At simulation: patients will have two MRI scans in addition to the standard planning CT scan. One MRI scan will be with bladder full, the other will be with bladder empty. During treatment: patients will have two or three MRI scans per week at their convenience. MRI scans will be scheduled close to radiotherapy treatment times to minimise inconvenience for the patient. On the same day they have an MRI scan, they will also have a CBCT at the time of treatment. None of the images will be used to alter or influence the treatment the patient is receiving. If the patient feels that the imaging schedule is too inconvenient, they will be given the option to reduce the frequency of the scanning or to withdraw from the study.All the images will be loaded onto a password secure server. The MRI scans will be fused to the planning CT scan using bone-to-bone matching. Tumour and intra-pelvic organs will be contoured on the MRI scans. The contours will be used to create surface meshes of each organ and with research software, we will be able to model the motion and deformation of these organs over the course of radiotherapy. Using this data, we will be able to simulate the dose impact of the organ motion on different intensity-modulated radiotherapy plans. We will also explore the impact of bladder & rectal filling on tumour motion.