Dose De-escalation in Prostate Radiotherapy Using the MRL

Inclusion Criteria: * Men aged ≥18 years * Histological confirmation of prostate adenocarcinoma requiring radical radiotherapy * Gleason score 3+3, 3+4 or 4+3 (Grade groups 1, 2 or 3) * MRI stage T2 or less (as staged by AJCC TNM 2018) * MRI-visible tumour(s) of Prostate Imaging-Reporting and Data System (PIRADS) v2 grade 3 or higher on T2 and diffusion-weighted imaging and/or dynamic contrast-enhanced imaging with concordant pathology * Tumour nodule visible on MRI occupying \<50% of prostate on any axial slice and \<50% total prostate volume * PSA \<20 ng/ml prior to starting androgen deprivation therapy (ADT) * Patients can be concurrently treated with androgen deprivation therapy if this would be standard of care. Luteinizing hormone-releasing hormone (LHRH) analogues or Bicalutamide are permitted. ADT is not mandatory where this would usually be omitted. * World Health Organisation (WHO) Performance status 0-2 * Ability of the participant understand and the willingness to sign a written informed consent form. * Ability/willingness to comply with the patient reported outcome questionnaires schedule throughout the study. Exclusion Criteria: * Contraindications to MRI (e.g. pacemaker, potentially mobile metal implant, claustrophobia) * IPSS 19 or higher * High grade disease (GG3) occult to MRI-defined lesion * Post-void residual \>100 mls, where known * Prostate volume \>90cc * Comorbidities which predispose to significant toxicity (e.g. inflammatory bowel disease) or preclude long term follow up * Unilateral or bilateral total hip replacement, or other pelvic metalwork which causes artefact on diffusion-weighted imaging * Previous pelvic radiotherapy * Patients needing \>6 months of ADT due to disease parameters. * Previous invasive malignancy within the last 2 years excluding basal or squamous cell carcinomas of the skin, low risk non-muscle invasive bladder