The purpose of this research is to investigate whether stereotactic body radiotherapy (SBRT), precise X-ray treatment, is best given in five treatments (also called fractions) over 10 days or in two treatments over 8 days. SBRT is an accurate way to deliver a high dose of radiotherapy to the prostate in a smaller number of doses. We have considerable experience with 5-dose SBRT and now wish to examine the feasibility and safety of delivering treatment over two, larger, doses. Previous work has shown it is theoretically possible to deliver two fraction SBRT on the MR-linac and previous studies have shown internal radiotherapy (brachytherapy) administered in two fractions to be a safe option for patients with low-risk prostate cancer.

All treatment within this trial will be delivered on a new, state of the art, radiotherapy machine called an MR-linac (Magnetic Resonance Linear Accelerator). It puts together an MRI scanner with a radiotherapy treatment machine called a Linear Accelerator. The use of the MR-linac means there is no extra radiation dose given when taking images (unlike computerized tomography (CT) scans or X-ray), enabling us to adapt the radiotherapy plan each day if needed to more precisely target the prostate. The results of the study will enable us to find out if the new, shorter treatment (2 doses of radiotherapy), has a similar level of side effects as the 5 dose treatment and is suitable for further study.

Inclusion Criteria:

1. Men aged ≥18 years
2. Histological confirmation of prostate adenocarcinoma requiring radical radiotherapy
3. Gleason score 3+4 or 4+3 (Grade groups 2 or 3)
4. MRI stage T3a or less
5. PSA <25 ng/ml prior to starting ADT (Androgen deprivation therapy)
6. Patients will be concurrently treated with androgen deprivation therapy (ADT) for at least 6 months, as per standard of care. Men who need longer courses of ADT (maximum 12 months) will be considered on a case-by-case basis, and bicalutamide monotherapy is accepted as an alternative to LHRH (luteinizing hormone-releasing hormone) analogues if required.
7. WHO (World Health Organisation) Performance status 0-2
8. Ability of the participant understand and the willingness to sign a written informed consent form.
9. 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. International Prostate Symptom Score (IPSS) 13 or higher
3. Post-void residual >100 mls
4. Prostate volume >80cc
5. Comorbidities which predispose to significant toxicity (e.g. inflammatory bowel disease) or preclude long term follow up
6. Unilateral or bilateral total hip replacement, or other pelvic metalwork which causes artefact on diffusion-weighted imaging
7. Previous pelvic radiotherapy
8. Patients needing 2-3 years of ADT due to disease parameters.
9. Previous invasive malignancy within the last 2 years excluding basal or squamous cell carcinomas of the skin, low risk non-muscle invasive bladder cancer (assuming cystoscopic follow up now negative) or small renal masses on surveillance.