In the clinical scenario of recurrent prostate cancer (PCa) post local therapy, current standard studies (bone scan and computed tomography) commonly fail to identify the recurrent disease location. In this study the investigator aims to prospectively map recurrent disease with the unique combination of whole-body MR anatomical imaging combined with a new high-sensitivity and PCa-specific PET probe (PSMA-targeted: [18F]DCFPyL) to provide precise localization information to target disseminated tumor deposits in men presenting with rising PSA after prostatectomy and radiotherapy (maximal local therapies). Moreover, we will consequently treat all identified disease with image-guided stereotactic ablative radiotherapy (SABR), which has shown tantalizing results achieving excellent tumor eradication rates with minimal toxicities. This study is uniquely positioned to enable the discovery of new biomarkers and the correlation of prognostic tests (e.g. genomic signatures) from the initial prostatectomy specimen with the PET-MR/CT imaging results and curative-intent treatment outcomes.

The significance of the proposed work towards a measurable impact in PCa care is important to emphasize. The study team believes this novel curative-intent approach will transform lives, as opposed to therapies that transiently impact incurable disease stages. Herein, the focus is on patients at the earliest point of the disease spectrum of recurrent PCa after curative-intent treatments. Our hypothesis is that PSMA-targeted [18F]DCFPyL PET-MR/CT allows earlier detection and localization of defined metastatic targets in these patients, at a stage amenable to image-guided curative-intent therapy.

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