Recipe Improvement and Mathematical Modelling of Polymer Gel Dosimeters

Abstract

A mathematical model for polymer gel dosimeters was extended to simulate the effects of radiation depth doses of various radiation beams on the mass of polymer formed. The influences of monomer diffusion and temperature variation were investigated and predicted by the model. Simulation results indicate that both diffusion and temperature effects are most noticeable at the depth of maximum dose. Diffusion effects are larger for steep depth-dose curves with large dose gradients, while temperature effects are larger for extensive depth-dose curves that deliver high doses of radiation to a greater depth. Based on simulation results, involving a maximum dose of 5 Gy, the amount of additional polymer formed due to diffusion is small, ranging from 0.1 % for 15 MV x-ray photons to 2.6 % for Co60 γ-radiation. This small amount of additional polymer should not cause significant problems for the accuracy of depth-dose calibration curves, particularly if the depth of maximum dose is avoided. Inaccuracies caused by temperature effects are expected to be smaller than those caused by diffusion. Experimental studies were undertaken to improve the radiation dose response using x-ray Computed Tomography (CT). A new polymer gel dosimeter recipe with enhanced dose response was achieved by using a large quantity of N-isopropyl acrylamide (NIPAM) (15 wt%) to help dissolve the N,N’-methylene bisacrylamide (Bis) crosslinker. The solubility of Bis was substantially increased, allowing for large quantities of dissolved NIPAM and Bis in the system. The new dosimeter exhibits an enhanced dose sensitivity and dose resolution for x-ray CT imaging, which holds promise for clinical applications. The dose resolution of approximately 0.1 Gy, for up to absorbed doses of 50 Gy, for the new recipe is superior to that for previous dosimeter formulations developed for x-ray CT.