Toward "on-line" X-ray computed tomography-based dosimetry using a new polymer gel with rapid response

A previously reported X-ray computed tomography (CT) based polymer gel dosimeter required a long waiting time before measurement and analysis. Current radiotherapy quality assurance needs both high-efficiency and high-accuracy measurements in a multidimensional system. Thus, we developed a new dGELTM X-ray CT-based polymer gel dosimeter with a rapid dose response and room-temperature stability. Here we report characterization of the gel dosimeter and its validation in a volumetric modulated arc therapy (VMAT) application. All samples of dGELTM were manufactured by a commercial company. The final gel solution was poured into cylindrical jars. All gel imaging was performed using a Varian TrueBeam cone-beam CT system. We found that increases in CT number were similar throughout the irradiated gel and that a rapid 5 HU increment could be measured immediately after a radiation dose of 5-10 Gy. Intra- and inter-batch reproducibility were good over the range of 5-35 Gy, although there was a significant difference between the upper and lower regions of the gel. The gel response was independent of the energy of the irradiation (6 MV and 10 MV) and of the dose rate in the 400-2400 cGy min -1 range. The gel response was positively correlated with the dose up to 35 Gy. The VMAT irradiation compared well with the planned doses, yielding a gamma pass rate of more than 90 % for multiple slices at isocenter and off -center. The gel was a solid at room temperature, and its CT number increased rapidly after irradiation, thus enabling "on-line" dosimetry.