IAEA/AAPM code of practice for the dosimetry of small static fields used in external beam radiotherapy

Transcription

1 IAEA/AAPM code of practice for the dosimetry of small static fields used in external beam radiotherapy Hugo Palmans MedAustron, Wiener Neustadt, Austria and National Physical Laboratory, Teddington, UK 1

2 Acknowledgements On behalf of the authors: Hugo Palmans (Chair), Pedro Andreo, Karen Christaki-Rosser, Saiful Huq and Jan Seuntjens With thanks to Rodolfo Alfonso, Roberto Capote, Joanna Izewska, Jonas Johansson, Warren Kilby, Per Kjäll, Rock Mackie, Ahmed Meghzifene, Wolfgang Ullrich, Stan Vatnitsky, Mania Aspradakis, Gregor Bruggmoser, Hugo Bouchard, Sylvie Derreumaux, George Ding, Diego Gonzalez-Castaño, Paolo Francescon, David Followill, Faustino Gomez, Malcolm McEwen, Chester Reft, Francisco Sanchez-Doblado, Otto Sauer, Timothy Zhu All the hard working people who have published relevant data 2

3 Overview 1. Introduction 2. Physics of small field dosimetry 3. Concepts and Formalism 4. Detectors and Equipment 5. Code of practice for reference dosimetry of machine-specific reference fields 6. Code of practice for relative dosimetry of small fields Appendix I. Beam quality correction factors for reference dosimetry and their uncertainty estimates Appendix II. Field output correction factors and their uncertainty estimates 3

4 History First meeting 2006 Alfonso et al Formalism for: machine-specific reference dosimetry plan-class specific reference dosimetry small field output factors Reviewer comments 2015 Publication of CoP in

5 #papers used - SF data MLC based units 7 6 #papers used

6 1. Introduction Need - scope Brief overview Quantities and symbols - abbreviations 6

7 2. Physics of small field dosimetry Small field conditions - Loss of lateral charged particle equilibrium - Partial occlusion of the primary source - Detector size too large Definition of field size (FWHM) Spectral hardening Beam quality Detector response Energy range of interest Absorbed dose standards Current practice 7

8 3. Concepts and formalism Concepts: definition of field size: FWHM machine-specific reference field lateral charged particle equilibrium range (r LCPE ) volume averaging beam quality Formalism: msr reference dosimetry measurements in plastic phantoms determination of field output factors 8

9 3. Formalism - lateral charged particle equilibrium range rr LCPE rr LCPE cccc = TPR 20,10 (10) cccc = %dddd 10,10 X The machine-specific reference field should extend at least a distance rr LCPE beyond the outer boundaries of the reference ionization chamber 9

10 3. Formalism (Alfonso et al) / D w in machine specific reference (msr) fields Chamber calibrated specifically for the msr field ff DD msr ww,qqmsr ff = msr ff MMQQmsr NN msr DD,ww,QQmsr Chamber calibrated for the conventional reference field and generic correction factors are available ff DD msr ww,qqmsr ff = msr ff MMQQmsr NN ref ff DD,ww,QQ0 msr,ff ref kkqqmsr,qq 0 Chamber calibrated for the conventional reference field and generic correction factors not available ff DD msr ww,qqmsr ff = msr ff MMQQmsr NN ref DD,ww,QQ0 ff ref ff kkqq,qq0 kk msr,ff ref QQmsr,QQ 10

11 3. Formalism for FFF beam Identical formalism: ff DD msr ww,qqmsr ff = msr ff MMQQmsr NN ref DD,ww,QQ0 ff ref ff kkqq,qq0 kk msr,ff ref QQmsr,QQ 11

12 3. Formalism for FFF beam Identical formalism: ff DD msr ww,qqmsr ff = msr ff MMQQmsr NN ref DD,ww,QQ0 ff ref ff kkqq,qq0 kk msr,ff ref QQmsr,QQ But to clarify the calculation: ff DD msr ww,qqmmmmmm FFFFFF ff = msr ff MMQQmmmmmm FFFFFF NN ref DD,ww,QQ0 WWFFFF ff ref kkqq WWWWWW,QQ WWWWWW 0 ff ref kkqq FFFFFF,QQ WWWWWW ff msr,ff ref kkqqmmmmmm FFFFFF,QQ FFFFFF with BQI QQ FFFFFF = BQI QQ WWFFFF 12

