De-escalation of Post-mastectomy Irradiation in Hormone Receptor-Positive Breast Cancer with One to Three Positive Nodes

Background: The 21-gene recurrence score (RS) is used to predict benefit from chemotherapy in hormone receptor (HR)-positive breast cancer with one to three positive lymph nodes. Prospective–retrospective studies have shown that the RS is prognostic for both systemic and locoregional recurrence in tamoxifen-treated patients. We aimed to assess whether RS could be utilized to predict a survival benefit from postmastectomy radiation therapy (PMRT). Patients and Methods: The National Cancer Database (NCDB) was used to identify women ≤ 75 years of age with HR+, HER2-negative, T1–3, N1, M0 breast cancer who underwent mastectomy and axillary staging with available RS during the years 2010–2016. Kaplan–Meier and Cox proportional hazards models were used to identify association between treatment and overall survival (OS). Univariate and multivariate analyses were used to identify variables correlating with PMRT and OS. Results: A total of 8907 patients were identified. Of the total, 3203 (36%) patients received adjuvant PMRT and 5704 (64%) did not. Across the entire cohort, 5-year OS was 97.5% for patients receiving PMRT and 96.8% for those who did not (P = 0.063). After adjusting for all covariates, in patients with RS ≤ 25, there was no statistically significant improvement in 5-year OS with the addition of adjuvant PMRT (97.5% versus 98.1% P = 0.093). Moreover, no survival benefit was seen with axillary node dissection (P = 0.58) or with the addition of chemotherapy (P = 0.312). Conclusions: In our cohort of patients with one to three positive nodes and a RS ≤ 25, omission of post-mastectomy radiation therapy had no impact on OS. Our results suggest that RS may be utilized in the individualized decision making on PMRT.