PSMA PET guided salvage radiotherapy among patients with prostate cancer in the post-prostatectomy setting: A single center post-hoc analysis

5009 Background: Prostate-specific membrane antigen (PSMA) positron emission tomography (PET) shows improved sensitivity and specificity for detection of locoregional and distant metastatic prostate cancer (PCa) compared to conventional imaging, especially at lower PSA levels as is often the case in the biochemically recurrent (BCR), post radical prostatectomy (RP) setting. Providers are now utilizing PSMA PET findings to guide their salvage radiotherapy (sRT) treatment fields and doses, although it is not well understood how PSMA PET guided sRT impacts patient outcomes. Methods: This was a post-hoc analysis of 5 prospective studies of PSMA PET conducted at UCLA from 2016 to 2021 that included patients with recurrent PCa following RP. Patients were included in this retrospective study if they initiated sRT within 3 months of PSMA PET, had at least 12 months of follow up after sRT completion, had available sRT treatment details, and did not have distant metastases (DM) by conventional imaging on upfront staging. Patients treated with palliative RT were excluded. BCR following sRT was defined as an increase in PSA of 0.2 ng/ml above the post sRT nadir. Metastasis directed therapy (MDT) was defined as sRT to all PSMA+ N1 and M1 lesions. Baseline patient demographics, PSMA PET findings, sRT & ADT treatment details, and patient outcome data were collected. Results: 176 patients were included in this study. Median time between RP and PSMA PET was 38 months (range 1-329). Median PSA at the time of the PSMA PET was 0.625 ng/mL (range 0.063-35). PSMA PET was positive in 128 patients (73%): 21 (12%) miT+N0M0, 55 (31%) miTxN1M0 and 52 (30%) miTxNxM1 with 19 (11%) miTxNxM1a, 31 (18%) miTxNxM1b, and 2 (1%) miTxNxM1c. Median number of lesions seen on positive PSMA scans was 1 (range 1-8). 39 (22%) patients were subsequently treated with sRT to the prostate bed (PB) only, 59 (34%) to PB + pelvic lymph nodes (PLNs), 33 (19%) to PLNs only, 7 (4%) to PB + PLNs + DM, 7 (4%) to PLNs + DM, and 31 (18%) to DM only. 59 (34%) patients were treated with concurrent ADT at a median duration of 6 months (range 1-39). At a median follow-up of 32 months (range 12-70) after sRT, 80 patients (45%) did not develop BCR or imaging relapse (IR) following sRT, 24 patients (14%) developed BCR but not IR, 1 patient (<1%) developed IR only, and 70 patients (40%) developed both BCR and IR. The median time to BCR and IR following sRT was 15 months (range 1-48) and 19 months (range 6-61), respectively. 1 year post sRT biochemical recurrence free survival was 77%. Of the 83 patients treated with MDT, 32 (39%) did not develop subsequent disease relapse. Conclusions: This post-hoc analysis assessed the outcomes of 176 patients treated with PSMA PET guided salvage RT, proving it to be an effective method for treating both pelvic and extrapelvic recurrent PCa. Further investigation is needed to assess the full extent of patient outcomes in this population.