Polymer Gel Dosimetry in Magnetic Resonance-Image Guided Radiation Therapy

The integration of a magnetic resonance (MR) scanner with a radiation therapy treatment unit allows for adaptive radiotherapy guided by MR imaging (MR-IGRT) and has been proposed as an advanced radiotherapy treatment modality. The delineation of the tumor and adjacent organs is greatly improved by the soft tissue contrast available from MR images. With novel integrated devices, MR images can be acquired to assess inter- and intra-fraction changes allowing for a more accurate adaptation of the patient’s radiation treatment plan and consequently a better dose delivery to the tumor.

The need for MR-IGRT treatment plan verification with 3D dosimeters arose due to the presence of the strong magnetic field altering dose depositions in 3D. Furthermore, vendors only provide MR-compatible electronics that measure dose distributions at best in quasi-3D. The implementation of 3D dosimetry into clinical dosimetry requires attention to safety, accuracy, and rigorous validation prior to any routine patient treatment.

Polymer gel detectors have previously been shown to be a useful tool to capture complex 3D dose distributions with steep dose gradients using an MR scanner for readout. Their properties make them favorable devices for the development of quality assurance standards in MR-IGRT.

The basic dosimetric properties of the BANG3-Pro polymer gel provided by MGS Research were characterized with the Elekta Versa HD™ linear accelerator and compared to the characterizations performed with a GMW electromagnet installed in the linear accelerator vault and with the Elekta MR-Linac, a 7 MV linear accelerator integrated with a 1.5 T MR scanner. Additionally, the feasibility of the BANG3-Pro polymer gel was investigated for treatment plan verification following the modified clinical workflow for the MR-Linac. The results of this dissertation provided important insights into the use of the BANG3-Pro polymer gels in MR-IGRT.