13 3. Formalism for FFF beam Identical formalism: ff DD msr ww,qqmsr ff = msr ff MMQQmsr NN ref DD,ww,QQ0 ff ref ff kkqq,qq0 kk msr,ff ref QQmsr,QQ But to clarify the calculation: ff DD msr ww,qqmmmmmm FFFFFF ff = msr ff MMQQmmmmmm FFFFFF NN ref DD,ww,QQ0 WWFFFF ff ref kkqq WWWWWW,QQ WWWWWW 0 ff ref kkqq FFFFFF,QQ WWWWWW ff msr,ff ref kkqqmmmmmm FFFFFF,QQ FFFFFF with BQI QQ FFFFFF = BQI QQ WWFFFF ff ss ww,aaaaaa and kk rrrrrr vvvvvv QQ FFFFFF 13

14 3. Volume averaging kk vvvvvv = AA AA ww xx, yy dddddddd ww xx, yy OAr(xx, yy)dddddddd Compare with Kawachi et al 2008 kk vvvvvv = AA AA dddddddd OAr(xx, yy)dddddddd 14

15 3. Use of plastic phantoms ff DD mmmmmm ff ww,qqmmmmmm zzrrrrrr = MM mmmmmm ff rrrrrr ff mmmmmm,ff rrrrrr ww,pppppppppppppp pppppppppppppp,qqmmmmmm zzeeee,pppppppppppppp NN DD,ww,QQ0 kk QQmmmmmm,QQ kk 0 QQmmmmmm Exception for GammaKnife: kk ww,pppppppppppppp ff mmmmmm,ff rrrrrr QQmmmmmm integrated in kk QQmmmmmm,QQ 0 15

16 3. Formalism / determination of field output factors Field output factor relative to the reference field ff ΩΩ clin,ff msr QQclin,QQ msr = MM ff clin QQ clin ff kk clin,ff msr ff MM mmmmmm QQclin,QQ msr QQmmmmmm Field output factor relative to the reference field using the intermediate field method ff clin,ff msr = MM ff clin QQ clin ff kk clin,ff int ff int QQclin,QQ int ΩΩ QQclin,QQ msr MM QQint det ff MM int QQint ff int,ff msr kk ff MM mmmmmm QQint,QQ msr QQmmmmmm IC 16

17 4. Instrumentation TABLE 3. SPECIFICATIONS FOR REFERENCE-CLASS IONIZATION CHAMBERS FOR REFERENCE DOSIMETRY OF msr FIELDS. Parameter Chamber settling Leakage Polarity effect Recombination correction Specification Monitoring chamber response with accumulated dose, equilibrium should be reached in less than 5 minutes; the initial and equilibrium reading should agree within 0.5 %. Smaller than 0.1 % of the chamber reading Smaller than 0.4% of the chamber reading. The polarity energy dependence should be less than 0.3% between 60 Co and 10 MV photons. 1. The correction must be linear with dose per pulse 2. Initial recombination (the dose rate or dose-per-pulse independent part of the total charge recombination) must be below 0.2% at polarizing voltages around 300 V. 3. For pulsed beams, a plot of 1/M Q (charge reading) vs 1/V (polarizing voltage) should be linear at least for practical values of V. 4. For continuous beams, a plot of 1/M Q vs 1/V 2 should be linear, describing the effect of general recombination. The presence of initial recombination disturbs the linearity but this is normally a small effect, which may be neglected. 5. The difference in the initial recombination correction obtained with opposite polarities should be less than 0.1%. Chamber stability Change in calibration coefficient over a typical recalibration period of 217 years below 0.3 %. Same figure for long term (>5 y) stability Chamber material Wall material not exhibiting temperature and humidity effects

18 4. Instrumentation TABLE 4. CHARACTERISTICS OF CYLINDRICAL IONIZATION CHAMBERS [1] FOR REFERENCE DOSIMETRY OF msr FIELDS f msr 6 cm 6 cm. Ionization chamber type Cavity volume (cm 3 ) Cavity length (mm) Cavity radius (mm) Wall material Wall thickne ss (g cm - 2 ) Central electrode material b Waterpro of Capintec PR-06C/G Farmer C C-552 N Exradin A2 Spokas C C-552 Y Exradin A12 Farmer C C-552 Y 18

