Characteristics and Prognosis of Estrogen Receptor Low-Positive Breast Cancer

PURPOSE: The updated American Society of Clinical Oncology/College of American Pathologists guideline for estrogen receptor (ER) testing recommends that breast cancer with ER expression in 1–10% of tumor cells should be reported as ER-low positive (ER(low)), although limited data are available on the overall benefits of endocrine therapy. We investigated the clinicopathological characteristics and clinical outcomes of ER(low) breast cancer and to compare them with those of ER-negative (ER(neg)) and ER-high (> 10% of tumor cells, ER(high)) breast cancers. METHODS: Consecutive patients with invasive breast cancer who underwent curative surgery between November 2007 and December 2014 were included. Clinicopathological characteristics and disease-free survival (DFS) of ER(low) tumors were compared with those of ER(neg) and ER(high) tumors. RESULTS: Of the 2,309 cases included, 46 (2%), 643 (27.8%), and 1,620 (70.2%) were ER(low), ER(neg), and ER(high), respectively. ER(low) tumors were associated with no special type of histology (p = 0.011), advanced pT (p = 0.017), pN (p = 0.009) and anatomic stages (p < 0.001), high grade (p < 0.001), negative/low progesterone receptor (PR) status (p < 0.001), human epidermal growth factor receptor 2 positivity (p < 0.001), high Ki-67 (p < 0.001), and recurrence (p = 0.006) compared to ER(high) tumors. DFS was significantly dependent on ER status, and ER(low) tumors showed poorer DFS than ER(high) tumors (p = 0.001), however, there was no significant survival difference between ER(low) and ER(neg) tumors. Furthermore, DFS in ER(high) patients was affected by hormone therapy (p < 0.001), while it was not affected in ER(low) patients. CONCLUSION: Patients with ER(low) breast cancer have clinicopathological characteristics that differ from those with ER(high) tumors. Although this study was limited by the small sample size of the ER(low) group, no benefit from hormone therapy was observed in the ER(low) group compared with the ER(high) group.