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 $\varepsilon _{{\rm LET}}^{{\rm RGD}}$ is proposed.\nThe 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 $\varepsilon _{{\rm LET}}^{{\rm RGD}}$ 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 $k_{{\rm Q},{{{\rm Q}}_{0}}}^{{\rm RGD}}$ and the LET quenching effect of the RGD $k_{{\rm LET}}^{{\rm RGD}}$ . The proposed method was evaluated by measuring D w at different depths in a 200 MeV proton beam.\nFor both non-modulated and modulated proton beams, $k_{{\rm Q},{{{\rm Q}}_{0}}}^{{\rm RGD}}$ decreases rapidly where R res is less than 4 cm. The difference in $k_{{\rm Q},{{{\rm Q}}_{0}}}^{{\rm RGD}}$ 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. $\varepsilon _{{\rm LET}}^{{\rm RGD}}$ decreases rapidly at a LET range from 1 to 2 keV µm−1. In the evaluation experiments, D w using RGDs, $D_{{\rm w},{\rm Q}}^{{\rm RGD}}$ 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 $D_{{\rm w},{\rm Q}}^{{\rm RGD}}$ in a proton beam was estimated to investigate the potential of RGD postal dosimetry in proton therapy.\nThese 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