Defining the biology of estrogen receptor-low-positive breast cancer

Analysis of The Cancer Genome Atlas, the National Cancer Database, and the Dana-Farber/Brigham Cancer Center institutional database was carried out to better define the biology of ER-low-positive (1-10%)/HER2-negative breast cancer. In TCGA, 82.6% of these tumors ware basal and 4.3% were luminal. Among patients with ER-low-positive/HER2-negative disease in the NCDB with availabe Oncotype DX Breast Recurrence Score (RS) results, 67% had an RS ≥ 26. In the DF/BCC database, only two patients with ER-low-positive/HER2-negative breast cancer and an RS were identified; both with RS ≥ 26; analysis of archived tissue of patients with ER-low-positive/HER2-negative tumor treated at DF/BCC found RS ≥ 26 in 97.4%. The three-year breast cancer-specific survival was 97.0% (95% CI 96.9-97.1). The authors conclude that most ER-low-positive-HER2-negative breast cancers are basal-like with RS ≥ 26%.