Feasibility of a normoxic N-vinylpyrrolidone-based polymer gel (VIPET) dosimeter for three-dimensional proton beam measurements

Abstract

Background: Gel dosimeters enable three-dimensional dose measurement in x-ray and charged-particle therapies. A normoxic N-vinylpyrrolidone-based polymer gel (VIPET) dosimeter is expected to provide high-precision proton dose measurements. However, reports on the fundamental performance of VIPET gel dosimeters in proton beam measurement are limited, and the accuracy of position and dose measurements still needs to be determined.

Purpose: We evaluated the accuracy of the VIPET gel dosimeter in proton beam measurement.

Methods: Proton beams of 190 MeV were delivered at dose rates of 2 and 8 Gy/min, and N-vinylpyrrolidone-based polymer gel dosimeters containing an inorganic salt as a sensitizer (iVIPET) were irradiated with a 10 × 10 cm² field and doses of up to 30 Gy. Magnetic resonance imaging was used for imaging. Key parameters assessed included R2 - dose linearity, dose uncertainty and resolution, dose reproducibility, energy (linear energy transfer [LET]) dependence, dose rate dependence, dose uniformity, and stopping power ratio (SPR).

Results: A strong linear relationship was observed in the 0-30 Gy range. Dose uncertainties were in the range 1%-3%. Three sets of percentage depth dose measurements showed good agreement within 1.6%. Energy (LET) dependence led to measured peak doses being 23% lower than the treatment planning system (TPS) calculations. Proton energy effects in the plateau region were limited and dose rate effects were not recognized. Beam profiles in the axes parallel and perpendicular to the beam axis showed variations of approximately 1.2% and 1.3%, respectively, compared with the TPS calculations. The SPR was consistent with the TPS derived from CT values (1.03).

Conclusions: The VIPET dosimeter showed high reproducibility and uniformity in range and dose measurements of proton therapy beams without energy dependency in the plateau region or dose rate dependency in 2-8 Gy/min. The dosimeter also showed energy (LET) dependence comparable with that reported in previous studies.

Keywords: VIPET; polymer gel dosimeter; proton beam.