An introduction to IAEA TRS-483

Transcription

1 An introduction to IAEA TRS-483 P Andreo, Professor of Medical Radiation Physics Karolinska University Hospital and Karolinska Institute, Stockholm, Sweden Journées Scientifiques de la SFPM Toulouse 2018

2 IAEA TRS-483 (2017) A Code of Practice for small MV field dosimetry IAEA-AAPM working group years P Andreo-Introduction to IAEA TRS-483 2

3 1. Introduction Contents of IAEA TRS 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 P Andreo-Introduction to IAEA TRS-483 3

4 2. Physics: When is a field small? At least one of the following conditions is fulfilled: Fundamental condition a. Loss of Lateral Charged-Particle Equilibrium (LCPE) Machine-related issues b. Partial source occlusion Detector-related issues c. Mismatch of detector vs field size => perturbation effects much larger than in broad beams IPEM Report 103 (2010) P Andreo-Introduction to IAEA TRS-483 4

5 Lateral Charged-Particle Equilibrium Range r LCPE : minimum beam radius for D=K col r LCPE (cm) = a Q b where Q is TPR 20,10 or %dd(10) X a and b are fit coeffs to MC data P Andreo-Introduction to IAEA TRS-483 5

6 2b. Partial source occlusion Broad photon beam Narrow photon beam IPEM Report 103 (2010) penumbra overlap P Andreo-Introduction to IAEA TRS-483 6

7 2c. Detector related issues Ion chambers have been the backbone of RT dosimetry 1) Not suitable in high-dose gradients or non-uniform beams 2) Constraints regarding size versus sensitivity 3) Require small fluence perturbation corrections 4) Require a region of uniform fluence around the detector Item (4) poses an additional detector-size constraint, never of concern in broad beams, but of great importance in small beams P Andreo-Introduction to IAEA TRS-483 7

8 Detector-size related problems: Volume-averaging effect Detector reading provides a signal averaged over its volume Field size < chamber Ø inner Exradin A16 diameters outer Dose relative to central axis Fluence over detector not uniform detector Gaussian beam profile FWHM = 10 mm Measured beam profile with a 5 mm long detector Distance to central axis / mm Meltsner et al. Med Phys 36 (2009 ) 339 Wuerfel Med Phys Int 1 (2013 ) 81 P Andreo-Introduction to IAEA TRS-483 8

9 2c. Detector related issues (cont.) Volume averaging is critical for ion chamber dosimetry of small fields (also in FFF beams), but Correction can be minimized if a small chamber is used and chamber-to-field edge distance is at least r LCPE MC-calculated overall perturbation factors include the averaging effect (experimental data must be corrected) Solid-state and other small detectors Small size vs sensitivity overcomes some ion chamber constraints, including most volume averaging issues Appropriate for relative dosimetry Issues arise due to detector material, design, etc sometimes require quite large correction factors P Andreo-Introduction to IAEA TRS-483 9

10 2. Physics => small field practical condition For a given beam quality Q, the distance L from the detector outer boundary to the field edge is smaller than r LCPE (Q) L d 2 r LCPE +d L To achieve CPE (not in small fields): FWHM 2 r LCPE +d P Andreo-Introduction to IAEA TRS

11 3. Concepts and formalism The basis of TRS W Kilby, P Kjäll, T R Mackie, H Palmans, K Rosser, J Seuntjens, W Ullrich and S Vatnitsky P Andreo-Introduction to IAEA TRS

12 3. Concepts TRS-483 deals with machine-specific reference field (msr) and small field dosimetry msr fields are for REFERENCE DOSIMETRY, D ref f msr is the field size closest to f ref =10x10, or the largest field available in a given machine f msr is not a small field!! (FWHM 2 r LCPE +d) D ref determination similar to TRS-389 small fields deal with RELATIVE DOSIMETRY, D rel f D rel requires field output factors, Ω clin,f msr Qclin,Q msr f requires field output correction factors,k clin,f msr Qclin,Q msr P Andreo-Introduction to IAEA TRS

13 3. Formalism for static fields f recall that k msr f ref Qmsr Q P Andreo-Introduction to IAEA TRS

