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Correction of stopping power and LET quenching for radiophotoluminescent glass dosimetry in therapeutic proton beam.

Research paper by Weishan W Chang, Yusuke Y Koba, Tetsurou T Katayose, Keisuke K Yasui, Chihiro C Omachi, Masatsugu M Hariu, Hidetoshi H Saitoh

Indexed on: 06 Oct '17Published on: 06 Oct '17Published in: Physics in medicine and biology



Abstract

To measure the absorbed dose to water <i>D</i><sub>w</sub> in proton beams by radiophotoluminescent glass dosimeter (RGD), a method with the correction for the change of the mass stopping power ratio (SPR) and the linear energy transfer (LET) dependence of radiophotoluminescent (RPL) efficiency ε<sup>RGD</sup><sub>LET</sub> has been proposed. 
 The calibration coefficient in terms of <i>D</i><sub>w</sub> for RGDs (GD-302M, Asahi Techno Glass) was obtained using a <sup>60</sup>Co γ-ray. The SPR of water to RGD was calculated by Monte Carlo simulation, and <i>ε</i><sup>RGD</sup><sub><i>LET</i></sub> was investigated experimentally using a 70 MeV proton beam. For clinical usage, the residual range <i>R</i><sub>res</sub> was used as a quality index to determine the correction factor for beam quality <i>k</i><sup>RGD</sup><sub>Q, Q0</sub> and LET quenching effect of RGD <i>k</i><sup>RGD</sup><sub>LET</sub> . The proposed method was evaluated by measuring <i>D</i><sub>w</sub> at different depths in a 200 MeV proton beam. 
 For both non-modulated and modulated proton beams, <i>k</i><sup>RGD</sup><sub>Q, Q0</sub> decreases rapidly where <i>R</i><sub>res</sub> is less than 4 cm. The difference of <i>k</i><sup>RGD</sup><sub>Q, Q0</sub> between a non-modulated and a modulated proton beam is less than 0.5 % for the <i>R</i><sub>res</sub> range from 0 cm to 22 cm. <i>ε</i><sup>RGD</sup><sub><i>LET</i></sub> decreases rapidly at an LET range from 1 to 2 keV μm<sup>-1</sup>. In the evaluation experiments, <i>D</i><sub>w</sub> by RGDs <i>D</i><sup>RGD</sup><sub>w,Q</sub> showed good agreement with that by an ionization chamber and the relative difference was within 3 % where <i>R</i><sub>res</sub> is larger than 1 cm. The uncertainty budget for <i>D</i><sup>RGD</sup><sub>w,Q</sub> in a proton beam was estimated to investigate the potential of RGD postal dosimetry in proton therapy.
 These results demonstrate the feasibility of RGD dosimetry in a therapeutic proton beam and the general versatility of the proposed method. In conclusion, the proposed methodology for RGD in proton dosimetry is applicable where R<sub>res</sub> > 1 cm and RGD is feasible for being a postal audit dosimeter for proton therapy.