Applications of Monte Carlo simulations to radiation dosimetry

Transcription

1 Applications of Monte Carlo simulations to radiation dosimetry D.W.O. Rogers Carleton Laboratory for Radiotherapy Physics. Physics Dept, Carleton University, Ottawa ICTP, Trieste, Nov 14,

2 Papers in PMB and Med Phys with Monte Carlo in title or abstract 2/64

3 Radiation dosimetry in radiotherapy primary standards air kerma, absorbed dose electron & photon beams beta-ray fields clinical dosimetry protocols dose in a water tank TG51, TG61, TG43, TRS-398 radiotherapy treatment planning dose in a (CT) patient 3/64

4 radiation dosimeters many types of radiation dosimeters for radiotherapy ion chambers- the work horse for clinical reference dosimetry and air kerma primary standards calorimeters for absorbed dose primary standards free air chambers for x-ray air kerma standards TLDs LiF diodes, MOSFETS radiographic and radiochromic films chemical (Fricke) dosimeters Monte Carlo calculations have been used to elucidate all of these. 4/64

5 Ion chambers Farmer ion chamber from John McCaffrey, NRC 5/64

6 Cavity theory: stopping-power ratios Relates dose in cavity to dose in medium. sprs are fundamental to med gas -dosimetry protocols -primary standards 6/64

7 What is (L/ρ)? A Spencer-Attix spr - stopping-power ratio 7/64

8 Dosimetry in a phantom P wall, P gr, P fl, P cel all 1% or less effects -major variation comes from spr for complete definitions of P wall etc see 8/64

9 Electron beam depth-dose curve 12 MeV 9/64

10 sprs in electron beams Ding et al, Med Phys 22(1995) /64

11 Realistic electron beam sprs BEAM code used to simulate realistic accelerator beams Ding et al Med Phys 22 (1995)489 11/64

12 Effects of realistic sprs Ding et al MP 22(1995)489 12/64

13 How to use realistic sprs David Burns noted: changing d ref simplifies everything. d ref = 0.6 R (cm) The basis of electron beam dosimetry in IAEA TRS-398 and AAPM TG-51 clinical protocols Burns et al MP 23(1996)383 13/64

14 Realistic sprs: d ref =0.6R Burns et al MP 23(1996)383 14/64

15 Photon beams: specifying beam quality NAP -nominal accelerating potential %dd(10) -percentage depth dose at 10 cm depth in a 10x10 cm 2 field on surface at SSD 100 cm %dd(10) X -the photon component of %dd(10) TG-51 (i.e., ignoring electron contamination) TPR ratio of absorbed doses at depths 20 TRS-398 and 10 cm in a water phantom, measured with a constant source-chamber distance of 100 cm and a field size of 10x10 cm 2 at the plane of the chamber 15/64

16 sprs for photon beams filled: heavily filtered open: lightly filtered Kalach and Rogers 30 (2003) /64

17 sprs for photon beams filled: heavily filtered open: lightly filtered Kalach and Rogers 30 (2003) /64

18 What happens without a flattening filter? For IMRT, flattening filter is not needed (Titt et al, Med Phys 33(2006) 3270). A single fit handles both sets of beams using %dd(10) x. Major effect is on %dd(10) x due to non-flat beams Xiong and Rogers, in prep, 2007 Based on full BEAM simulations. 18/64

19 Flattening filter free: TPR Two sets of k Q values will be needed, one for with flattening filters, one for machines without them. 19/64

20 Summary: protocol dosimetry the major quantity which varies in protocol dosimetry is the stopping power ratio hence the discussion of it but other aspects of protocols such as TG-51 and TRS-398 which are based on MC calculated values P wall for plane parallel chambers in Co-60 beams P cel for aluminium electrodes relationship between I 50 and R 50 in e- beams plus on-going research on other aspects P wall for all beams, P repl, effective point of measurement 20/64

21 Primary standards of air-kerma in Co-60 Primary standards in Co-60 beams are based on cavity ion chambers and S-A cavity theory D gas D wall /D gas D air /D wall for complete definitions see 21/64

