Purpose: To determine prevalence, clinicopathological characteristics, initial treatments, and outcomes associated with low estrogen receptor (ER)-expressing invasive breast cancer.Methods: This retrospective, noninterventional database study included patients undergoing surgery with curative intent for invasive ductal or lobular breast cancer. Patients were treated between January 2003-December 2012. Demographics, clinicopathologic characteristics, initial treatments, and outcomes were abstracted from patient records. Patients were categorized using immunohistochemistry to determine ER, progesterone receptor, and human epidermal growth factor receptor 2 (HER2) levels. ER-positive patients were subclassi ed as ER-Low (1%-10%) and ER-High (>10%) according to Allred Proportion Score. Disease-free survival (DFS) and overall survival (OS) were estimated by the Kaplan-Meier method and compared among groups by log-rank test.Results: 5930 patients were included (median follow-up, 80.9 months). Of all patients included, 117 (2.0%) had ER-Low tumors, 63 (53.8%) of whom had HER2− tumors and 54 (46.2%) HER2+ tumors. Five-year DFS and OS were highest in the ER-High/HER2− cohort (94.0% and 98.6%, respectively) and lowest in the triple-negative breast cancer (TNBC; 81.3% and 90.1%) and ER-Low/HER2− (85.7% and 92.1%) cohorts. Menopausal status, elevated Ki-67, higher nuclear grade, higher tumor stage, presence of lymphovascular invasion, greater regional lymph node involvement, and larger tumor size were all potential prognostic factors for shorter DFS and OS.
Conclusion:Patients with ER-Low/HER2− breast cancer had similar clinicopathological characteristics, treatments, and outcomes as patients with TNBC irrespective of disease setting. Further research is needed to understand predictive and prognostic factors associated with ER-Low/HER2− disease.