This is a Phase II, single arm, multi-centre, prospective clinical trial evaluating next generation Stereotactic Ablative Radiotherapy (SABR) for low, intermediate, and eligible high-risk prostate cancer. Eligible patients will receive next generation prostate SABR incorporating toxicity reduction strategies

This is a Phase II, single arm, multi-centre, prospective clinical trial evaluating next generation Stereotactic Ablative Radiotherapy (SABR) for low, intermediate, and eligible high-risk prostate cancer. Eligible patients will receive next generation prostate SABR incorporating toxicity reduction strategies: urethral/trigone sparing, rectal hydrogel spacer, and neurovascular bundle preservation [as per Desai POTEN-C trial, Desai et al., 2025].

Treatment will prioritise the dominant intraprostatic lesion (DIL) while sparing surrounding organs at risk (OARs). Planned doses are: DIL 40-50 Gy in 5 fractions delivered on alternate days, prostate CTV 35 Gy, PTV 33.25 Gy, and urethral PRV 32.5 Gy. Eligible high-risk and some intermediate-risk patients may receive 6-12 months of androgen deprivation therapy (ADT)/hormonal therapy at the physician's discretion, provided they have not received >14 weeks prior to registration.

Radiotherapy planning will include advanced image-guided verification with fiducial markers, pre-treatment dosimetry to minimise dose to OARs, and adherence to protocol-defined constraints for urethra, rectum, and neurovascular bundles. Peri-rectal spacers will be inserted 7-10 days prior to CT-simulation to reduce rectal dose. Dose prioritisation allows full coverage of the DIL while sparing urethra, bladder trigone, and neurovascular bundles to minimise genitourinary, rectal, and sexual toxicity.

Follow-up assessments will include clinical evaluation, toxicity reporting using v5 NCI CTCAE, and patient-reported outcome measures (PRO/QoL) at 2-, 4-, 8-, and 12-weeks post-treatment, 6 and 9 months, and annually up to 5 years. Data on biochemical/clinical failure, progression-free survival, and initiation or re-initiation of ADT will also be collected.

A total of 136 patients will be enrolled to achieve 122 evaluable participants, allowing detection of a statistically significant reduction in Grade ≥2 late genitourinary toxicity compared to historical SABR controls.

Inclusion Criteria:

1. Written informed consent obtained prior to any study-related procedures

2. Males ≥ 18 years of age

3. ECOG performance status (PS) 0-2

4. Biopsy-proven prostate adenocarcinoma without neuro-endocrine differentiation (within 18 months prior to registration, unless on active surveillance and re-biopsy not clinically indicated)

5. Gleason score ≤ 4+3

6. Clinical and/or MRI stage T1c-T3a, N0-X, M0-X

7. PSA ≤ 30 ng/ml (within 60 days prior to registration / prior to starting androgen-deprivation therapy (ADT/hormone therapy) [PSA ≤ 15 ng/ml for patients on 5-alpha reductase inhibitors]

8. Patients belonging to one of the following risk groups:

   • Low risk - patients meeting all of the following criteria:

     • Gleason ≤ 6
     • Clinical stage T1c-T2a
     • PSA < 10 ng/ml (within 60 days prior to registration)

   • Intermediate risk - patients meeting any of the following criteria, assuming no high-risk features apply:

     • Gleason 7 (3+4 or 4+3)
     • MRI stage T2b-T2c (N0, M0-X)
     • PSA 10-20 ng/ml (within 60 days prior to registration)

   • High risk - patients with tumours that meet a maximum of one of the following criteria:

     • MRI stage T3a (N0, M0)
     • PSA >20 - ≤30 ng/ml (within 60 days prior to registration)

Exclusion Criteria:

1. Previous malignancy within the last 2 years (except basal cell carcinoma (BCC) or squamous carcinoma of the skin), or if previous malignancy is expected to significantly compromise 5 year survival
2. Prior pelvic radiotherapy
3. Any prior active treatment for prostate cancer (with the exception of ADT). Patients previously on active surveillance are eligible if they continue to meet all other eligibility criteria.
4. Life expectancy <5 years.
5. Bilateral hip prostheses or any other implants/hardware that would introduce substantial CT artefacts
6. Medical conditions likely to make radiotherapy inadvisable e.g. inflammatory bowel disease, significant urinary symptoms.
7. Anticoagulation with warfarin/bleeding tendency making fiducial placement or surgery unsafe in the opinion of the clinician. Note: Anti-platelet agents e.g. aspirin, clopidogrel and DOACs such as apixaban, rivaroxaban are not contraindications to trial entry.
8. Participation in another concurrent treatment protocol for prostate cancer (not including QoL, survivorship, exercise or registry studies).