Abstract
There have been developments in radiation therapy treatment techniques, which lead to an increase in the complexity of these treatments. The aim is to deliver highly conformal three-dimensional (3D) dose distributions, such as stereotactic radiosurgery (SRS). Polymer gel dosimetry offers three-dimensional (3D) dosimetry techniques for dose verification of dose distributions. N-isopropyl-acrylamide (NIPAM) polymer gel was the latest to develop and can be prepared under a normal atmospheric environment and has lower toxicity compared with the highly toxic polymer gels used earlier. NIPAM polymer gel using X-ray computed tomography (CT) was experimentally investigated in terms of its X-ray CT dose response, sensitivity and dose resolution. The effect of radiation beam type, radiation beam energy and radiation beam dose rate on X-ray CT dose response have also been studied. The temporal stability of NIP AM polymer gel has been examined over several days post-irradiation. The change in the polymer gel dosimeter’s physical and electron densities as a function of absorbed dose was also investigated. In this study two new prototype phantoms were designed and constructed for imaging and irradiation of polymer gel dosimeters to provide simplicity and practicality for clinical dosimetry. The dosimetric and water equivalence properties of four NIP AM based polymer gel dosimeter formulations have been studied by examining their physical properties, interaction probability, radiation transport parameters and performing Monte Carlo modelling of depth doses. NIPAM polymer gel dosimeter irradiated at different doses using 6 MeV photon beam and 400 MU min-1 dose rate were found to have higher CT dose response (up to 37. 8% at 10 Gy dose point) than results reported in the literature for NIP AM gel using similar concentration. The CT dose sensitivity of NIP AM polymer gel was found to be 0. 405+-0. 014 H Gy-1, which is 26. 2% higher than the reported sensitivity of 0. 321+-0. 008 H Gy-1 with similar NIP AM gel concentration. The maximum change in physical density as a function of absorbed dose for polymer gel dosimeters was found to be up to ~1. 0% for an absorbed dose of 20 Gy. The variations observed in the CT dose response curves for NIP AM gel dosimeters irradiated at different radiation dose rates were small. Therefore, NIP AM polymer gel dosimeters were found to be insensitive to variation in radiation beam dose rate for a 6 MeV photon beam. NIP AM polymer gel dosimeters showed very good temporal stability at different post-irradiation imaging times up to 14 days. The variations of NIP AM polymer gel dosimeters CT dose response were within experimental uncertainty for all dose rates and imaging times studied. Due to its low concentration NIP AM polymer gel had lower dose resolution of -1. 54 Gy compared with -0. 052 Gy for the highest NIP AM concentration published in the literature. The MDD was found to be -2. 5 Gy. NIPAM3% polymer gel formulation (the lowest concentration) had the most dosimetric and water equivalence properties over the energy range investigated. NIPAM15% had the least water equivalent properties due to its higher monomer concentration. NIPAM3% was found to be more water equivalent than other X-ray CT polymer gel dosimeters. In summary, X-ray CT polymer gel dosimetry is a low contrast and low sensitivity (and hence low dose resolution) technique due the fact that only small density changes occur within the irradiated polymer gel dosimeter. This has been the limiting factor of X-ray CT polymer gel dosimetry from gaining widespread acceptance for clinical dosimetry. Observations have shown that careful handling should be applied when transporting polymer gel dosimeters because of their sensitivity to movement shocks, when they may lose their gelling matrix integrity. Hence, it is recommended to establish polymer gel dosimetry as an in-house dosimetry technique, where full polymer gel dosimetry procedures can be performed on-site within the clinical facilities to eliminate errors.