Para-Aortic Radiation Therapy for Oligorecurrent Prostate Cancer

Abstract

<h3>Purpose/Objective(s)</h3> Patients with oligorecurrent prostate cancer (PCa) limited to the pelvic lymph nodes (LNs) are often treated with whole pelvis radiotherapy (RT) with excellent clinical outcomes, as demonstrated in a phase 2 clinical trial. For oligorecurrent PCa in the para-aortic (PA) LNs, although the standard of care is systemic therapy only, RT may also be beneficial. The purpose of this study is to retrospectively assess the safety and efficacy of RT to the PA LNs (PA-RT) in this population. <h3>Materials/Methods</h3> We identified a sequential cohort of patients with oligorecurrent PCa to the PA LNs (≤5) after pelvic RT at our institution from 2015 to 2021. Eligible patients had oligorecurrent PCa to the PA LNs treated with elective, conventionally fractionated PA-RT plus simultaneous integrated boost (SIB) to LN+ disease. The primary endpoint was defined as progression-free survival (PFS) at 2 years using Kaplan-Meier estimation. PFS was defined as the time from PA-RT to the first event: biochemical failure (PSA 50% above post-treatment nadir and at least 4 ng/mL), escalation of therapy, clinical progression, or death. Secondary endpoints included 2-year biochemical failure-free survival (BFFS), 2-year overall survival (OS) and treatment-related toxicity. <h3>Results</h3> In total, 34 patients were included (median age 66 years), and 82.4% were status post-prostatectomy. The median time from diagnosis to PA-RT was 5.7 years. The median PSA at PA-RT was 3.15 ng/mL (IQR 1.30-5.90), and 23.5% had castration-resistant PCa (CRPC). All patients were treated to the PA region with 50 Gy (88.2%) or 45 Gy (11.8%) in 25 daily fractions. LN+ disease received a SIB to a median dose of 62.5 Gy (range 60-65 Gy). Most received photon-based RT, while 21.1% were treated with proton therapy. Nearly all (97.1%) patients had androgen deprivation therapy (ADT) monotherapy (median duration 20 months), and 52.9% had ADT plus abiraterone. The median follow-up time from PA-RT was 21.5 months. The primary endpoint, PFS at 2-years, was 83.4% (95% CI: 68.6-100%). 2-year BFFS was 90.4% and OS 100%. 8 patients had castration resistant PCa with trend to worse PFS (p=0.14). There were no grade 3 or higher acute toxicities. There were 10 (29.4%) grade 2 acute toxicities, 8 were gastrointestinal. There were 2 (5.9%) grade 3 chronic toxicities (small bowel obstruction and hematuria requiring intervention). There were 4 (11.8%) chronic grade 2 toxicities. <h3>Conclusion</h3> PA-RT for oligorecurrent PCa has low toxicity with encouraging early disease control. These preliminary results show similar outcomes to RT for oligorecurrent PCa limited to the pelvis; however, these results require validation in prospective studies.