3D gel dosimeters offer the potential for real-time quality assurance in MR-IGRT

Abstract

To investigate the use of 3D gel dosimeters for real-time quality assurance (QA) in MR-image guided radiotherapy (MR-IGRT). Polymer gel dosimeters were prepared in cylindrical glass vials (5 cm diameter, 4 cm height) by MGS Research Inc. (Madison, CT) and kept at room temperature. Fricke-type gel dosimeters were made in-house (8.5 cm diameter, 6 cm height) and stored at 4⁰C for 24 hours prior to irradiation. Two separate irradiation setups were used for this study. Polymer gels were irradiated at isocenter distance at 6 cm depth inside a full phantom. A dose of 15 Gy was delivered with an integrated 1.5 T MRI – 7 MV linear accelerator system (MR-Linac). Fricke gels were irradiated in air to 10 Gy with a gantry angle of 0⁰ and to an additional 20 Gy with a gantry angle of 270⁰ in the center of the dosimeter. MR images were acquired in real-time for both setups using a balanced-Fast Field Echo (b-FFE) sequence. The sequence was started 30 seconds before irradiation and was stopped 30 seconds and 1 minute after irradiation for polymer gels and Fricke gels, respectively. Polymer gels were imaged with TR/TE = 3.4/1.7 ms and a temporal resolution of 277 ms. TR/TE = 4.4/2.2 ms and a temporal resolution of 1800 ms was selected for Fricke gels. Signal intensities (SI) inside and outside the radiation field were measured on all MR images. The difference in SI was calculated and normalized to the maximum value that was measured at the end of image acquisition. For both polymer and Fricke gels, the difference in SI was measured as early as the beam was turned on. The SI difference measured with polymer gels increased logarithmically up to 97% of the maximum value at the completion of the exposure. The ongoing polymerization of the gel until 30 seconds after beam-off was indicated by the slow, continuous logarithmic increase in SI differences and appeared to have reached a plateau at the end of image acquisition. The oxidation reaction of the Fricke gels continued after the exposure was completed, reaching a maximum at 1 minute following beam off. The SI difference increased linearly up to 32% of the maximum value with the gantry at 0⁰ and continued to increase linearly from 33% to 90% of maximum after the gantry was rotated to 270⁰. The appearance of a difference in SI at beam initiation showed that both gels reacted immediately to the delivered radiation. These results encourage further investigation and use of the gels for QA of MR-IGRT treatment units such as the MR-Linac, and especially for real-time QA. An optimization of the real-time imaging parameters is recommended to lower the noise of the MR images and obtain more qualitative and quantitative data.