19 4. Instrumentation TABLE 7. SILICON DIODE, DIAMOND. LIQUID IONIZATION CHAMBER AND ORGANIC SCINTILLATOR DETECTORS FOR SMALL FIELD DOSIMETRY. Detector Sensitiv e volume (mm 3 ) Geometric form of sensitive area a Diameter or side length of sensitive area (mm) Thickness of sensitive volume (mm) Reference point (from flat face/tip) (mm) Shielde d IBA-PFD3G diode 0.19 disc < 0.9 Yes IBA-EFD3G diode 0.19 disc < 0.9 No IBA-SFD diode disc < 0.9 No PTW liquid ion 1.7 disc Yes chamber PTW diode b 0.03 disc Yes PTW diode b 0.03 disc No PTW diode 0.03 disc Yes PTW diode 0.03 disc No PTW diode 0.3 disc No PTW natural 1-6 variable < No diamond PTW CVD diamond disc No b Sun Nuclear Edge square Yes Detector Exradin W1 (Standard Imaging) 2.4 cylinder No 19

20 5. Reference conditions Table 9: REFERENCE CONDITIONS FOR THE DETERMINATION OF ABSORBED DOSE TO WATER IN HIGH-ENERGY PHOTON BEAMS ON CYBERKNIFE MACHINES. Influence quantity Phantom material Phantom shape and size Chamber type Measurement depth z ref Reference value or reference characteristics Water At least 30 cm x 30 cm x 30 cm Cylindrical z max Reference point of chamber on the central axis at the centre of the cavity volume Position of reference point at the measurement depth z ref of chamber SDD 80 cm Field shape and size Circular, maximum available, fixed collimator (6 cm diameter) 20

21 5. Overall correction factors for ionization chambers Table 12: DATA FOR THE CONVENTIONAL FIELD (10 cm 10 cm) FOR REFERENCE IONIZATION CHAMBERS IN LINACS WITH FLATTENING FILTER (WFF), AS A FUNCTION OF THE BEAM QUALITY INDICES TPR AND. Ion chamber TPR 20,10 (10) = dd(10,10) X = Capintec PR-06C/G Farmer Exradin A2 Spokas Exradin A12 Farmer

22 5.Equations for beam quality in nonstandard reference fields for TPR 20,10 10 = TPR 20,10 TPR 20,10 10 for %dddd 10,10 = TPR 20,10 SS +cc(10 SS) 1+cc(10 SS) = %dddd 10,10 X = %dddd 10 X %dddd 10,SS +80cc(10 SS) %dddd 10,10 = 1+cc(10 SS) TPR 20,10(s) PDD 10(s) MV 21 MV 18 MV 15 MV 12 MV 10 MV 8 MV 6 MV 5 MV MV (b) s / cm MV MV 18 MV MV 12 MV MV 8 MV 65 6 MV 60 5 MV 4 MV 22 (d) s / cm

23 5. Equivalent square msr field size 23

24 5. Other Equipment: ionization chambers and phantoms Correction for influence quantities: see IAEA TRS-398 Plastic phantoms 24

25 Ch 6 Practical implementation relative dosimetry Required equipment, detectors, phantoms Measurements of profiles and field output factors Correction factors for determination of output factors 25

26 Ch 6. Equivalent square small field size Cranmer-Sargison et al 2011 RO 109:350 Rectangular field: SS clin = AA BB Circular field: SS clin = rr ππ = 1.77rr 26

27 6. Alignment Table 22: DETECTOR ORIENTATION, WITH RESPECT TO THE BEAM CENTRAL AXIS, FOR RELATIVE DOSIMETRY IN SMALL PHOTON FIELDS. Detector type Detector s geometrical reference Lateral beam profiles Field output factors Cylindrical micro ion chamber axis parallel or perpendicular perpendicular Liquid ion chamber axis perpendicular parallel Silicon shielded diode axis parallel parallel Silicon unshielded diode axis parallel parallel Diamond detector axis parallel parallel Radiochromic Film film surface perpendicular perpendicular Scan orientations for beam profiles Three methods for alignment with field 27

