A modified polymer gel for radiotherapy dosimetry: assessment by MRI and MRS

The characteristics of a new formulation of polymer gel are assessed for MRI-based radiotherapy dosimetry. The gel, based on the first BANG gel formulation, replaces acrylamide with the less toxic monomer sodium methacrylate. The relationship between MR T2 relaxation time and radiation dose for the gel formulation was studied using spin-echo imaging. Proton magnetic resonance spectroscopy was also used to assess the gel composition as a function of dose. The effect of gel pH on the dose-response and baseline R2 was then investigated. A calibration performed on gel without pH modulation (pH = 6.6) revealed a dose-response of 0.14 s(-1) Gy(-1) within the range 0-8 Gy. The baseline R2 increases with pH above neutrality, rising from 1.2 s(-1) at pH = 5.1 to 5.0 s(-1) at pH = 10.1. The dose-response is also pH dependent, having a minimum value of 0.09 s(-1) Gy(-1) at pH = 10.1 and peaking at 0.21 s(-1) Gy(-1) at pH = 7.7. Undertaking proton spectroscopy on the gels enabled resonances associated with the monomer and co-monomer to be studied. By integrating the peaks from the respective monomers and normalizing to the signal at 0 Gy it was shown that only 50% of the methacrylate monomer was used at 10 Gy, whereas 80% of the co-monomer was used at this dose. The data indicate that this gel has a reduced toxicity and a comparable dose response to the previously reported BANG gel. In addition, the performance of the gel can be optimized by controlling the pH. MR spectroscopy revealed that the crosslinking co-monomer is consumed more readily than the monomer, which is in agreement with previous compositional studies.