14 3. Formalism for D w in msr fields (D ref ) a) Chamber calibrated specifically for the msr field f D msr w,qmsr f = msr f MQmsr N msr D,w,Qmsr b) Chamber calibrated for the conventional reference field and generic correction factors are available f D msr w,qmsr f = msr f MQmsr N ref f D,w,Q0 msr,f ref kqmsr,q 0 c) Chamber calibrated for the conventional reference field and generic correction factors not available f D msr w,qmsr f = msr f MQmsr N ref D,w,Q0 f ref f kq,q0 k msr,f ref Qmsr,Q P Andreo-Introduction to IAEA TRS

15 3. Formalism for D w in small fields (D rel ) The absorbed dose in a small field, f clin, different from the reference field, f msr, is obtained from the reference dose as D D f f f, f w, Q w, Q Q, Q clin msr clin msr clin msr clin msr where is the field output factor P Andreo-Introduction to IAEA TRS

16 3. Formalism for D w in small fields (D rel ): field output factor The concept of field output factor is re-defined as a strict dose ratio fclin fclin D clin, msr w, Q M clin Qclin clin, msr clin, msr fmsr fmsr k clin, msr f f f f Q Q Q Q Dw, Q MQ msr msr conventional OF f k clin,f msr Qclin,Q msr is the field output correction factor, i.e., corrects the ratio of detector readings for perturbations that depend on the field size P Andreo-Introduction to IAEA TRS

17 f The field output correction factor, k clin,f ref Qclin,Q ref Is the essential component for relative dosimetry Not to be confused with a beam quality factor Many values published in the last decade for different detectors Obtained with Monte Carlo and experimentally Both methods have pros and cons P Andreo-Introduction to IAEA TRS

18 f The field output correction factor, k clin,f ref Qclin,Q ref Data in TRS-483 are obtained as an statistical average of published values, MC and experimental, until ~2015 (refs list in Tables 35 and 36) Tables for different detectors and machines: 23 - CyberKnife 24 TomoTherapy 25 GammaKnife 26 Linacs 6 MV WFF y FFF, MLC or SRS cones 27 Ditto for 10 MV (max MV in TRS-483) P Andreo-Introduction to IAEA TRS

19 Field output correction 6MV Output correction factor, (a) Ionization chambers Exradin A14SL micro Shonka Exradin A16 micro IBA/Wellhöfer CC01 IBA/Wellhöfer CC04 IBA/Wellhöfer CC13/IC10/IC15 fclin fclin, fref Qclin fclin, f Qclin, Qref f Q ref clin, Q M k Q PTW Flexible PTW Semiflex PTW PinPoint PTW PinPoint 3D Equivalent square field size / cm M ref ref ref Output correction factor, Solid-state & other detectors (b) IBA PFD3G shielded diode IBA EFD3G unshielded diode IBA SFD unshielded diode PTW shielded diode PTW unshielded diode PTW shielded diode PTW unshielded diode fclin fclin, fref Qclin fclin, f Qclin, Qref f Q ref clin, Q M k Q PTW unshielded diode PTW natural diamond PTW CVD diamond PTW liquid ion chamber Sun Nuclear EDGE Detector Standard Imaging W1 plastic sct Equivalent square field size / cm M ref ref ref Note the log scale in the abscissa axis below about 2.5 cm field size Data from IAEA TRS-483 (2017) obtained as mean weighted values of MC and experimental published values P Andreo-Introduction to IAEA TRS

20 4. Detectors and equipment For reference dosimetry of msr fields Ionization chamber characteristics Phantoms For relative dosimetry of small and other (non-reference) fields General characteristics of detectors of different type (diode, diamond, LIC, etc) Phantoms P Andreo-Introduction to IAEA TRS

21 4. Detectors and equipment (msr fields) Table 3. Specifications for reference-class ionization chambers for msr field dosimetry Chamber settling Leakage and polarity Recombination correction Chamber stability Chamber wall material P Andreo-Introduction to IAEA TRS