22 How accurately can we calculate ion chamber response? Fano cavity chamber, - walls and gas the same material (assume graphite) with a density ratio of about establish kerma to graphite in a parallel 60 Co beam. Fano s theorem => no fluence correction (traditionally ignored, but in principle needed). All other K = 1.00 ie we can check our D gas calculation 22/64

23 How accurately can we calculate ion -cover of EGSnrc manual -against own cross sections -ESTEPE is max fractional step size chamber response? (cont) This is the toughest test I know for any electron-photon Monte Carlo code 23/64

24 How accurately can we calculate ion chamber response? (cont) against measured data Kawrakow & Rogers, MC2000, p135 based on data of Nilsson et al, IAEA Proceedings, /64

25 K wall : attenuation and scatter K air eqn ignores attenuation and scatter in chamber walls D gas or Monte Carlo K wall scores without / D gas with scatter and attenuation Or regenerate interacting photons & ignore scattered photons 25/64

26 K wall : non-linear extrapolation Rogers & Bielajew, PMB 35 (1990) /64

27 Some measured confirmations of MC K wall rotate the chamber in Co-60 response*k wall =response/a wall should be constant graphite walled chamber at NRC 27/64

28 Response vs angle of Mark IV If A wall is correct, R/A wall should be constant. It is, within 0.3% despite 8% variation. (residual 0.3% is a K an effect) McCaffrey et al PMB 49(2004) /64

29 PTB/OMH: cylindrical chamber radial axial axis of rotation 45 measured response vs wall thickness. Should all extrapolate to same value. Only the calculated K wall correction gave a constant response Büermann et al PMB 48 (2003) /64

30 K an : axial non-uniformity Bielajew developed an analytic theory to account for point sources not parallel beams (PMB 35(1990)501 & 517) A brute force MC calculation with a parallel beam or a point source, confirms the analytic theory. The corrections are all very small for Co-60 sources at 1 m from typical chambers 30/64

31 Revision of air-kerma standards Using EGSnrc calculated K wall and K an values, revise the reported values Rogers and Treurniet, 1999 (NRC Report PIRS-663) extending work of Bielajew and Rogers, PMB 37(1992) /64

32 Revision of air-kerma standards (cont) Note: the BIPM baseline moved up by 0.3% Monte Carlo => world s air kerma standards increased 0.8% (double stated uncertainty) Rogers & Treurniet 1999 NRC Report 32/64

33 How accurate are calculations? If we are going to use Monte Carlo calculated factors, we need to know their uncertainty How sensitive are they to: -algorithm/computer code used -cross sections -spectrum used -size of source Rogers & Kawrakow Med Phys 30 (2003)521 33/64

34 Calculated response of NRC 3C chamber 34/64

35 K wall for NRC 3C 35/64

36 (L/ρ) for different algorithms 36/64

37 K wall vs incident spectrum 37/64

38 K an vs incident spectrum 38/64

39 spr vs incident spectrum 39/64

40 K an for 3C vs source radius 40/64

41 Uncertainty estimates (%) spr K wall K K an comp Stats <0.01 < Algorithm Spectrum 0.01 < e- X-sec γ X-sec Rogers & Kawrakow Med Phys 30 (2003)521 41/64

42 Verification of cavity theory? Can Monte Carlo verify the accuracy of cavity theory? EGSnrc can calculate D gas to 0.1% (proof: Fano cavity calculations) Cavity theory assumes that photon interactions in the cavity do not occur But Ma and Nahum showed they did. PMB 36(1991)413 So does cavity theory hold for Ir-192 or lower energy photon beams? 42/64

43 Accuracy of Spencer-Attix cavity theory Another thought/computational experiment For a parallel beam incident on a stemless chamber filled with dry air spectrum SPRRZnrc DOSRZnrc CAVRZnrc CAVRZnrc EGSnrc 43/64

44 Accuracy of Spencer-Attix cavity theory Only this good because graphite and air so similar. Calculations used = 10 kev for spr. Using larger values brings value within 0.1% of unity Borg et al, Med Phys 27(2000) /64

