Abstract Purpose/Objectives: Post-mastectomy radiation therapy (PMRT) is known to benefit patients with pathologic N2+ breast cancer, but its impact on pN1 disease remains uncertain. The primary objective of this study was to examine whether PMRT improves recurrence-free survival (RFS) and overall survival (OS) in cN1 breast cancer patients. Various clinical factors were also assessed for associations with survival outcomes. Materials/Methods: A retrospective review of pT1N1M0 and pT2N1M0 breast cancer patients treated at one institution from 2016-2022 was conducted. RFS was defined as the elapsed time from diagnosis to recurrence, death, or last follow-up. OS was defined as the elapsed time from diagnosis to death or last follow-up. The Kaplan-Meier method was used to estimate RFS and OS, and groups were compared using the log-rank test. Cox proportional hazards regression models assessed associations between clinical factors—including age, race, receptor status, tumor size, laterality, and adjuvant therapies—and RFS and OS. Results: Among 57 patients, 22 (38.6%) received PMRT and 35 (61.4%) did not. There were no significant differences in baseline demographic or clinical characteristics between the two groups (p .05). Median RFS and OS were higher in the PMRT group (133 months and not estimable NE) than in the non-PMRT group (120 and 195 months); however, the differences were not statistically significant (p = 0.256 and p = 0.154, respectively). Univariable Cox analyses across the entire cohort found hormone therapy to be significantly associated with improved RFS (HR = 0.43, 95% CI: 0.21-0.90, p = 0.026) and OS (HR = 0.13, 95% CI: 0.04-0.50, p = 0.003). Receptor subtype (ER+/PR+/HER2− vs. triple negative) was also associated with longer OS (HR = 0.25, 95% CI: 0.08-0.80, p = 0.019), but not with RFS (p = 0.839). In subset analyses of 25 non-PMRT patients, tumors located in the left breast were significantly associated with improved RFS (HR = 0.25, 95% CI: 0.09-0.73, p = 0.011) and OS (HR = 0.22, 95% CI: 0.05-0.89, p = 0.033), compared to right-sided tumors. Additionally, having two or more positive lymph nodes (vs. one: HR = 2.89, 95% CI: 1.15-7.27, p = 0.024) and hormone therapy (HR = 0.15, 95% CI: 0.03-0.70, p = 0.016) were significant predictors of RFS and OS, respectively. Conclusions: PMRT did not significantly improve RFS or OS in patients with N1 breast cancer, though a trend toward longer survival was observed. Hormone therapy, receptor subtype, tumor laterality, and nodal burden emerged as predictors associated with survival outcomes. These findings highlight the importance of individualized treatment decisions based on tumor biology and the adjuvant therapy context. Further investigation with a larger sample size is required to clarify the role of PMRT in patients with pathologic N1 breast cancer. Citation Format: R. M. Narasimhan, X. Zhao, S. Han, A. S. Saini, K. Samimi, I. Ogobuiro, C. S. Taswell, C. Takita. A Single-Institutional Analysis of Survival Outcomes and Predictors in pN1 BreastCancer: Is There a Role for Post-Mastectomy Radiation? abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-07-27.
Narasimhan et al. (Tue,) studied this question.