A real-world assessment of the robustness of gel dosimetry

Abstract

The purpose of this multi-institutional study was to evaluate the robustness of a transported gel dosimeter. Six vials were filled with a normoxic N-vinylpyrrolidone-based polymer gel dosimeter containing inorganic salt as a sensitizer (iVIPET), manufactured by Triangle Products (Kashiwa, Japan). Three vials were sent to the home institution (National Cancer Center Hospital East, Chiba, Japan) and three to an external institution. At each institution, vials received a dose of 0, 5, or 10 Gy using a 10 MV X-ray beam. After irradiation, the gel in the vial was imaged using a 3 T MRI scanner, and an R2 (spin–spin relaxation rate) map was created. The dose–R2 relationship was then analyzed using linear regression and the dose uncertainty computed. Thirty-four institutions participated in the study from July 2020 to July 2021. At the home institution, dose uncertainties were constant throughout the year (95% confidence interval [mean ± 1.96σ]: 3.3 ± 2.1% at 5 Gy and 4.0 ± 2.5% at 10 Gy). The ratio of the dose uncertainties measured at the external institutions and home institution was 1.1 ± 0.4 at 5 Gy (r = 0.88) and 1.0 ± 0.3 at 10 Gy (r = 0.81). Dose uncertainties were approximately 4% within 4 d of transportation but exceeded 10% when the gels were irradiated 7 d after fabrication. Temperature variations during transportation resulted in negligible uncertainties in absorbed dose; however, extended transportation times may increase dose uncertainty. Thus, polymer gel dosimeters should be used within 2–4 d of fabrication.