Author: Yoshihiko Hoshino, Kenji Hotta, Taeko Matsuura, Ai Nakaoka, Hidenobu Tachibana 👨🔬
Affiliation: Hokkaido University, Department of Radiology, Gunma University Hospital, Radiation Safety and Quality Assurance Division, National Cancer Center Hospital East 🌍
Purpose: Gel dosimeters enable three-dimensional dose measurements, but no reports have evaluated the fundamental performance of VIPER-type gel dosimeters in proton beam measurements. Therefore, we evaluated the fundamental performance of the iVIPET gel dosimeter in proton beam measurements.
Methods: A normoxic N-vinylpyrrolidone-based polymer gel (VIPET) dosimeter enclosed in a glass vial was placed at a depth of 5 cm in a water-equivalent cuboid phantom. Using a 190 MeV mono-energy proton beam, the phantom was irradiated at doses of 0-30 Gy, and the dose linearity, dose uncertainty, and dose resolution were determined. Three gel dosimeters were irradiated with 5 Gy to assess dose reproducibility, and the percentage depth dose (PDD) was measured. Energy dependency was evaluated by comparing the PDDs at 150 and 190 MeV, and dose rate dependency was examined at dose rates of 2 and 8 Gy/min. Linear energy transfer (LET) dependency was assessed by comparing the PDD calculation. The stopping power ratio (SPR) was calculated from the range shift. Dose uniformity was evaluated by comparing the measured beam profile of a spread-out Bragg peak irradiated from opposing lateral fields measured in a cube container with the profile on the TPS.
Results: The PDD of the three dosimeters showed consistent ranges, confirming dose reproducibility. No energy or dose rate dependency was observed. LET dependency was identified, with measured PDD peak doses approximately 23% lower than TPS predictions. Dose uncertainty and resolution were 1-3% and 0.18-0.34 Gy, respectively. The SPR was equivalent to the value estimated from CT numbers, and the difference between measured and TPS beam profiles was approximately 1%.
Conclusion: Despite showing LET dependency, the VIPET gel dosimeter demonstrated no energy or dose rate dependency in proton beam measurements. It achieved high dose reproducibility, linearity, and uniformity while maintaining water equivalence, enabling high-precision measurements.