22 4. Detectors and equipment (msr fields) Table 4. CHARACTERISTICS OF CYLINDRICAL IONIZATION CHAMBERS FOR REFERENCE DOSIMETRY OF msr FIELDS 6 cm 6 cm Ionization chamber type Cavity volume (cm 3 ) Cavity length (mm) Cavity radius (mm) Wall material Wall thickness (g cm -2 ) Central electrode material Waterproof Capintec PR-06C/G Farmer C C-552 N Exradin A2 Spokas C C-552 Y Exradin A12 Farmer C C-552 Y Table 5. CHARACTERISTICS OF CYLINDRICAL IONIZATION CHAMBERS FOR REFERENCE DOSIMETRY OF msr FIELDS < 6 cm 6 cm P Andreo-Introduction to IAEA TRS

23 4. Detectors and equipment (small fields) TABLE 7. SILICON DIODE, DIAMOND, LIQUID ION CHAMBER AND ORGANIC SCINTILLATOR DETECTORS FOR SMALL FIELD DOSIMETRY Detector Sensitive volume (mm 3 ) Geometric form of sensitive area Diameter or side length of sensitive area (mm) Thickness of sensitive volume (mm) Reference point (from flat face/tip) (mm) Shielded 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 chamber 1.7 disc Yes 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 diamond 1-6 variable < No PTW CVD diamond disc No Sun Nuclear Edge Detector square Yes Exradin W1 (Standard Imaging) 2.4 cylinder No P Andreo-Introduction to IAEA TRS

24 5. CoP for D ref of msr fields Dosimetry equipment Ionization chambers => condition on r LCPE : FWHM 2 r LCPE + d Reference conditions Linacs conv, CyberKnife, TomoTherapy, GammaKnife Determination of D w Application of the formalism f Data for k ref f Q (WFF) and k msr,f ref Qmsr (FFF). Q 0 = 60 Co used Equivalent square field (BJR-25; new for FFF and CyberKnife) Beam quality determination in non-standard reference fields Measurements in plastic phantoms (if more convenient) Corrections for influence quantities Cross-calibration in the msr field P Andreo-Introduction to IAEA TRS

25 5. Reference conditions (example) 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 Reference value or reference characteristics Water At least 30 cm x 30 cm x 30 cm Cylindrical Measurement depth z ref 10 g/cm 2 Reference point of chamber Position of reference point of chamber SDD Field shape and size On the CAX, at the centre of the cavity volume At the measurement depth z ref 80 cm Circular, maximum available, fixed collimator (6 cm diameter) P Andreo-Introduction to IAEA TRS

26 f 5. Factors k ref f Q and msr,f ref kqmsr WFF and FFF beams for TABLE 12: DATA FOR THE CONVENTIONAL FIELD f ref (10 cm 10 cm) FOR REFERENCE CHAMBERS IN WFF LINACS Ion chamber TPR 20,10 (10) = dd(10,10) X = Capintec PR-06C/G Farmer Exradin A2 Spokas Exradin A12 Farmer TABLE 13: DATA FOR THE CONVENTIONAL FIELD f ref (10 cm 10 cm) FOR REFERENCE CHAMBERS IN FFF LINACS Ion chamber TPR 20,10 (10) = dd(10,10) X = Capintec PR-06C/G Farmer Exradin A2 Spokas Exradin A12 Farmer P Andreo-Introduction to IAEA TRS

27 5. Beam quality in non-standard reference square fields (S) To get TPR 20,10 10 TPR 20,10 from TPR 20,10 S TPR 20,10 10 = TPR 20,10 S +c(10 S) 1+c(10 S) c= , 4 S 12 TPR 20,10(s) MV 21 MV 18 MV 15 MV 12 MV 10 MV 8 MV 6 MV 5 MV 4 MV s / cm (b) MV To get %dd 10,10 %dd 10 X from %dd 10, S %dd 10,10 = %dd 10,S +80c(10 S) 1+c(10 S) c= , 4 S 12 PDD 10(s) MV 18 MV 15 MV 12 MV 10 MV 8 MV 6 MV 5 MV 4 MV s / cm (d) P Andreo-Introduction to IAEA TRS

28 6. CoP for D rel in small fields Equipment: detectors and phantoms Detector alignment Measurement of beam profiles at z ref Determination of D rel Reference conditions Equivalent square small field size Determination of field output factors Tables of field output correction factors P Andreo-Introduction to IAEA TRS

