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 (ROD), 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 epsilon(RGD)(LET) is proposed. The calibration coefficient in terms of Dw for RGDs (GD-302M, Asahi Techno Glass) was obtained using a Co-60 gamma-ray. The SPR of water to the ROD was calculated by Monte Carlo simulation, and epsilon(RGD)(LET) 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 k(Q,Q0)(RGD) and the LET quenching effect of the RGD k(LET)(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, k(Q,Q0)(RGD) decreases rapidly where R-res is less than 4cm. The difference k(Q,Q0)(RGD) 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. EN decreases rapidly at a LET range from 1 to 2 keV mu m(-1). In the evaluation experiments, D-w using RGDs, D-w, q(RGD) showed good agreement with that obtained using an ionization chamber and the relative difference was within 3% where R, was larger than 1 cm. The uncertainty budget for D-w, q(RGD) in a proton beam was estimated to investigate the potential of ROD 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 ROD is feasible as a postal audit dosimeter for proton therapy.