The feasibility of using a conventional flexible RF coil for an online MR-guided radiotherapy treatment

The purpose of this paper is to evaluate the impact of a flexible radiofrequency coil on the treatment delivery of an online MR-guided radiotherapy treatment. For this study, we used a Synergy MR body coil (Philips, Best) in combination with the current MRL prototype of the UMC Utrecht. The compatibility of the coil is evaluated in two steps. First, we evaluated the dosimetric impact of the MR coil on both a simple and a complex irradiation strategy for treating spinal bone metastases. This tumor site will likely be chosen for the first in-man treatments with the UMC Utrecht MRL system. Second, we investigated the impact of the treatment beam on the MRI performance of the body coil. In case a single posterior-anterior rectangular field was applied, dose to the target volume was underestimated up to 2.2% as a result of beam attenuation in the MR coil. This underestimation however, decreased to 1% when a stereotactic treatment strategy was employed. The presence of the MR coil in or near the distal site of the treatment beam decreased the exit dose when a magnetic field was present. The MRI performance of the coil was unaffected as the result of the radiation. It is feasible to use the Synergy MR body coil for an online MR-guided radiotherapy treatment without any modification to the coil or attenuation correction methods in the planning stage. The effect of the MRI coil on the dose delivery is minimal and there is no effect of the treatment beam on the SNR of the acquired MRI data.