Absorbed Dose and Air Kerma Primary Standards

Transcription

1 Absorbed Dose and Air Kerma Primary Standards Paris, 9-11 May, 2007 Procedure for absorbed dose to water determination in high energy photon and electron beams by ferrous sulphate dosimeter at M. Pimpinella, A.S. Guerra, S. La Civita and R.F. Laitano Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, ENEA-CR Casaccia, Roma (Italy)

2 Overview -Characteristics of the ferrous sulphate transfer dosimeter -Monte Carlo calculation of correction factors for the dosimeter non-water equivalence -Results for the correction factors in high energy photon and electron beams -Uncertainty budget on D w

3 ferrous sulphate dosimeter - Ferrous sulphate solution: 0.4 mol/l of H 2 SO 4, 10-3 mol/l of Fe(NH 4 ) 2 (SO 4 ) 2 and 10-3 mol/l of NaCl - Small sealed glass ampoule: only 7.8 mm diameter, 24 mm height, wall thickness 0.5 mm, volume of the dosimetric solution about 1.1 cm 3 flame sealed vertical beams solution level

4 Calibration of clinical beams -The ferrous sulphate dosimeters are used for in-situ calibrations of clinical beams in terms of D w (esp. electron beams with E<10 MeV) -The ferrous sulphate dosimetry is particularly useful for calibration of electron beams with high dose per pulse (> 10 mgy per pulse) as those produced by some linear accelerators used in Italy for intraoperative (IORT) radiotherapy (NOVAC7, Liac) -A set of 27 dosimeters is mailed to the radiotherapy center. The dosimeters are irradiated in a water phantom and sent back to for readings -The returns to the customer a calibration certificate reporting the absorbed dose to water delivered to each dosimeter

5 Absorbance readings -Varian Cary 400 UV-VIS spectrophotometer -Absorbance reading at wavelength of 304 nm with bandwidth of 1 nm - Sample compartment temperature stable within ± 0.1 C -Varian quartz micro cells of about 80 μl (4 mm width and 2 mm high) -Typical reproducibility of the absorbance readings: 0.2%

6 Dosimeter calibration at Co-60 gamma rays -Each new batch of ferrous sulphate solution is calibrated against the absorbed dose to water primary standard for the 60 Co gamma radiation 0.40 ΔA (ODU) y = E-03x E-05 R 2 = E-01 Dosimeter calibration coefficient at Co-60 gamma rays N w = D w / ΔA Dw (Gy) N w long-tem reproducibility: 0.2%

7 Absorbed dose measurements at the quality Q -D w determination in high energy photon and electron beams [D w ] Q = [ΔA] Q N w F Q where the correction factor F Q takes into account the energy dependence of the calibration coefficient [ PFeSO ] [ P ] [ Pwall ] [ P ] [ G ] Co [ G ] Q 4 Q Q 60 FQ = FeSO 4 Co 60 wall Co 60

8 The correction factor F Q FQ = [ PFeSO ] [ P ] FeSO4 [ Pwall ] [ P ] wall Co 60 [ G] Co [ G] Q 4 Q Q 60 Co 60 P FeSO 4 converts the absorbed dose to ferrous sulphate solution in absorbed dose to water (it includes the ratio of the interaction coefficients and the fluence perturbation due to the replacement of water by ferrous sulphate solution) P wall takes into accout the perturbation effects due to the dosimeter wall G is the radiation chemical yield of ferric ions

9 Monte Carlo determination of the correction factors P FeSO4 and P wall beam direction beam direction beam direction D w water wall D FeSO4 pyrex wall D FeSO4,pyrex water water water P FeSO4 = D w / / D FeSO4 P wall = D FeSO4 / D FeSO4, pyrex D w,d and FeSO D 4 FeSO 4,pyrex were obtained scoring the energy deposited by the radiation beam in the volume of interest

10 Monte Carlo simulation water phantom z y incident beam Simulation based on the EGSnrc/DOSRZnrc code ECUT, AE = MeV PCUT, AP = MeV -Electron beams: energy from 3 MeV to 20 MeV scoring region at the depth of maximum dose; realistic input spectra (Varian Clinac, Philips SL75 and Therac AECL 20 - data from NRC Report PIRS-439-; Hitesys NOVAC7 - beam simulation by BEAMnrc code) -Photon beams: from Co-60 to 24 MV scoring region at the depth of 5 cm or 10 cm in water; input spectrum of the Co-60 beam input spectra from Mohan et al (1985)

11 Results: correction factors P FeSO4 and P wall at d max in electron beams 1.04 P Q = P FeSO4 P wall PQ monoenergetic electrons Hitesys NOVAC7 Varian Clinac 2100C Philips SL75 20 Therac AECL R 50 (g cm -2 ) R 50 > 2.4 g cm -2 : weak energy dependence; 1.000< P Q <1.006 R 50 < 2.4 g cm -2 : strong energy dependence; 0.998< P Q <1.028 Remark: ΔP Q up to 1% for electron beams with similar R 50 but different spectral distribution

12 Correction factors P FeSO4 and P wall as function of depth (electron beam, R 50 = 2.1 g cm -2 ) correction factor P Q P wall P FeSO d max R z (g cm -2 ) z < d max : z > d max : weak P Q dependence on the depth; the variation of P Q is about 0.3% per mm strong P wall dependence on the depth; P wall = at depth R 50 ; P FeSO4 increases from to 1.013

13 Correction factors P FeSO4 and P wall in photon beams P Q P wall correction factor P FeSO TPR 20,10 -P FeSO4 approximately constant and equal to ± variation of P wall : < P wall < P Q decreases from (Co-60) to (TPR 20,10 = 0.80)

14 Uncertainty on D w Uncertainty source relative uncertainty 1σ (%) Type A Type B N w determination 0.7 G energy independence 0.5 Storage effect 0.2 ΔA measurement T r correction 0.2 T irr correction 0.1 F Q factor quadratic sum combined uncertainty 1.1

15 Conclusions -The correction factors for the ferrous sulphate dosimeter have been determined electron beams: 1.2 g cm -2 < R 50 < 8.5 g cm -2 at d max photon beams: < TPR 20/10 < 0.8 at 5 cm or 10 cm -In these ranges of qualities the combined standard uncertainty on D w measurements is 1.1% -The ferrous sulphate dosimetry is currently used for clinical beam calibrations in terms of D w as well for research activity as a method independent of dose per pulse