Dose rate properties of NIPAM-based x-ray CT polymer gel dosimeters.

In this work we investigate radiation dose rate dependencies of N-isopropylacrylamide (NIPAM) based polymer gel dosimeters (PGDs) used in conjunction with x-ray computed tomography imaging for radiotherapy dose verification. We define four primary forms of dose rate variation: constant mean dose rate where beam on and beam off times both vary, variable mean dose rate where beam on time varies, variable mean dose rate where beam off time varies and machine dose rate (MU min(-1)). We utilize both small (20 mL) vials and large volume (1L) gel containers to identify and characterize dose rate dependence in NIPAM PGDs. Results indicate that all investigated constant and variable mean dose rates had negligible affect on PGD dose response with the exception of machine dose rates (100-600 MU min(-1)) which produced variations in dose response significantly lower than previously reported. Explanations of the reduced variability in dose response are given. It is also shown that NIPAM PGD dose response is not affected by variations in dose rate that may occur in modulated treatment deliveries. Finally, compositional changes in NIPAM PGDs are investigated as potential mitigating strategies for dose rate-dependent response variability.