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PSMA PET is being increasingly used in restaging of biochemical recurrence after definitive therapy for prostate cancer. It has also regenerated the debate on the role of elective nodal irradiation (ENI) in localized prostate cancer. The Roach formula estimates the risk of regional lymph node involvement (LNI) based on preoperative PSA and Gleason score. Extension of radiation fields to include pelvic nodes in patients with LNI risk >15% has previously been recommended. More recently the POP-RT trial demonstrated a benefit of whole pelvic radiotherapy (RT) over prostate only RT in patients with Roach LNI risk >20%. We carried out a retrospective review of patients with PSMA PET confirmed pelvic nodal recurrences post-prostatectomy to determine whether they would have met the Roach formula criteria for ENI if they had had primary prostate RT instead of prostatectomy. We also sought to determine whether these nodal recurrences were within standard ENI fields.

We identified patients who underwent PSMA PET imaging at our institution for biochemical failure post-prostatectomy. Within that group, we identified patients with PSMA PET confirmed pelvic nodal recurrences only, who were referred to our institution for consideration of salvage pelvic radiotherapy. Preoperative and postoperative clinical parameters were reviewed. Roach LNI risk was calculated using preoperative data.

Thirty patients with PSMA PET confirmed pelvic nodal recurrences were identified. Median age at radical prostatectomy was 61 years (range 49-70). Median pre-op PSA was 7.9 ng/ml (range 3.7 – 48). At diagnosis, 50% of patients were clinical stage T3a or T3b and 50% were Gleason 8, 9 or 10. Preoperative staging bone scans and CTs were done in 73% and 50% of patients respectively. At surgery, 40% were Gleason 8, 9 or 10 and 77% were pathologic stage T3a or T3b. Pathologic nodal staging was Nx in 57% of patients, N0 in 30% and N1 in 13%. The median lymph node yield in patients who did have nodes removed was 4 (range 1-16). Margins were positive in 53% of patients. Postoperative PSA was detectable in 83.3%.

Median PSA at the time of PSMA PET was 0.7 ng/ml (range 0.17 – 8.3) and median PSA doubling time was 5.4 months (range 1.1 – 42.8). The median time from surgery to PSMA PET confirmed pelvic nodal recurrence was 12.4 months. The median number of positive nodes identified on PSMA PET was 1. Preoperative Roach LNI risk was >15% in 77% of patients and >20% in 63% of patients. There were 54 nodal recurrences identified; 48 (89%) were within standard ENI fields and 6 (11%) were not.

The Roach formula is used in estimating risk of regional LNI in prostate cancer but there remains ongoing debate as to the ideal cut off level that should be used to identify patients who would benefit most from whole pelvic radiotherapy. PSMA PET is being increasingly used to identify regional and distant metastatic disease and using it to individualize and tailor RT treatment may improve outcomes.