Patterns of Failure After Prostate-Only Radiotherapy in High-Risk Prostate Cancer: Implications for Refining Pelvic Nodal Contouring Guidelines.

PURPOSE: To study prostate specific membrane antigen - positron emission tomography (Ga68 PSMA-PETCT) based patterns of relapse at biochemical failure (BCF) after prostate-only radiotherapy (PORT) in high-risk (HR) prostate cancer and its implications on pelvic contouring recommendations.

METHODS AND MATERIALS: Patients with clinico-radiological high-risk node-negative prostate cancer treated with curative PORT and androgen deprivation therapy (ADT), either within the POP-RT randomised trial or off trial, who underwent a Ga68 PSMA-PETCT upon BCF were included. Patterns of regional and distant recurrence on Ga68 PSMA-PETCT were studied. Pelvic nodal recurrences were mapped with reference to the superior border of pubic symphysis. Pelvic lymph nodal caudal border (PLNcb ) recommendations in the published contouring guidelines (RTOGcb , GETUGcb , PIVOTALcb , NRGcb , GFRUcb) were evaluated.

RESULTS: Of the total 262 patients screened, 68 eligible patients were included (POP-RT trial 35 patients; off-trial 33 patients). Median follow-up was 91 months (IQR, 72-117) and median time to BCF was 65 months (IQR, 49-83). Regional and distant recurrence was seen in 31 (46%) and 31 (46%) patients, respectively. Of the nodal recurrences, nearly half (46%, 14/31) had no distant metastases and 64% (20/31) had a failure in the common iliac nodal region. The lower-most nodal recurrence was 20 mm cranial to the top of pubic symphysis (RTOGcb , GETUGcb , GFRUcb ) and 10 mm cranial to the PIVOTALcb . The PLNcb recommended by NRG guideline (NRGcb ) had an inter-patient variability of 32 mm, ranging from 16 mm above to 16 mm below the top of pubic symphysis, and the lower most nodal recurrence ranged from 4 mm to 36 mm cranial to NRGcb .

CONCLUSION: Pelvic failures accounted for a major proportion of recurrences after prostate-only radiotherapy, with the caudal most nodal recurrence being 20 mm cranial to the top of pubic symphysis. This could have implications in defining the caudal border of contouring recommendations.