Global and spatial dosimetric characteristics of N-vinylpyrrolidone-based polymer gel dosimeters as a function of medium-term post-preparation and post-irradiation time

It is useful in practice to know the best time intervals between gel preparation, irradiation and imaging for optimal dosimetric response. Knowing how long after preparation and/or irradiation a gel dosimeter can still produce reliable results is also practically helpful. This study aimed to answer these two important but little-investigated questions for normoxic N-vinylpyrrolidone-based polymer gel dosimeters (VIPET) during a nine-day time period. We evaluated the global dose response, reproducibility and high-dose-gradient spatial integrity of this gel dosimetry system for up to 6 Gy dose and assessed the medium-term post-preparation and post-irradiation stability of these key properties with relatively short time intervals of 3 days. After synthesis, several batches of VIPET vials were irradiated with 6 MV photon beams and imaged on a 3 T magnetic resonance imaging (MRI) scanner in different time and dose schemes and were subsequently analysed. Statistical analysis of the experimental data showed acceptable relative dosimetric characteristics in terms of dose sensitivity, offset (R0) values, and linearity of synthesized gel over the course of a nine-day post-preparation and post-irradiation time. The results showed reasonably good reproducibility of dose sensitivity (standard error of measurement, SEM: 0.009) and R0 value (SEM: 0.023). Further, the measurements indicated satisfactory spatial stability of relative dose distribution in high-dose-gradient regions throughout the nine-day studied period (variations in beam penumbra widths and off-axis positions being typically less than 1 mm). These findings provide further evidence to the suitability of the VIPET gel for practical three-dimensional dosimetry in clinical settings such as modern radiotherapy.