
arrived at?
• Ask: What is the size restriction on an ionization
chamber for msr dosimetry?
msr fields: selection of chambers
• CPE conditions exist when one of the edges of the field extends at least a distance rLCPE beyond the outer boundaries of the ionization chamber. If the size of the detector is d, the FWHM of the field has to fulfil the condition:
FWHM ≥ 2 rLCPE + d
rLCPE (in cm) = 0.07797•%dd(10,10)x – 4.112
rLCPE (in cm) = 8.369•TPR20,10(10) – 4.382 d
rLCPE
CPE condition for field size
• Consider a 6 MV beam. It’s TPR20,10(10) = 0.677 • rLCPE = 8.369 0.677 – 4.382 = 12.8 mm • PTW 30013 Farmer type chamber:
• cavity length l = 23mm • cavity radius r = 3.1 mm • wall thickness twall= 0.057 g/cm2 • With ρ (PMMA) = 1.19 g/cm3, twall= 0.48mm • In the longitudinal direction, the chamber outer size will be dl = l + twall =
23.48 mm (say 23.5mm) • In the radial direction dr = 2( r + twall) = 7.2mm • As dl > dr, the largest detector size is dl
• Eq. for FWHM yields a FWHM = 2 x 12.8 + 23.5 = 49.1 mm
For a PTW 30013 chamber: FWHM ≥ 4.9 cm
Example
• Chambers must meet specifications for reference class
ionizations chambers. Table 3 in the CoP
• Refers to chamber settling time, polarity effect, leakage,
recombination correction, chamber stability, chamber material
• fmsr ≥ 6 cm x 6 cm
• Chambers listed in Table 4 meet this criteria
msr fields: selection of chambers
• Farmer type chambers: • WFF beams: Farmer type chambers listed in Table 4 meets this
criteria
• FFF beams use a chamber with a length shorter than the length of Farmer type chambers
• If you have to use a Farmer type chamber a correction for the non-uniformity of the beam profile should be used. For 6 MV beam this can be about 1.5%
msr fields: selection of chambers
• For field sizes smaller than 6 cm x 6 cm similar
analysis led to the chambers listed in Table 5
(including Gamma Knife)
• These are chambers with volumes smaller than 0.3
cc (chamber length 7 mm)
msr fields: selection of chambers
• For reference dosimetry in msr fields, you will need to determine “equivalent square msr field” sizes. For non-square field sizes, the corresponds to the field for which the phantom scatter is the same.
• Tables 15-17 tells you how to do this. This is needed to calculate TPR20,10(10) or %dd(10,10)x using Palman’s equation.
Equivalent square msr field sizes
15
Table 15 (Tables 16 & 17 are for FFF beams)
• Choice of an appropriate detector for small field dosimetry measurements depends on the parameter to be measured.
