1256 - Elective Pelvic Nodal Radiation in Prostate Cancer with and without Mesorectal or Extra-Pelvic Disease

Purpose/Objective(s): For men with regional lymph node (LN) spread of prostate cancer, radiotherapy (RT) with elective LN coverage is an established standard of care. However, for men with mesorectal (MR), peri-arotic (PA), or oligometastatic bony (OMB) spread, there is less known about outcomes following RT to all visible disease with elective LN RT.

Materials/Methods: A multi-site, single institution retrospective study was conducted including 1,166 men (n=353 PET staged) with positive pelvic LN with or without MR LN, PA LN, or OMB (1-5 sites) spread. Patients were treated 2015-2024 with elective pelvic LN RT (45-46.8 Gy in 1.8 Gy/Fx) with simultaneous integrated boost to involved lymph nodes (2.2-2.25 Gy/Fx), and prostate/prostate bed. Metastases directed RT was delivered to all men with OMB disease. ADT used for 98% (n=1,145) and ARPI for 59% (n=693). Outcomes included overall survival (OS), distant metastases free survival (DMFS), and biochemical progression free survival (bPFS) evaluated using Kaplan-Meier analysis. Multivariable (MVA) cox-proportional hazard models assessed associations between clinicodemographic and treatment variables with outcomes.

Results: After median f/u 31 mo, 5yr OS, DMFS, and bPFS for men with pelvic LN-only disease was 90%, 73%, and 61% respectively; pelvic + MR LN only spread (n=128; 11%) was 86%, 53%, and 44% respectively; pelvic + PA LN spread without OMBD (n=221; 19%) was 82%, 63%, and 57% respectively, and for pelvic + OMB (n=109; 9%) was 71%, 47%, and 44% respectively. On MVA, compared to patients with pelvic LN-only disease, there was worse OS, DMFS, and bPFS for patients with OMB but not significantly different for men with PA or MR LN spread. Of the 56 patients who progressed after PA LN RT, 18 (32%) had a failure within the radiation field; of the 204 patients who progressed after pelvic-only RT, 29 (14%) had a failure within the radiation field (p=0.002).

Conclusion: In the largest report on elective pelvic LN RT in prostate cancer, we showed that men with PA and MR LN spread do not have significantly worse outcomes than standard pelvic LN-only spread. However, patients treated with PA fields had higher rates of in-field failure compared to pelvic-only fields, possibly related to continued uncertainty about optimal field design and dosing. Furthermore, we highlight that almost half of men with pelvic LN and OMBD are free from biochemical progression following radiation to all gross disease and elective pelvic nodes.