Dose-dEscalated MR-Guided Radiotherapy for Localized Prostate Cancer

Inclusion Criteria: 1. Men aged ≥18 years 2. Histological confirmation of prostate adenocarcinoma requiring radical radiotherapy 3. Gleason score 3+3, 3+4 or 4+3 (Grade groups (GG) 1, 2 or 3) 4. MRI stage T3a or less (as staged by AJCC TNM 2018). MRI must be performed within a year of randomisation 5. MRI-visible tumour(s) of PIRADS v2 grade 3 or higher and able to be delineated on T2 and diffusion-weighted imaging +/- dynamic contrast-enhanced imaging. Tumour nodule visible on MRI should be considered able to be boosted by treating clinician and \<2.5cm in maximal dimension 6. The MRI-defined lesion must be confirmed as malignant on biopsies (Gleason grade must be within the limits expressed in inclusion factor 3) 7. Short course (\< 6 months) concurrent androgen deprivation therapy (antiandrogens or LHRH analogues) allowed though not mandated as per the discretion of the treating physician. 8. PSA \<20 ng/ml prior to starting ADT, if used 9. WHO Performance status 0-2 10. Ability of the participant understand and the willingness to sign a written informed consent form. 11. Ability/willingness to comply with the patient reported outcome questionnaires schedule throughout the study. Exclusion Criteria: 1. Contraindications to MRI (e.g. pacemaker, potentially mobile metal implant, claustrophobia) 2. IPSS Score \> 19 3. High grade disease (GG3) occult to MRI-defined lesion. 4. Prostate volume \>90cc 5. Comorbidities which predispose to significant toxicity (e.g. inflammatory bowel disease) or preclude long term follow up 6. Hip replacement, or other pelvic metalwork which causes significant artefact on diffusion-weighted imaging 7. Previous pelvic radiotherapy 8. Patients needing \>6 months of ADT due to disease parameters, as per the discretion of the treating physician 9. Previous invasive malignancy within the last 2 years where this is likely to shorten lifespan the following will remain eligible: basal or squamous carcinomas of the skin, low risk non-muscle invasive bladder cancer (assuming cystoscopic follow up now negative) or small renal masses on surveillance.