• Note: NO ideal detector exists for measurements in small fields
• Use two or three different types of suitable detectors so that redundancy in results can provide more assurance that no significant errors in dosimetry are made
Relative dosimetry: Detectors
• Assume that detectors used for large field dosimetry will not perform well in small fields
• Ion chambers: major issues are volume averaging and substantial perturbations in the absence of LCPE, signal to background ratio for small volume ionization chambers
• Below certain field sizes, volume averaging effects become unacceptably large. Below these field sizes only liquid ion chamber and solid state detectors are suitable for dosimetry, but even those exhibit substantial perturbations for the smallest field sizes
Relative dosimetry: Detectors
• Output correction factors are given as a function of
the size of the square fields. For non-square fields,
one determines a “equivalent square small field” for
which output corrections are the same
Output correction factor
• Field output correction factors are given as a
function of Collimator setting for CyberKnife and
Gamma Knife (Tables 23 and 25) and as a function of
“equivalent square” for Tomotherapy, MLC and SRS
cones for 6 and 10 MV beams in Tables 24, 26 and 27
Output correction factor
Table 23: Output correction factors for CyberKnife
Table 26: Output correction factors for 6MV in WFF and FFF beams
• For relative dosimetry ensure placement of the
detector in the center of the radiation field
Practical considerations
• Correct and incorrect orientations of detectors
for measurements of beam profile
Practical considerations
• The absorbed dose to water at the reference depth zref in water
for the fmsr field in a beam of quality as Qmsr and in the absence
of the ionization chamber is given by:
• is the chamber reading corrected for influence quantities
• is the absorbed dose to water calibration coefficient of the chamber at beam quality Qmsr
msr
msr
msr
msr
msr
msr
fQwD
fQ
fQw NMD ,,, ⋅=
Chamber calibrated specifically for the msr field
Formalism: Preferred option
• The absorbed dose to water at the reference depth zref in water for the fmsr field
in a beam of quality as Qmsr and in the absence of the ionization chamber is given
by:
• is the chamber reading corrected for influence quantities • is the absorbed dose to water calibration coefficient of the chamber at
beam quality Q0 in the ref field fref = 10x10 cm2
• is a correction factor that accounts for the differences between the
response of an ionization chamber in the field fref and beam quality Qo and the field fmsr and beam quality Qmsr
Chamber calibrated in a conventional reference field and generic values of beam quality corrections factors available
Formalism: Option b
Table 12: vs %dd(10,10)x and TPR20,10(10) for WFF beams
Table 13: vs% dd(10,10)x and TPR20,10(10) for FFF beams
Table 14: for GammaKnife
Beam quality
TPR20,10(S) %dd(10,S)x
• Equations for beam quality in non-standard reference fields
(Palmans 2012 Med Phys 39:5513)
0.55
0.60
0.65
0.70
0.75
0.80
0.85
2 4 6 8 10 12
s / cm
TP
R20
,10(
s)
(b)4 MV
10 MV8 MV
6 MV
5 MV
25 MV21 MV18 MV15 MV12 MV
Beam quality
C=(16.15 ± 0.12) x 10 -3
• Field output factor relative to reference field (ref stands here for
a conventional reference or msr field)
where is the so-called output correction factor, which
can be determined as a directly measured value, an
experimentally generic value or a Monte Carlo calculated
generic value
Field output factor
• Method 1: Field output factor relative to msr field is given by
• Method 2: Field output factor relative to msr field using
intermediate field or ‘daisy chaining’ method
where
Assumed to be unity
Field output factor
Field output correction factor
34
Field output correction factor
• Rectangular small fields with uneven in-plane and cross-plane FWHM, the equivalent square field size is given by
Sclin = √ A.B 0.7 < A/B < 1.4 • For circular small fields with FWHM radius r Sclin = r√ π = 1.77r
Equivalent square field size
Application of TRS-483
What did we measure?
• TPR20,10(10) and %dd(10)x in msr fields for a 6MV
beam based on measurements in different field sizes
in a TrueBeam STx linac
• Polarity effect in a GammaKnife Perfexion machine
• Reference and relative dosimetry (Field output
factors) in GammaKnife Perfexion, CyberKnife M6
with Incise MLC and TrueBeam STx machines
Measured …..
