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 ${\epsilon }_{LET}^{RGD}$ 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 ${\epsilon }_{LET}^{RGD}$ 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 $k_{Q,Q_0}^{RGD}$ 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, $k_{Q,Q_0}^{RGD}$ decreases rapidly where R _res is less than 4 cm. The difference in $k_{Q,Q_0}^{RGD}$ 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. ${\epsilon }_{LET}^{RGD}$ decreases rapidly at a LET range from 1 to 2 keV µm⁻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 _res was larger than 1 cm. The uncertainty budget for $D_{w,Q}^{RGD}$ 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. (paper)