Implementation of the IAEA-AAPM Code of Practice for the dosimetry of small static fields used in external beam radiotherapy M. Saiful Huq, PhD, FAAPM, FInstP Dept. of Radiation Oncology, University of Pittsburgh Cancer Institute and UPMC Hillman Cancer Center, Pittsburgh, Pennsylvania, USA
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. Collaborators
Contributors to IAEA TRS-483 Missing: Ahmed Meghzifene and Stan Vatnitsky
TRS-483 CoP 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 N D,w,Qo or N D,w,Qmsr in a standard s laboratory s reference beam of quality Q o or Q msr. It also provides guidance for measurements of field output factors and lateral beam profiles at the measurement depth
Reference dosimetry: msr field 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, f msr Dimension of f msr field Should be as close as possible to the conventional reference field
msr fields for common radiotherapy machines f msr should extend at least a distance r LCPE beyond the outer boundaries of the reference ionization chamber FWHM 2r LCPE + d Machine type CyberKnife TomoTherapy GammaKnife BrainLab micromlc add on SRS cone add-ons 6 cm diameter fixed collimator 5 cm x 10 cm field msr field 1.6 cm or 1.8 cm diameter collimator helmet, all sources simultaneously out For example 9.8 cm x 9.8 cm or 9.6 cm x 10.4 cm The closest to a 10 cm x 10 cm equivalent square msr field achievable
msr fields: selection of chambers How were the field sizes for the msr dosimetry arrived at? Ask: What is the size restriction on an ionization chamber for msr dosimetry?
CPE conditions exist when one of the edges of the field extends at least a distance r LCPE 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: CPE condition for field size FWHM 2 r LCPE + d r LCPE r LCPE (in cm) = 8.369 TPR 20,10 (10) 4.382 d r LCPE (in cm) = 0.07797 %dd(10,10) x 4.112
Example Consider a 6 MV beam. It s TPR 20,10 (10) = 0.677 r LCPE = 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 t wall = 0.057 g/cm 2 With ρ (PMMA) = 1.19 g/cm 3, t wall = 0.48mm In the longitudinal direction, the chamber outer size will be d l = l + t wall = 23.48 mm (say 23.5mm) In the radial direction d r = 2( r + t wall ) = 7.2mm As d l > d r, the largest detector size is d l Eq. for FWHM yields a FWHM = 2 x 12.8 + 23.5 = 49.1 mm For a PTW 30013 chamber: FWHM 4.9 cm
msr fields: selection of chambers 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 f msr 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)
Equivalent square msr field sizes For reference dosimetry in msr fields, you will need to determine equivalent square msr field sizes. For nonsquare 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 TPR 20,10 (10) or %dd(10,10) x using Palman s equation.
15 Table 15 (Tables 16 & 17 are for FFF beams)
Relative dosimetry: Detectors 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
Output correction factor 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
Table 23: Output correction factors for CyberKnife
Table 26: Output correction factors for 6MV in WFF and FFF beams
Practical considerations 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
Formalism: Preferred option Chamber calibrated specifically for the msr field The absorbed dose to water at the reference depth z ref in water for the f msr field in a beam of quality as Q msr and in the absence of the ionization chamber is given by: fmsr fmsr fmsr D w, Q = M msr Q N msr,, D w Q is the chamber reading corrected for influence quantities is the absorbed dose to water calibration coefficient of the chamber at beam quality Q msr msr
The absorbed dose to water at the reference depth z ref in water for the f msr field in a beam of quality as Q msr and in the absence of the ionization chamber is given by: Formalism: Option b Chamber calibrated in a conventional reference field and generic values of beam quality corrections factors available is the chamber reading corrected for influence quantities is the absorbed dose to water calibration coefficient of the chamber at beam quality Q 0 in the ref field f ref = 10x10 cm 2 is a correction factor that accounts for the differences between the response of an ionization chamber in the field f ref and beam quality Q o and the field f msr and beam quality Q msr
Table 12: vs %dd(10,10) x and TPR 20,10 (10) for WFF beams
Table 13: vs% dd(10,10) x and TPR 20,10 (10) for FFF beams
Table 14: for GammaKnife
Beam quality TPR 20,10 (S) %dd(10,s) x
Beam quality Equations for beam quality in non-standard reference fields 0.85 TPR 20,10(s) 0.80 0.75 0.70 0.65 25 MV 21 MV 18 MV 15 MV 12 MV 10 MV 8 MV 6 MV C=(16.15 ± 0.12) x 10-3 0.60 0.55 5 MV 4 MV (b) 2 4 6 8 10 12 s / cm (Palmans 2012 Med Phys 39:5513)
Field output factor 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 correction factor
34 Field output correction factor
Equivalent square field size Rectangular small fields with uneven in-plane and cross-plane FWHM, the equivalent square field size is given by S clin = A.B 0.7 < A/B < 1.4 For circular small fields with FWHM radius r S clin = r π = 1.77r
Application of TRS-483 What did we measure?
