Evaluation of radiophotoluminescent glass dosimeter response for therapeutic spot scanning proton beam: suggestion of linear energy transfer-based correction

Abstract

A radiophotoluminescent glass dosimeter (RGD) is used for a postal audit of a photon beam because of its various excellent characteristics. However, it has not been used for scanning proton beams because its response characteristics have not been verified. In this study, the response of RGD to scanning protons was investigated to develop a dosimetry protocol using the linear energy transfer (LET)-based correction factor. The responses of RGD to four maximum-range-energy-pattern proton beams were verified by comparing it with ionization chamber (IC) dosimetry. The LET at each measurement depth was calculated via Monte Carlo (MC) simulation. The LET correction factor ( $k_{\mathrm{LET}}^{\mathrm{RGD}}$ ) was the ratio between the uncorrected RGD dose ( $D_{\mathrm{raw}}^{\mathrm{RGD}}$ ) and the IC dose at each measurement depth. $k_{\mathrm{LET}}^{\mathrm{RGD}}$ can be represented as a function of LET using the following equation: $k_{\mathrm{LET}}^{\mathrm{RGD}}\left(\mathrm{LET}\right)=-0.035\left(\mathrm{LET}\right)+1.090$ . $D_{\mathrm{raw}}^{\mathrm{RGD}}$ showed a linear under-response with increasing LET, and the maximum dose difference between the IC dose and $D_{\mathrm{raw}}^{\mathrm{RGD}}$ was 15.2% at an LET of 6.07 keV/μm. The LET-based correction dose ( $D_{\mathrm{LET}}^{\mathrm{RGD}}$ ) conformed within 3.6% of the IC dose. The mean dose difference (±SD) of $D_{\mathrm{raw}}^{\mathrm{RGD}}$ and $D_{\mathrm{LET}}^{\mathrm{RGD}}$ was -2.5 ± 6.9% and 0.0 ± 1.6%, respectively. To achieve accurate dose verification for scanning proton beams using RGD, we derived a linear regression equation based on LET. The results show that with appropriate LET correction, RGD can be used for dose verification of scanning proton beams.

Keywords: glass dosimeter; linear energy transfer; postal audit; proton dosimetry; under-response correction.