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-enhancedimaging 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 wouldbe standard of care. Luteinizing hormone-releasing hormone (LHRH) analogues orBicalutamide 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 informedconsent form.
• Ability/willingness to comply with the patient reported outcome questionnairesschedule 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 boweldisease) or preclude long term follow up
• Unilateral or bilateral total hip replacement, or other pelvic metalwork which causesartefact 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 cellcarcinomas of the skin, low risk non-muscle invasive bladder