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

To measure the absorbed dose to water D _w in proton beams using a 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 efficiency is proposed. The calibration coefficient in terms of D _w for RGDs (GD-302M, Asahi Techno Glass) was obtained using a ⁶⁰Co γ-ray. The SPR of water to the RGD was calculated by Monte Carlo simulation, and was investigated experimentally using a 70 MeV proton beam. For clinical usage, the residual range R _res was used as a quality index to determine the correction factor for the beam quality and the LET quenching effect of the RGD . The proposed method was evaluated by measuring D _w at different depths in a 200 MeV proton beam. For both non-modulated and modulated proton beams, decreases rapidly where R _res is less than 4 cm. The difference in between a non-modulated and a modulated proton beam is less than 0.5% for the R _res range from 0 cm to 22 cm. decreases rapidly at a LET range from 1 to 2 keV μm^-1. In the evaluation experiments, D _w using RGDs, showed good agreement with that obtained using an ionization chamber and the relative difference was within 3% where R _res was larger than 1 cm. The uncertainty budget for 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 RGDs in proton dosimetry is applicable where R _res > 1 cm and the RGD is feasible as a postal audit dosimeter for proton therapy.