Measured.. TPR 20,10 (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
TPR 20,10 (10) for 6 MV beam msr Field Size (S)(cm 2 ) Measured TPR 20,10 (S) Calculated TPR 20,10 (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 0.7 * Measured using CC13 chamber. Calculated using Palman s equation TPR 20,10 0.68 0.66 0.64 0.62 0.6 Calculated TPR20,10 (10) Measured TPR20,10 (S) 3x3 4x4 6x6 8x8 10x10 Field Size (cm 2 )
%dd(10,10) x for 6 MV beam msr Field Size (cm 2 ) 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 * Measured using CC13 chamber. Calculated using Palman s equation % dd 70 68 66 64 62 60 Calculated %dd (10,10)X Measured %dd (S) 3x3 4x4 6x6 8x8 10x10 Field Size (cm 2 )
Conclusion on Beam quality index measurements Palman s equation accurately calculates TPR 20,10 (10) or %dd(10,10) x from measured values of of TPR 20,10 (s) and %dd(10,s) for various field sizes
Polarity effect Co-60 beam 16 mm collimator 3 Exradin A16 chamber, 1 Exradin A14 chamber, 1 PTW 31016 chamber, 2 Capintec PR05P chamber
Polarity effect 1.01 Polarity factor 1.005 1 0.995 0.99 0.985 0.98 0.975 0.97 V (volts) A16SN100075 A16SN040907 A16SN031113 PTW31016 A14 PR05P9546 PR05P7837 0 100 200 300 400 500 600 700 800 V (volts)
Reference dosimetry
GammaKnife - IconTM
Gamma Knife (Icon) 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
CyberKnife M6 TM Incise TM MLC
CyberKnife : 6X FFF Chambers Dw(z max )/MU TRS-483 (cgy/mu) Dw(z max )/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%
TrueBeam TM STx
TRS-483 : 6X Chamb er- Serial # M correc ted (nc) N D,w 10 9 (Gy/C) %dd(10, D w /M 10) x U at z max TPR 20,10 (10) D w /M U at z max D w (%dd) x / D w (TPR) PTW 30013-1551 PTW 30013-0262 PTW 30013-0905 EXR A12-020581 14.46 5.397 66.38 0.991 1.007 0.668 0.993 1.009 0.998 14.57 5.343 66.38 0.991 1.004 0.668 0.993 1.006 0.998 14.54 5.37 66.38 0.991 1.008 0.668 0.993 1.010 0.998 15.75 4.915 66.38 0.995 1.002 0.668 0.995 1.003 0.999
TRS-483 : 6X FFF Chamb er- Serial # M correc ted (nc) N D,w 10 9 (Gy/C) %dd(10, 10)x D w /M U at z max TPR 20,10 (10) D w /M U at z max D w (%dd)x/ D w (TPR) PTW 30013-1551 PTW 30013-0262 PTW 30013-0905 EXR A12-020581 13.77 5.397 63.89 0.995 1.007 0.632 0.995 1.007 1.000 13.88 5.343 63.89 0.995 1.005 0.632 0.995 1.005 1.000 13.85 5.37 63.89 0.995 1.008 0.632 0.995 1.008 1.000 14.98 4.915 63.89 0.998 1.001 0.632 0.998 1.001 1.000
TRS-483 : 10X Chamb er- Serial # M correc ted (-nc) N D,w 10 9 (Gy/C) %dd(10, 10)x D w /M U at z max TPR 20,10 (10) D w /M U at z max D w (%dd)x/ D w (TPR) PTW 30013-1551 PTW 30013-0262 PTW 30013-0905 EXR A12-020581 15.97 5.397 73.15 0.979 1.004 0.740 0.980 1.004 0.999 16.13 5.343 73.15 0.979 1.004 0.740 0.980 1.004 0.999 16.05 5.37 73.15 0.979 1.004 0.740 0.980 1.004 0.999 17.33 4.915 73.15 0.985 0.998 0.740 0.986 0.999 0.999
TRS-483 : 10X FFF Chamb er- Serial # M correc ted (-nc) N D,w 10 9 (Gy/C) %dd(10, 10)x D w /M U at z max TPR 20,10 (10) D w /M U at z max D w (%dd)x/ D w (TPR) PTW 30013-1551 PTW 30013-0262 PTW 30013-0905 EXR A12-020581 15.33 5.397 71.39 0.985 1.005 0.707 0.987 1.007 0.998 15.48 5.343 71.39 0.985 1.005 0.707 0.987 1.007 0.998 15.38 5.37 71.39 0.985 1.004 0.707 0.987 1.005 0.998 16.60 4.915 71.39 0.991 0.997 0.707 0.992 0.998 0.999
TRS-483 vs TRS-398 : 6X Chamber- Serial # TPR 20,10 (10) D w /MU at z max D w (TRS398)/ D w (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 FFF Chamber- Serial # TPR 20,10 (10) D w /MU at z max D w (TRS398)/ D w (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 : 10X Chamber- Serial # TPR 20,10 (10) D w /MU at z max D w (TRS398)/ D w (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 FFF Chamber- Serial # TPR 20,10 (10) D w /MU at z max D w (TRS398)/ D w (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
Relative dosimetry Field output factor
CyberKnife
CyberKnife
