Modelling of polyacrylamide gel dosimeters with spatially non-uniform radiation dose distributions

Abstract

Over the past decade there has been much interest in the development of three-dimensional gel dosimeters to aid in the determination of the distribution and magnitude of absorbed dose in clinical radiation therapy. The most widely used dosimeter for verification of spatial dose distributions is the polyacrylamide gel (PAG) dosimeter. In this article, a detailed fundamental model is developed to describe the chemical and physical phenomena in this gel dosimeter, based on the radiation-induced copolymerization of acrylamide and bisacrylamide monomers in water and gelatin. The model predicts the concentrations of acrylamide, bisacrylamide, pendant double bonds (PDB), cyclized groups and cross-link units as well as temperature at different times and positions within the dosimeter, and accounts for the formation of insoluble microgels. Dosimetry experiments are simulated in which the spatial distribution of the radiation dose is non-uniform in the horizontal direction. The middle section of the dosimeter is irradiated, and the two ends are not, resulting in large concentration gradients near the edges of the irradiated zone. The resulting system of partial differential equations (PDEs) is solved numerically, and a stretching transformation is applied to ensure accurate model predictions. The model is able to simulate the effects of radiation dose, dose rate and dosimeter recipe on edge enhancement and temporal instability, two problems that have plagued the users of polymer gel dosimeters. The mechanistic scheme summarized in the model and the associated simulations provide important knowledge that should assist medical physicists in understanding existing dosimetry systems and in designing improved dosimeters in the future.