Implementation of the IAEA-AAPM Code of Practice for the dosimetry of small static fields used in external beam radiotherapy

Transcription

1 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

2 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

3 Contributors to IAEA TRS-483 Missing: Ahmed Meghzifene and Stan Vatnitsky

4 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

5 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

6 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

7 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

8 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?

9 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) = TPR 20,10 (10) d r LCPE (in cm) = %dd(10,10) x 4.112

10 Example Consider a 6 MV beam. It s TPR 20,10 (10) = r LCPE = = 12.8 mm PTW Farmer type chamber: cavity length l = 23mm cavity radius r = 3.1 mm wall thickness t wall = 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 = 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 = 49.1 mm For a PTW chamber: FWHM 4.9 cm

11 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

12 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%

13 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)

14 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 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 15 Table 15 (Tables 16 & 17 are for FFF beams)

16 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

17 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

18 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

19 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

20 Table 23: Output correction factors for CyberKnife

21 Table 26: Output correction factors for 6MV in WFF and FFF beams

22 Practical considerations For relative dosimetry ensure placement of the detector in the center of the radiation field

23 Practical considerations Correct and incorrect orientations of detectors for measurements of beam profile

24 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

25 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

26 Table 12: vs %dd(10,10) x and TPR 20,10 (10) for WFF beams

27 Table 13: vs% dd(10,10) x and TPR 20,10 (10) for FFF beams

28 Table 14: for GammaKnife

29 Beam quality TPR 20,10 (S) %dd(10,s) x

30 Beam quality Equations for beam quality in non-standard reference fields 0.85 TPR 20,10(s) MV 21 MV 18 MV 15 MV 12 MV 10 MV 8 MV 6 MV C=(16.15 ± 0.12) x MV 4 MV (b) s / cm (Palmans 2012 Med Phys 39:5513)

31 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

32 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

33 Field output correction factor

34 34 Field output correction factor

35 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

36 Application of TRS-483 What did we measure?

37 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

38 TPR 20,10 (10) for 6 MV beam msr Field Size (S)(cm 2 ) Measured TPR 20,10 (S) Calculated TPR 20,10 (10)* 3x x x x x * Measured using CC13 chamber. Calculated using Palman s equation TPR 20, Calculated TPR20,10 (10) Measured TPR20,10 (S) 3x3 4x4 6x6 8x8 10x10 Field Size (cm 2 )

39 %dd(10,10) x for 6 MV beam msr Field Size (cm 2 ) Measured %dd (S) Calculated %dd (10,10) X * 3x x x x x * Measured using CC13 chamber. Calculated using Palman s equation % dd Calculated %dd (10,10)X Measured %dd (S) 3x3 4x4 6x6 8x8 10x10 Field Size (cm 2 )

40 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

41 Polarity effect Co-60 beam 16 mm collimator 3 Exradin A16 chamber, 1 Exradin A14 chamber, 1 PTW chamber, 2 Capintec PR05P chamber

42 Polarity effect 1.01 Polarity factor V (volts) A16SN A16SN A16SN PTW31016 A14 PR05P9546 PR05P V (volts)

43 Reference dosimetry

44 GammaKnife - IconTM

45 Gamma Knife (Icon) Chamber Serial number Dose rate TRS-483 Dose rate TRS-398 Difference (%) Exradin A Exradin A

46 CyberKnife M6 TM Incise TM MLC

47 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 A PTW Sl. No PTW Sl. No PTW Sl. No PTW Sl. No Avg ± sd ± 0.4% ± 0.4%

48 TrueBeam TM STx

49 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 PTW PTW EXR A

50 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 PTW PTW EXR A

51 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 PTW PTW EXR A

52 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 PTW PTW EXR A

53 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 PTW PTW EXR A

54 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 PTW PTW EXR A

55 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 PTW PTW EXR A

56 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 PTW PTW EXR A

57 Relative dosimetry Field output factor

58 CyberKnife

59 CyberKnife

60 TrueBeam STx

61 TrueBeam STx : 6X Uncorrected ratio of readings X mean = Sun Nuclear Edge PTW Equivalent square field size, Sclin (cm) Field output factors X mean = uncorr = - 2% Sun Nuclear Edge PTW Equivalent square field size, Sclin (cm) 61

62 TrueBeam STx : 6X Uncorrected ratio of readings X mean = Sun Nuclear Edge PTW Field output factors X (IMF) mean = uncorr = - 2% Sun Nuclear Edge PTW Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 62

63 TrueBeam STx : 6X Field output factors X, PTW60017 Field output factors X, PTW60017 mean = uncorr = - 3.6% mean = uncorr = - 0.9% Uncorrected Corrected Corrected (IFM) Uncorrected Corrected Corrected (IF) Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 63

64 TrueBeam STx : 6XFFF Uncorrected ratio of readings X FFF mean = Sun Nuclear Edge PTW Field output factors X FFF mean = uncorr = - 2% Sun Nuclear Edge PTW Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 64

65 TrueBeam STx : 6XFFF Uncorrected ratio of readings X FFF mean = Sun Nuclear Edge PTW Field output factors X FFF (IFM) mean = uncorr = - 2% Sun Nuclear Edge PTW Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 65

66 TrueBeam STx : 6XFFF Field output factors XFFF, PTW60017 Field output factors XFFF, PTW60017 mean = uncorr = - 3.5% mean = uncorr = - 0.6% Uncorrected Corrected Corrected (IF) Uncorrected Corrected Corrected (IF) Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 66

67 TrueBeam STx : 10X Uncorrected ratio of readings X mean = Sun Nuclear Edge PTW Field output factors X mean = uncorr = - 2% Sun Nuclear Edge PTW Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 67

68 TrueBeam STx : 10X Uncorrected ratio of readings X mean = Sun Nuclear Edge PTW Field output factors X (IFM) mean = uncorr = - 2% Sun Nuclear Edge PTW Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 68

69 TrueBeam STx : 10X 1 10X, PTW X, PTW60017 Field output factors Field output factors mean = uncorr = % Uncorrected Corrected Corrected (IF) Uncorrected Corrected Corrected (IF) Equivalent square field size, Sclin (cm) mean = uncorr = - 3.9% Equivalent square field size, Sclin (cm) 69

70 TrueBeam STx : 10X FFF Uncorrected ratio of readings XFFF PTW Field output factors X FFF PTW Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 70

71 TrueBeam STx : 10X FFF Uncorrected ratio of readings XFF PTW Field output factors X FFF (IFM) PTW Equivalent square field size, Sclin (cm) Equivalent square field size, Sclin (cm) 71

72 TrueBeam STx : 10X FFF 1 10XFFF, PTW XFFF, PTW60017 Field output factors Field output factors mean = uncorr = % Uncorrected Corrected Corrected (IF) Equivalent square field size, Sclin (cm) mean = uncorr = - 3.9% Uncorrected Corrected Corrected (IF) Equivalent square field size, Sclin (cm) 72

73 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