TrueBeam STx
TrueBeam STx : 6X Uncorrected ratio of readings 1 0.9 0.8 0.7 0.6 0.5 6X mean = 0.728 Sun Nuclear Edge PTW 60017 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) Field output factors 1 0.9 0.8 0.7 0.6 0.5 6X mean = 0.713 uncorr = - 2% Sun Nuclear Edge PTW 60017 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) 61
TrueBeam STx : 6X Uncorrected ratio of readings 1 0.9 0.8 0.7 0.6 0.5 6X mean = 0.728 Sun Nuclear Edge PTW 60017 0 2 4 6 8 10 Field output factors 1 0.9 0.8 0.7 0.6 0.5 6X (IMF) mean = 0.715 uncorr = - 2% Sun Nuclear Edge PTW 60017 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 62
TrueBeam STx : 6X Field output factors 1 0.9 0.8 0.7 6X, PTW60017 Field output factors 0.75 0.7 0.65 0.6 6X, PTW60017 mean = 0.591 uncorr = - 3.6% mean = 0.714 uncorr = - 0.9% 0.6 0.5 Uncorrected Corrected Corrected (IFM) 0 2 4 6 8 10 0.55 0.5 Uncorrected Corrected Corrected (IF) 0 0.5 1 1.5 2 Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 63
TrueBeam STx : 6XFFF Uncorrected ratio of readings 1 0.9 0.8 0.7 0.6 6X FFF mean = 0.743 Sun Nuclear Edge PTW 60017 Field output factors 1.0 0.9 0.8 0.7 0.6 6X FFF mean = 0.728 uncorr = - 2% Sun Nuclear Edge PTW 60017 0.5 0 2 4 6 8 10 0.5 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 64
TrueBeam STx : 6XFFF Uncorrected ratio of readings 1 0.9 0.8 0.7 0.6 6X FFF mean = 0.743 Sun Nuclear Edge PTW 60017 Field output factors 1 0.9 0.8 0.7 0.6 6X FFF (IFM) mean = 0.732 uncorr = - 2% Sun Nuclear Edge PTW 60017 0.5 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) 0.5 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) 65
TrueBeam STx : 6XFFF Field output factors 1 0.9 0.8 0.7 6XFFF, PTW60017 Field output factors 0.75 0.7 0.65 0.6 6XFFF, PTW60017 mean = 0.607 uncorr = - 3.5% mean = 0.730 uncorr = - 0.6% 0.6 0.5 Uncorrected Corrected Corrected (IF) 0 2 4 6 8 10 0.55 0.5 Uncorrected Corrected Corrected (IF) 0 0.5 1 1.5 2 Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 66
TrueBeam STx : 10X Uncorrected ratio of readings 1 0.9 0.8 0.7 0.6 0.5 10X mean = 0.692 Sun Nuclear Edge PTW 60017 0 2 4 6 8 10 Field output factors 1.0 0.9 0.8 0.7 0.6 0.5 10X mean = 0.677 uncorr = - 2% Sun Nuclear Edge PTW 60017 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 67
TrueBeam STx : 10X Uncorrected ratio of readings 1 0.9 0.8 0.7 0.6 0.5 10X mean = 0.692 Sun Nuclear Edge PTW 60017 0 2 4 6 8 10 Field output factors 1 0.9 0.8 0.7 0.6 0.5 10X (IFM) mean = 0.678 uncorr = - 2% Sun Nuclear Edge PTW 60017 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 68
TrueBeam STx : 10X 1 10X, PTW60017 0.75 10X, PTW60017 Field output factors 0.9 0.8 0.7 Field output factors 0.7 0.65 0.6 mean = 0.675 uncorr = - 1.5 % Uncorrected Corrected Corrected (IF) 0.6 0.5 Uncorrected Corrected Corrected (IF) 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) 0.55 0.5 mean = 0.506 uncorr = - 3.9% 0 0.5 1 1.5 2 Equivalent square field size, Sclin (cm) 69
TrueBeam STx : 10X FFF Uncorrected ratio of readings 1 0.9 0.8 0.7 0.6 10XFFF PTW 60017 Field output factors 1 0.9 0.8 0.7 0.6 10X FFF PTW 60017 0.5 0 2 4 6 8 10 0.5 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 70
TrueBeam STx : 10X FFF Uncorrected ratio of readings 1 0.9 0.8 0.7 0.6 10XFF PTW 60017 Field output factors 1 0.9 0.8 0.7 0.6 10X FFF (IFM) PTW 60017 0.5 0 2 4 6 8 10 0.5 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 71
TrueBeam STx : 10X FFF 1 10XFFF, PTW60017 0.75 10XFFF, PTW60017 Field output factors 0.9 0.8 0.7 Field output factors 0.7 0.65 0.6 mean = 0.675 uncorr = - 1.5 % 0.6 0.5 Uncorrected Corrected Corrected (IF) 0 2 4 6 8 10 Equivalent square field size, Sclin (cm) 0.55 0.5 mean = 0.506 uncorr = - 3.9% Uncorrected Corrected Corrected (IF) 0 0.5 1 1.5 Equivalent square field size, Sclin (cm) 72
Summary 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