Polymer gel dosimetry: a promising 3D quality assurance tool for magnetic resonance-image guided radiotherapy

Safe and accurate delivery of radiation therapy treatment has been the focus of quality assurance (QA) programs. Advances in treatment delivery require development of new quality assurance tools and ongoing evaluation of existing tools. Magnetic resonance-image guided radiotherapy (MR-IGRT) has been proposed as a new advanced treatment modality in which either a linear accelerator or cobalt sources are combined with a diagnostic MR scanner. Delineation of tumors and adjacent, critical organs is improved by superior soft tissue contrast available on MR images. MR-IGRT devices enable MR image acquisition simultaneously with treatment for an assessment of the tumor volume and its response to treatment in real time. The presence of a strong magnetic field brings new challenges to the selection of QA detectors as the response of ion chambers and electronic devises can be influenced under such conditions. MRcompatibility is key when choosing detectors. Polymer gel dosimeters are a useful tool for capturing complex 3D dose distributions with steep dose gradients. The polymerization as a response to radiation dose can be measured with an MR scanner as spin-spin relaxation rates (R2) increase linearly with increasing radiation dose. These properties make them favorable detectors for volumetric QA in MR-IGRT.