Endocrine Therapy Omission in Estrogen Receptor–Low (1%-10%) Early-Stage Breast Cancer

PURPOSE: Adjuvant endocrine therapy (ET) improves overall survival (OS) in estrogen receptor (ER)-positive early-stage breast cancer (BC). However, the benefit of ET for those with ER-low BC (ER 1%-10%) is unclear. METHODS: Using the National Cancer Database, we studied patients with high-risk stage I to III, ER-low BC (defined as immunohistochemistry 1%-10%) who received (neo)adjuvant chemotherapy and did or did not initiate ET. OS was analyzed with ET initiation as a time-dependent covariate using Cox proportional hazards regression. RESULTS: = .84). CONCLUSION: In ER-low, early-stage BC, ET omission is associated with significantly worse OS, especially in patients with RD after NAC and those with higher (6%-10%) ER levels. Until prospective data are available, patients with ER-low BC should be counseled regarding the potential benefit of ET.