Prostate Cancer Subclinical Metastatic Ablative MR-guided Radiotherapy

3.1.1 - Age ≥ 18 years 3.1.2 - ECOG performance status of 0-2 3.1.3 - Absence of significant comorbidities rendering patient nor suitable for curative ablative approaches 3.1.4 - No history of active non-skin malignancy precluding management of their prostate cancer 3.1.5 - Histological evidence of prostate adenocarcinoma on previous radical prostatectomy.

3.1.6 - No use of any form of hormonal therapy in the previous 12 months, or intention to start HT at time of enrollment; and no use of ADT as salvage therapy.

3.1.7 - Normal serum testosterone level ascertained within 4-6 weeks of enrollment, as deemed by treating physician 3.1.8 - Absence of known metastatic disease on conventional imaging 3.1.8.1 - Radiological studies without evidence of regional or distant metastases: CT abdomen-pelvis and bone scan within previous 6 months (prior to PSMA PET scan or consent) or at discretion of the treating physician 3.1.8.2 - Patients with disease detected on PSMA PET-CT, performed at UHN as part of the Provincial or Institutional registry with disease amenable to SABR or surgery 3.1.9 - No contraindications to CT or PET as per Joint Department of Medical Imaging policies 3.1.10 - Able to lie supine at least 60 minutes to comply with imaging and treatment.

3.1.11 - Absence of impaired renal function (calculated GFR \> 30mL/min) 3.1.12 - Absence of sickle cell disease or other hemoglobinopathies 3.1.13 - No other medical conditions deemed by the PI to make patient ineligible for PET/MR scanning or treatment (SABR or surgery) 3.1.14 Rising PSA after maximal local therapies (radical prostatectomy and either adjuvant or salvage radiotherapy): 3.1.14.1.1 - Three documented PSA rises, at least 1 month apart from post radiotherapy.

3.1.14.1.2 - PSA value \>0.1 and \< 3 ng/mL, within 4-6 weeks of enrollment. Salvage ADT to be started when PSA reaches a value of 6.0 ng/mL or greater

For the OM\^2 group:

* Met inclusion criteria 3.1.1-3.1.8.1 above
* Underwent ablative therapies (SABR or surgery) for oligometastatic prostate cancer following radical prostatectomy and radiotherapy with biochemical response meeting criteria for either complete or partial biochemical response
* Six months or more since last OM-directed ablative therapy.
* Rising PSA as defined by PSA of nadir post ablative therapy + 2 ng/mL, or nadir post ablative therapy + 1 ng/mL in those where \>12 months have elapsed since ablative therapy; in two repeated measures at least 2 weeks apart