msr Field Size
(S)(cm2)
Measured TPR20,10 (S)
Calculated TPR20,10 (10)*
3x3 0.631 0.669 4x4 0.634 0.666 6x6 0.645 0.666 8x8 0.656 0.667
10x10 0.667 0.667
* Measured using CC13 chamber. Calculated using Palman’s equation
0.6
0.62
0.64
0.66
0.68
0.7
3x3 4x4 6x6 8x8 10x10
TPR
20,1
0
Field Size (cm2)
Calculated TPR20,10 (10)Measured TPR20,10 (S)
TPR20,10(10) for 6 MV beam
* Measured using CC13 chamber. Calculated using Palman’s equation
%dd(10,10)x for 6 MV beam
msr Field Size (cm2)
Measured %dd (S)
Calculated %dd (10,10)X *
3x3 60.48 65.79
4x4 61.71 66.15
6x6 63.80 66.65
8x8 65.34 66.76
10x10 66.54 66.54
60
62
64
66
68
70
3x3 4x4 6x6 8x8 10x10
% d
d
Field Size (cm2)
Calculated %dd (10,10)X
Measured %dd (S)
• Palman’s equation accurately calculates TPR20,10(10) or
%dd(10,10)x from measured values of of TPR20,10(s) and
%dd(10,s) for various field sizes
Conclusion on Beam quality index measurements
Polarity effect
Co-60 beam 16 mm collimator
3 Exradin A16 chamber, 1 Exradin A14 chamber, 1 PTW 31016 chamber, 2 Capintec PR05P chamber
V (volts)
Pola
rity
fact
or
0.97
0.975
0.98
0.985
0.99
0.995
1
1.005
1.01
0 100 200 300 400 500 600 700 800
A16SN100075
A16SN040907
A16SN031113
PTW31016
A14
PR05P9546
PR05P7837
Polarity effect
V (volts)
Reference dosimetry
GammaKnife® - IconTM
Chamber Serial number
Dose rate TRS-483
Dose rate TRS-398
Difference (%)
Exradin A16 040907 3.235 3.177 1.8
Exradin A16 092725 3.230 3.171 1.8
Gamma Knife (Icon)
CyberKnife M6TM InciseTM MLC
Chambers Dw(zmax)/MU TRS-483 (cGy/MU)
Dw(zmax)/MU TRS-398 (cGy/MU)
Ratio TRS-398TRS-483
Exradin A12
1.016 1.009 0.993
PTW 30013 Sl. No. 1551
1.014
1.010
0.996
PTW 30013 Sl. No. 0262
1.018
1.013
0.995
PTW 30013 Sl. No. 0343
1.017
1.012
0.995
PTW 30013 Sl. No. 0905
1.024
1.019
0.995
Avg ± sd 1.018 ± 0.4% 1.012 ± 0.4%
CyberKnife : 6X FFF
TrueBeamTM STx
Chamber-Serial #
Mcorrected (nC)
ND,w
109(Gy/C) %dd(10,10)x
Dw/MU at zmax
TPR20,10(10)
Dw/MU at zmax
Dw (%dd)x/Dw(TPR)
PTW 30013-1551
14.46
5.397
66.38
0.991
1.007
0.668
0.993
1.009
0.998
PTW 30013-0262
14.57
5.343
66.38
0.991
1.004
0.668
0.993
1.006
0.998
PTW 30013-0905
14.54
5.37
66.38
0.991
1.008
0.668
0.993
1.010
0.998
EXR A12 -020581
15.75 4.915 66.38 0.995 1.002 0.668 0.995 1.003 0.999
TRS-483 : 6X
Chamber-Serial #
Mcorrected (nC)
ND,w
109(Gy/C) %dd(10,10)x
Dw/MU at zmax
TPR20,10(10)
Dw/MU at zmax
Dw (%dd)x/Dw(TPR)
PTW 30013-1551
13.77
5.397
63.89
0.995
1.007
0.632
0.995
1.007
1.000
PTW 30013-0262
13.88
5.343
63.89
0.995
1.005
0.632
0.995
1.005
1.000
PTW 30013-0905
13.85
5.37
63.89
0.995
1.008
0.632
0.995
1.008
1.000
EXR A12 -020581
14.98 4.915 63.89 0.998 1.001 0.632 0.998 1.001 1.000
TRS-483 : 6X FFF
Chamber-Serial #
Mcorrected (-nC)
ND,w
109(Gy/C) %dd(10,10)x
Dw/MU at zmax
TPR20,10(10)
Dw/MU at zmax
Dw (%dd)x/Dw(TPR)
PTW 30013-1551
15.97
5.397
73.15
0.979
1.004
0.740
0.980
1.004
0.999
PTW 30013-0262
16.13
5.343
73.15
0.979
1.004
0.740
0.980
1.004
0.999
PTW 30013-0905
16.05
5.37
73.15
0.979
1.004
0.740
0.980
1.004
0.999
EXR A12 -020581