45 The use of silicon diode detectors a common assumption is that diode detectors measure dose directly ie no spr correction etc but sprs actually change quite a bit as the beam quality changes Why don t we need to correct for this? 45/64

46 water/silicon stopping powers are not constant calculate ratio of dose in small active region of diode detector isolated from rest of detector to dose to water at same location. Use CSnrc which uses correlated sampling Wang Med Phys 34 (2007) /64

47 model of diode detector (Scanditronix EFD) McKerracher and Thwaites Radioth Oncol 79(06) /64

48 dose water/dose silicon active region Wang Med Phys 34 (2007) /64

49 effect of backscatter from rest of chip Wang Med Phys 34 (2007) /64

50 diode response at d max vs field size mostly a change in spr effect as d max changes Wang Med Phys 34 (2007) /64

51 Summary re diode detectors diodes measure dose directly within +-1% as a function of depth and beam quality in electron beams one exception - small radius electron beams the silicon backing of the active region and the epoxy play an important role in the flat response 51/64

52 P TP : the pressure-temperature correction for ion chambers PTP is constructed so t e- ion chamber independent of ρ So E dep (ρ) is proportional to the density ρ. E dep (ρ ο ) is independent of the density ρ. 52/64

53 P TP (cont) e- What happens if the electron does not cross the cavity? ion chamber independent of ρ E dep (ρ) is no longer proportional to the density ρ. Hence the standard P TP correction factor may no longer work. 53/64

54 Pressure vs. altitude 54/64

55 A4 EGSnrc Monte Carlo code NE2571 A12 NRC x- ray monitor cross-sections for DRY air of different densities calculate D cav (dose to air) standard P TP correction inherent in results PTB catalogued spectra 55/64

56 Thimble chamber calculations 56/64

57 Conclusions of P TP paper I there is a significant breakdown of the standard P TP correction for low energy photon beams basic cause: e- stopping in the cavity, not crossing magnitude of the effect depends on: mismatch of wall to air cross sections fraction of dose due to photon interactions in the cavity air a similar effect was reported in 2005 by the UW ADCL for well ion chambers for I /64

58 Experiments at NRC to demonstrate the effect with Malcolm McEwen complete BEAMnrc model to give x-ray spectrum 58/64

59 A variety of chambers studied A2 C552 aluminium A12 C552 NE2571 NE2505 A19 graphite dural, C552 Kawrakow s egs_view Calculations with cavity.cpp, using Kawrakow s C++ geometry package & interface to EGSnrc 59/64

60 Farmer-like chambers: 60 kv Closed symbols: P TP corrected measured responses open symbols: calculated responses 60/64

61 Effects of geometry details CAVRZnrc uses a cylindrical model cavity.cpp includes the conical end. These geometry differences have no effect in a Co-60 beam 61/64

62 Summary re P PT corrections measurements confirm the calculated breakdown of the P TP correction factor for low-energy x-rays EGSnrc is capable of reproducing air-kerma calibration coefficients well within 1% N K vs beam quality curves allow quantification of the size of impurity effects geometry details have some effects at these low energies although not at Co-60 impurities are important at low photon energies 62/64

63 Summary MC techniques play a fundamental role in radiation dosimetry sprs and other corrections for ion chambers used in clinical dosimetry correction factors for primary standards verification of cavity theory accuracy elucidation of detector response (eg was diode) investigation of pressure-temperature effects and much, much more TLDs, OSL, alanine,fricke, well chambers, brachytherapy dosimetry etc 63/64

64 Acknowledgements The work described here has been done in conjunction with many colleagues, grad students and research associates, without whom it wouldn t get done. the various works described involved: Iwan Kawrakow, David Burns, George Ding, Guoming Xiong, Nina Kalach, Jette Borg, Alex Bielajew, John McCaffrey, Joanne Truerniet, Lilie Wang and Dan La Russa, but many more were involved in the overall project of Monte Carlo in radiation dosimetry Support from the Canada Research Chairs program and 64/64