28 6. Alignment 28

29 Appendix I s w,air for FFF beams 29

30 Appendix I volume averaging FFF beams ff kk rrrrrr vvvvvv QQ FFFFFF = AA AA ww xx, yy dddddddd ww xx, yy OAr(xx, yy)dddddddd 30

31 Appendix I volume averaging FFF beams ff kk rrrrrr vvvvvv QQ = %dd 10,10 XX SSSSSS 2 LL 2 31

32 ff Appendix I kk msr,ff ref QQmsr,QQ 0 for FFF beams Table 12: DATA FOR THE CONVENTIONAL FIELD (10 cm 10 cm) FOR REFERENCE IONIZATION CHAMBERS IN LINACS WITH FLATTENING FILTER (WFF), AS A FUNCTION OF THE BEAM QUALITY INDICES TPR (10) AND %dd(10,10) X. Ion chamber TPR 20,10 (10) = dd(10,10) X = Capintec PR-06C/G Farmer Exradin A2 Spokas Exradin A12 Farmer Ion chamber TPR 20,10 (10) = dd(10,10) X = Table 13: DATA FOR THE CONVENTIONAL FIELD (10 cm 10 cm) FOR REFERENCE IONIZATION CHAMBERS IN FLATTENING FILTER FREE (FFF) LINACS, AS A FUNCTION OF THE BEAM QUALITY INDICES TPR (10) AND %dd(10,10) X, AND FOR THE CYBERKNIFE AND TOMOTHERAPY MACHINES. (NOTE THAT THE CORRESPONDENCE BETWEEN TPR (10) AND %dd(10,10) X IS DIFFERENT FROM THAT OR WFF BEAMS.) Capintec PR-06C/G Farmer Exradin A2 Spokas Exradin A12 Farmer

33 Validation CyberKnife Ionization chamber ff kk msr,ff ref QQmsr Reference Francescon Araki Kawachi (2012) (2006) (2008) Exradin A (1.007) Exradin A12S (0.998) NE NE 2561/ (0.996) PTW (0.999) PTW (1.002) PTW (1.003) PTW 30006/ (0.999) Francescon (2005) (1.006) (1.001) This CoP

34 Validation TomoTherapy Ionization chamber Reference Exradin A12 Sterpin (2012) ff kk msr,ff ref QQmsr Jeraj (2005) This CoP NE (0.997) PTW 30006/ (0.997)

35 Appendix II Output correction ff factors, kk clin,ff reeee QQclin,QQ reeee, for small fields Reference detectors, perturbation-free except for volume averaging k f clin, f msr Q clin, Q msr [ sfd] = M Q clin [ ref ] kvol[ ref ]/ M Q [ ref ] msr [ sfd] / M [ sfd] M Q msr Reference detector with known output correction factors k f clin, f msr Q clin, Q msr [ sfd] = M Q clin Q clin f [ ref ] clin, f msr kq [ ref ]/ [ ref ] clin, Q M msr Q msr M [ sfd] / M [ sfd] Q clin Q msr Monte Carlo calculated output correction factors k f clin, f msr Q clin, Q msr [ sfd] = D D det, Q clin w, Q clin / D w, Q msr [ sfd] / D [ sfd] det, Q msr 35

36 ff Appendix II Uncertainty of kk clin,ff reeee QQclin,QQ reeee Two components: 1. Based on literature, generic 1% to all individual data points, except 2% for experimental data for S < 1 cm and weighted averages of interpolated data 2. Spread of data as 95% confidence limits 36

37 ff Appendix II - kk clin,ff reeee QQclin,QQ reeee diodes 37

38 ff Appendix II - kk clin,ff reeee QQclin,QQ reeee diodes 38

39 ff Appendix II - kk clin,ff reeee QQclin,QQ reeee mini and micro IC 39

40 ff Appendix II - kk clin,ff reeee QQclin,QQ reeee energy dependence 40

41 ff Appendix II - kk clin,ff reeee QQclin,QQ reeee depth dependence 41

42 Appendix II - Uncertainties Square small field size S / cm unshielded diodes & PTW microdiamon d shielded diodes mini IC micro IC PTW natural diamond PTW liquid ion chamber

43 That s all Thank you 43