Brief summary:
Purpose: Prospective, single-site Phase II study testing whether PSMA-PET/MRI-guided, de-escalated salvage radiation reduces acute Grade ≥2 toxicity versus a 44% historical rate, while maintaining cancer control after prostatectomy.Population/Eligibility: Adult men ≥30 years with prior radical prostatectomy and biochemical persistence/recurrence per NCCN (persistent positive PSA after RP, or undetectable PSA that becomes detectable and rises on ≥2 determinations, or PSA >0.1 ng/mL). Must have a targetable PSMA-avid lesion in the prostate bed and/or pelvic lymph nodes and/or an MRI-defined lesion suspicious for local recurrence. KPS ≥80 or ECOG ≤2; life expectancy >5 years; able to consent. Exclude: Evidence of distant metastatic disease outside pelvic nodes (including osseous involvement), conditions that preclude radiation, or factors preventing protocol compliance.Interventions \& Evaluations: Baseline history/physical, vitals, performance status, labs (PSA, CBC w/diff, CMP/creatinine), pelvic MRI and PSMA-PET/CT; optional biopsy if feasible. External beam radiation therapy (LINAC/VMAT) with daily image guidance: pelvis 45 Gy in 25 fractions, followed by a sequential boost to PSMA/MRI-defined disease to 63-70.2 Gy in 10-14 additional fractions, with protocolized OAR constraints. All participants receive standard-of-care androgen deprivation therapy (ADT) for 6-24 months at the treating clinician's discretion. Weekly on-treatment visits; physician-assessed toxicities graded by CTCAE v5. Patient-reported outcomes (IPSS; FACT-P) at baseline and each in-person follow-up.Follow-up: Phone toxicity check 1 month post-RT; clinic at 4 months post-RT, then every 3 months thereafter until 24 months after completion of ADT. At each visit: H\&P, CTCAE toxicity assessment, and PSA. If biochemical failure occurs, imaging (PSMA-PET/CT, CT and/or MRI) is obtained per standard of care to assess clinical progression.Endpoints/Design: Primary endpoint: acute (≤4 months post-RT) Grade ≥2 toxicity (all types). Secondary endpoints: 2-year biochemical progression-free survival; chronic toxicity and patient-reported outcomes from 4-24 months; 24-month local control, locoregional control, distant metastasis, and overall survival. Simon optimal two-stage design with interim analysis after the first 18 patients complete RT (stop if ≥8 have Grade ≥2 acute toxicity); total planned enrollment up to 54.