Patterns of failure after radiotherapy (RT) in patients with prostate cancer re-staged with 18F-DCFPyL PSMA-PET: A regional cohort analysis.

45 Background: In Ontario, PSMA-PET is available to patients with prostate cancer who experience biochemical failure after definitive primary RT (pRT) or salvage/adjuvant RT (sRT) through a provincial registry study (PREP protocol). Here we review patterns of PSMA-PET-detected failure after primary (pRT) or salvage/adjuvant RT (sRT) radiotherapy (RT) in cohorts of PREP-registry patients from the Local Health Integration Network (LHIN) 4 region. Methods: With ethics approval, we retrospectively analyzed patients imaged with PSMA-PET in Hamilton, Ontario, Canada, after biochemical failure, following pRT or sRT. Demographic, clinical and treatment parameters as well as PSMA-PET results were collected. Results: Between April 2019 and December 2021, 104 pRT and 91 sRT patients were imaged with 18 F-DCFPyL PSMA-PET. Median PSA (ng/mL) before PSMA-PET for the two groups was 5.0 (IQR:3.1-8.9) and 1.3 (IQR:0.6-3.4). The percentages of Prostate or Prostatic bed RT alone (Pr-RT) and Pr + pelvic Lymph Node RT (Pr+LN-RT) RT treatments, were Pr-RT:76% vs Pr+LN-RT:24% for the pRT group and Pr-RT:49.4% vs Pr+LN-RT:50.6% for the sRT group, respectively. Rates of PSMA-PET detected local, pelvic lymph node and distant recurrence are shown. In the pRT group, increasing D’Amico risk category (low to high risk) was associated with lower risk for local and increased risk for pelvic and distant metastasis. Pelvic RT was associated with reduce risk for local recurrence in the pRT but not in the sRT group. Conclusions: In this cohort of patients PSMA-PET detected a considerable risk for local recurrence in patient treated with pRT. A high number of patients are detected with pelvic-only disease recurrence, but pelvic RT seems to be associated with reduction in this risk only in the pRT population. Studies with PSMA-PET at baseline, before pRT or sRT, may help discern better patients that would benefit from pelvic RT and systemic therapy.[Table: see text]