MRI guidance for breast cancer radiotherapy

This thesis evaluated the application of MRI guidance in different steps of the breast cancer radiotherapy workflow and describes how to handle several challenges for MRI-guided radiotherapy. The application of MRI is most beneficial in the setting of preoperative breast radiotherapy. In this setting, MRI is required for preoperative target definition and the thesis describes consensus guidelines for this purpose. An opportunity for the application of MRI lies in response assessment of changes in the tumor after preoperative radiotherapy. A challenge for MRI-guided radiotherapy is that radiotherapy plans are usually calculated on CT scans and MRI cannot be directly used for dose calculations. This thesis evaluated two different synthetic CT approaches that both resulted in accurate dose calculations for partial breast irradiation. Patient positioning for MRI-guided radiotherapy was also evaluated. A comparison of MRI acquired in prone position and supine position showed that dose to the ipsilateral lung was reduced in prone position. However, appropriate treatment plans for partial breast irradiation on an MR-Linac (a hybrid machine integrating MRI and radiotherapy delivery in a single machine) could be achieved in both positions. The current applications of MRI and the MR-Linac for breast radiotherapy are still limited. This is due to several remaining challenges, but in particular because of the limited value of MRI for the standard treatment in which the breast is irradiated after surgery.