29 6. Detector 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 Emphasize detector alignment with field CAX for measuring beam profiles P Andreo-Introduction to IAEA TRS

30 6. Detector orientation for scanning <- Ionization chamber Diode -> P Andreo-Introduction to IAEA TRS

31 6. Equivalent square small field size Rule based on fields having equal area (Cranmer-Sargison et al 2011) Rectangular fields (A B): S clin = A B Circular fields (r): S clin = r π = 1.77r P Andreo-Introduction to IAEA TRS

32 Appendix Beam quality correction factors for reference dosimetry and uncertainty estimates Procedures used in TRS-483 to obtain correction factors for WFF and FFF beams WFF beams Practically the k Q factors in TRS-398 Equivalent square fields details FFF beams Stopping powers Volume averaging (details and examples) Equivalent square fields details Similar uncertainties (~1%) in both beam types P Andreo-Introduction to IAEA TRS I

33 Appendix I Volume averaging in FFF beams k vol f ref FFF Q A A w( x, y) dxdy w( x, y) OAr( xy, ) dxdy OAR w(x,y) Negligible differences regarding the model used to describe w(x,y). Model (A) adopted P Andreo-Introduction to IAEA TRS

34 Appendix I Volume averaging in FFF beams Reciprocal of the correction factor k vol 6 MV 10 MV fref kvol TPR 20,10(10) k vol Q f Q ref SDD L ( CyberKnife) L ~ length of a Farmer ch Table 32 provides generic values of k vol (TPR) for different chambers/machines L P Andreo-Introduction to IAEA TRS

35 Appendix II f clin,f ref Field output correction factors k Qclin,Q ref for small fields and uncertainty estimates Procedures used in TRS-483 to derive its data Types of published experimental and MC data How mean values of the data were derived and their uncertainty estimated (95% c.l.) List of data used (35 publications, up to 2015) Graphs of data for ~30 different detectors (uncertainties given in Table 37) P Andreo-Introduction to IAEA TRS

36 f Appendix II Uncertainties in k clin,f msr Qclin,Q msr Table 37 Equivalent square field S / cm unshielded diodes & PTW microdiamond shielded diodes mini IC micro IC PTW natural diamond PTW liquid ion chamber P Andreo-Introduction to IAEA TRS

37 f Appendix II - k clin,f ref Qclin,Q ref diodes The horizontal line sets the limits ( ) within which correction factors are recommended P Andreo-Introduction to IAEA TRS

38 f Appendix II - k clin,f ref Qclin,Q ref micro chambers P Andreo-Introduction to IAEA TRS

39 f Appendix II - k clin,f ref Qclin,Q ref natural and micro diamond P Andreo-Introduction to IAEA TRS

40 Take home messages Physics of small field dosimetry can be complex msr and small fields should not be confused Keep track of the different k (beam quality vs output correction factors) Although relative dosimetry is conceptually simple, perturbation effects impact considerably field output factors Influence of detector design can be significant (e.g., silicon diodes) Comparing different detectors is best option for small field dosimetry P Andreo-Introduction to IAEA TRS

41 Acknowledgements Material for this presentation has been contributed by, or developed in collaboration with, different colleagues, particularly the members of the IAEA-AAPM Working Group* on ``Small and Non-standard Field Dosimetry". Special thanks to the co-authors of IAEA TRS-483 (underlined below). (*) R Alfonso, P Andreo, R Capote, K Christaki, S Huq, J Izewska, J Johansson, W Kilby, T R Mackie, A Meghzifene, H Palmans (Chair), J Seuntjens and W Ullrich P Andreo-Introduction to IAEA TRS

