3356 - Efficacy and Toxicity of PSMA-PET Guided Radiation Therapy to Newly Diagnosed Node Positive Prostate Cancer with Mesorectal Node Involvement

Purpose/Objective(s): The introduction of PSMA-PET imaging to stage newly diagnosed prostate cancer has led to an increased identification of involved mesorectal (MR) lymph nodes (LNs). These are outside of traditional contouring guidelines, and thus management of these patients remains controversial. There are no series specifically evaluating the management of newly diagnosed prostate cancer with MR LN involvement. Our study aims to evaluate the progression-free survival (PFS), metastasis-free survival (MFS), prostate cancer-specific survival (PCaSS), overall survival (OS), and gastrointestinal (GI) and genitourinary (GU) toxicity outcomes of patients with newly-diagnosed prostate cancer with PSMA-PET detected MR LNs treated with curative intent radiation therapy (RT).

Materials/Methods: A retrospective analysis was conducted on men treated at our integrated regional cancer center between 2016 and 2024, identified via our EMR. Newly diagnosed men with PSMA-PET-detected MR LNs and no distant metastases, who received curative-intent RT were included. All patients received conventional fractionation (CF) or hypofractionation (HF) using VMAT, and were prescribed 78-81Gy in 1.8-2.0Gy per Fx, or 60Gy in 3.0Gy per Fx respectively to the prostate, with simultaneous integrated boost to PET-positive LNs (median boost dose 55Gy for HF and 81Gy for CF). Conventional elective LNs and the MR space were electively treated in all men, with lateralized MR disease treated with unilateral MR elective treatment, and centralized MR involvement having the whole MR space treated electively. All patients were offered androgen deprivation therapy (ADT). All patients with PSA rise >1.0ug/L after cessation of ADT, or rising PSA on ADT, had restaging PETs. Progression was defined as PSA rising on ADT, PSA failure (nadir+2) after completion of ADT, or restaging PET positive relapse.

Results: 32 patients were eligible with a median follow up of 53 months. The median age was 72 (range 54-84), median initial PSA 21ug/L (range 2.67-190), median Gleason score of 9 (range 7-10), median number of PET-positive MR LNs was 1 (range 1-5), and median total LNs involved was 3 (range 1-13). 100% of men received ADT for a median duration of 3.5 years (range 0.5-3.5). Five-year OS, PCaSS, MFS, and PFS were 100, 100, 94, and 94% respectively. Only one patient failed, with pelvic (within initial elective LN contours) and extra-pelvic metastatic LN recurrence on restaging PET, but no MR failure. Acute grade 1, 2, and 3 GI toxicity occurred in 31, 28, and 0%, and GU in 34, 50, and 0%. Late grade 1, 2, and 3 GI toxicity occurred in 16, 13, and 0%, and GU in 35, 42, and 3%.

Conclusion: Outcomes are promising for patients treated with curative RT and ADT for PSMA-PET detected MR node-positive prostate cancer. Further research is needed to optimize contouring guidelines and treatment strategies for this patient subset.