Post-mastectomy Radiation Therapy in Breast Cancer Patients with Nodal Micrometastases

Background. Recent data support the use of post-mastectomy radiation therapy (PMRT) in women with one to three positive lymph nodes; however, the benefit of PMRT in patients with micrometastatic nodal disease (Nlmi) is unknown. We evaluated the survival impact of PMRT in patients with Nlmi within the National Cancer Database. Methods. The pattern of care and survival benefit of PMRT was examined in women with pT1-2Nlmi breast cancer who underwent mastectomy without neoadjuvant chemotherapy. Univariable and multivariable Cox proportional hazard models were employed for survival analysis, and subanalyses of high-risk patients and a propensity score-matched (PSM) cohort were completed. Results. From 2004 to 2014, we identified 14,019 patients who fitted the study criteria. PMRT was delivered in 18.5% of patients and its use increased over the study period. Patients treated with PMRT were younger, had better performance status and larger primaries, were estrogen receptor (ER)-negative, had higher grade, lymphovascular invasion and positive surgical margins, and more often received systemic therapy. PMRT was significantly associated with overall survival (OS) in univariable analysis (hazard ratio [HR] 0.75 [0.64-0.89]), but was not significant in multivariable analysis (adjusted HR 1.01 [0.84-1.20]). There was no survival benefit to PMRT in ER-negative, high-grade, and/or young patients. There were 2 (0.9%) death events in the sentinel lymph node biopsy (SLNB) + PMRT group versus 21 (2.9%) in the SLNB-alone group (log-rank p = 0.053), and 8 (3.9%) death events in the axillary lymph node biopsy (ALNB) + PMRT group versus 27 (3.6%) in the axillary lymph node dissection-alone group (p = 0.82). There was no significant association between PMRT and OS within the PSM subgroup. Conclusion. In this largest reported retrospective study, no OS differences were associated with PMRT, which suggests that PMRT may not benefit every patient with microscopic nodal disease.