17.33 4.915 73.15 0.985 0.998 0.740 0.986 0.999 0.999
TRS-483 : 10X
Chamber-Serial #
Mcorrected (-nC)
ND,w
109(Gy/C) %dd(10,10)x
Dw/MU at zmax
TPR20,10(10)
Dw/MU at zmax
Dw (%dd)x/Dw(TPR)
PTW 30013-1551
15.33
5.397
71.39
0.985
1.005
0.707
0.987
1.007
0.998
PTW 30013-0262
15.48
5.343
71.39
0.985
1.005
0.707
0.987
1.007
0.998
PTW 30013-0905
15.38
5.37
71.39
0.985
1.004
0.707
0.987
1.005
0.998
EXR A12 -020581
16.60 4.915 71.39 0.991 0.997 0.707 0.992 0.998 0.999
TRS-483 : 10X FFF
Chamber-Serial #
TPR20,10(10) Dw/MU at zmax
Dw(TRS398)/ Dw(TRS483)
PTW 30013-1551
0.668
0.992
0.993
1.008
1.009
0.999
PTW 30013-0262
0.668
0.992
0.993
1.005
1.006
0.999
PTW 30013-0905
0.668
0.992
0.993
1.008
1.010
0.999
EXR A12 -020581
0.668 0.995 0.995 1.003
1.003 1.000
TRS-483 vs TRS-398 : 6X
Chamber-Serial #
TPR20,10(10) Dw/MU at zmax
Dw(TRS398)/ Dw(TRS483)
PTW 30013-1551
0.632
0.996
0.995
1.008
1.007
1.001
PTW 30013-0262
0.632
0.996
0.995
1.006
1.005
1.001
PTW 30013-0905
0.632
0.996
0.995
1.009
1.008
1.001
EXR A12 -020581
0.632 0.998 0.998 1.001
1.001 1.000
TRS-483 vs TRS-398 : 6X FFF
Chamber-Serial #
TPR20,10(10) Dw/MU at zmax
Dw(TRS398)/ Dw(TRS483)
PTW 30013-1551
0.740
0.980
0.980
1.004
1.004
1.000
PTW 30013-0262
0.740
0.980
0.980
1.004
1.004
1.000
PTW 30013-0905
0.740
0.980
0.980
1.004
1.004
1.000
EXR A12 -020581
0.740 0.986 0.986 0.999 0.999 1.000
TRS-483 vs TRS-398 : 10X
Chamber-Serial #
TPR20,10(10) Dw/MU at zmax
Dw(TRS398)/ Dw(TRS483)
PTW 30013-1551
0.707
0.987
0.987
1.006
1.007
0.999
PTW 30013-0262
0.707
0.987
0.987
1.006
1.007
0.999
PTW 30013-0905
0.707
0.980
0.980
1.005
1.005
0.999
EXR A12 -020581
0.707 0.98 0.987 0.997 0.998 0.999
TRS-483 vs TRS-398 : 10X FFF
Relative dosimetry
Field output factor
CyberKnife
CyberKnife
TrueBeam STx
61
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
Sun Nuclear EdgePTW 60017
6X
mean = 0.713 ∆uncorr = - 2%
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Unc
orre
cted
ratio
of r
eadi
ngs
Equivalent square field size, Sclin (cm)
Sun Nuclear EdgePTW 60017
6X
mean = 0.728
TrueBeam STx : 6X
62
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
Sun Nuclear EdgePTW 60017
6X (IMF)
mean = 0.715 ∆uncorr = - 2%
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Unc
orre
cted
ratio
of r
eadi
ngs
Equivalent square field size, Sclin (cm)
Sun Nuclear EdgePTW 60017
6X
mean = 0.728
TrueBeam STx : 6X
63
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
UncorrectedCorrectedCorrected (IFM)
6X, PTW60017
0.5
0.55
0.6
0.65
0.7
0.75
0 0.5 1 1.5 2
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
UncorrectedCorrectedCorrected (IF)
6X, PTW60017
mean = 0.591 ∆uncorr = - 3.6%
mean = 0.714 ∆uncorr = - 0.9%
TrueBeam STx : 6X
64
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Unc
orre
cted
ratio
of r
eadi
ngs
Equivalent square field size, Sclin (cm)
Sun Nuclear EdgePTW 60017
6X FFF
mean = 0.743
0.5
0.6
0.7
0.8
0.9
1.0
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
Sun Nuclear EdgePTW 60017
6X FFF
mean = 0.728 ∆uncorr = - 2%
TrueBeam STx : 6XFFF
65