42 P Andreo-Introduction to IAEA TRS

43 SUMMARY ADDENDUM Determination of correction factors and uncertainty estimates in IAEA TRS-483 Appendices I and II

44 Appendix I Beam quality correction factors for reference dosimetry and uncertainty estimates f Details on the beam quality factors, k ref f Q and msr,f ref kqmsr, for reference dosimetry in linacs with WFF and FFF beams, including CyberKnife and TomoTherapy, and Gamma Knife 60 Co (ch 5) f k ref f Q ( k ref Q,Q0 with Q 0 = 60 Co and f ref = 10 cm 10 cm) from k Q in TRS-398 and TG-51 Table 12 For FFF beams, determine k vol, s w,air and eq. square field (using refs) new values Table 13 f k msr,f ref f Qmsr for Gamma Knife,( msr,f ref kqmsr,q 0 with Q0 = 60 Co) taken from refs Table 14 P Andreo-Introduction to IAEA TRS

45 Appendix I. Uncertainties of f msr,f ref k Q f ref and k Qmsr u c (k Q ) between 1% (TRS-398, conservative) and 0.5% (TG-51 Upd, optimistic) Equiv between TPR 20,10 (10) and %dd(10,10) x u c = 0.2% SPRs s w,air in FFF beams u c = 0.15% k vol in FFF beams u c = 0.2% Other factors assumed identical in WFF y FFF u c = 0.15% f Estimate generic value for u c (k ref f Q ) and u c (k msr,f ref Qmsr ) is 1%, conservative (1.06% TRS-398 and 0.6% TG-51 Upd) P Andreo-Introduction to IAEA TRS

46 Appendix II Field output correction factors for small fields and uncertainty estimates f Details on the correction factors k clin,f msr Qclin,Q msr for different detectors Experimental and MC data from 35 refs Three types of data, obtained with: 1. Perturbation-free detectors except for k vol (alanine, scintillation, radiochromic, etc) 2. Reference detectors with known correction factors 3. Correction factors calculated with Monte Carlo Re-adjusted data: field size corrected for divergence and equiv square (area) to correspond to f ref = 10 cm 10 cm at z ref =10 cm P Andreo-Introduction to IAEA TRS

47 Appendix II. Uncertainties associated to f clin,f msr published values of k Qclin,Q msr Publications are unclear when they provide (if they do!) uncertainities. Based on the refs, a type B uncertainty (u B1 ) is associated to all datasets according to: 1% in MC data for all field sizes 1% in experimental data for field sizes > 1 cm 1 cm 2% in experimental data for field sizes 1 cm 1 cm Estimations given in refs are not used to avoid biasing using u B1 common for all datasets of each type P Andreo-Introduction to IAEA TRS

48 f Appendix II. Values of k clin,f msr Qclin,Q msr For each detector, the weighted mean value (with the u B1 of each data point, exp or MC) of all the data for all field sizes S, is obtained fitting the function: f k clin,f msr Qclin,Q msr S = 1 + d e10 a b + c (S 10) 1 + d e S a b Coeff d can only take the values d=+1 or d=-1 f The function forces a value k clin,f msr Qclin,Q msr =1 for the reference field of 10 cm 10 cm Data outside the 99% confidence interval predicted by the fit are filtered, and the fit is re-done P Andreo-Introduction to IAEA TRS

49 Appendix II. Uncertainty of the calculated f clin,f msr (fitted) weighted mean values k Qclin,Q msr For each detector and field size, an overall u B for all the data (exp and MC) is determined from the limiting values ±L, assumed to correspond to the 95% c.l. of a Gaussian u B = L/2 This is a conservative compromise between rectangular, u B = L/ 3, and triangular, u B = L/ 6, distributions for the las constraints discussed for the exp and MC data The combined standard uncertainty is obtained by quadrature of the u B1 and the generic u B, being dominated by the latter (aprox one order of magnitude larger than u B1 ) Table 37 gives the combined standard uncertainty (relative, %) for different detectors and field sizes P Andreo-Introduction to IAEA TRS

50 Appendix II Graphs of the f clin,f msr uncertainties of k Qclin,Q msr Six figures (figs ), a total of 25 plots Beams of 6 MV, z ref = 10 cm and f ref = 10 cm 10 cm Distintion made between MC and experimental values The thick lines and uncertainty bars are the weighted mean values and their expanded uncertainty with k = 2 (95% c.l.) The horizontal line defines the limits ( ) for which correction factors are recommended (±5%) Data marked with an arrow are excluded after the 1st fit, as they differ by more tan 3s of the mean value. A second fit is done. P Andreo-Introduction to IAEA TRS