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Unc
orre
cted
ratio
of r
eadi
ngs
Equivalent square field size, Sclin (cm)
Sun Nuclear EdgePTW 60017
6X FFF
mean = 0.743
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
Sun Nuclear EdgePTW 60017
6X FFF (IFM)
mean = 0.732 ∆uncorr = - 2%
TrueBeam STx : 6XFFF
66
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
UncorrectedCorrectedCorrected (IF)
6XFFF, PTW60017
0.5
0.55
0.6
0.65
0.7
0.75
0 0.5 1 1.5 2
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
UncorrectedCorrectedCorrected (IF)
6XFFF, PTW60017
mean = 0.607 ∆uncorr = - 3.5%
mean = 0.730 ∆uncorr = - 0.6%
TrueBeam STx : 6XFFF
67
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Unc
orre
cted
ratio
of r
eadi
ngs
Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
10X
mean = 0.692
0.5
0.6
0.7
0.8
0.9
1.0
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
Sun Nuclear EdgePTW 60017
10X
mean = 0.677 ∆uncorr = - 2%
TrueBeam STx : 10X
68
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Unc
orre
cted
ratio
of r
eadi
ngs
Equivalent square field size, Sclin (cm)
Sun Nuclear Edge
PTW 60017
10X
mean = 0.692
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
Sun Nuclear EdgePTW 60017
10X (IFM)
mean = 0.678 ∆uncorr = - 2%
TrueBeam STx : 10X
69
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
UncorrectedCorrectedCorrected (IF)
10X, PTW60017
0.5
0.55
0.6
0.65
0.7
0.75
0 0.5 1 1.5 2
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
UncorrectedCorrectedCorrected (IF)
10X, PTW60017
mean = 0.506 ∆uncorr = - 3.9%
mean = 0.675 ∆uncorr = - 1.5 %
TrueBeam STx : 10X
70
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Unc
orre
cted
ratio
of r
eadi
ngs
Equivalent square field size, Sclin (cm)
PTW 60017
10XFFF
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
PTW 60017
10X FFF
TrueBeam STx : 10X FFF
71
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Unc
orre
cted
ratio
of r
eadi
ngs
Equivalent square field size, Sclin (cm)
PTW 60017
10XFF
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
PTW 60017
10X FFF (IFM)
TrueBeam STx : 10X FFF
72
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
UncorrectedCorrectedCorrected (IF)
10XFFF, PTW60017
0.5
0.55
0.6
0.65
0.7
0.75
0 0.5 1 1.5
Fiel
d ou
tput
fact
ors
Equivalent square field size, Sclin (cm)
UncorrectedCorrectedCorrected (IF)
10XFFF, PTW60017
mean = 0.506 ∆uncorr = - 3.9%
mean = 0.675 ∆uncorr = - 1.5 %
TrueBeam STx : 10X FFF
• For the GammaKnife msr beam, differences in references dosimetry using TRS-483 and TRS-398 can be up to 2% assuming depth scaling is taken into consideration
• For linac WFF and FFF beams, the values of Dw/MU following TRS-483 are consistent within better than 1% with those obtained using TRS-398
• The small field dosimetry of certain msr (reference) and most relative (using field output factor) beams can be significantly improved when the correction factors or different detectors included in TRS-483 are appropriately incorporated into their dosimetry
Summary