Prognosis and endocrine therapy efficacy in early breast cancer with estrogen receptor low expression.

[PURPOSE] To investigate the prognosis and endocrine therapy (ET) efficacy in patients with estrogen receptor (ER)-low breast cancer.

[METHODS] This retrospective cohort study included patients diagnosed with early-stage breast cancer at a single institution in China from January 2010 to December 2020. Clinicopathological features, survival outcomes, and the effect of ET were compared among patients with ER-low, ER-high, and ER-negative breast cancer. Analyses were stratified by human epidermal growth factor receptor 2 (HER2) status (positive or negative).

[RESULTS] A total of 2951 patients were included, consisting of 131 (4.4%) ER-low, 2040 (69.1%) ER-high, and 780 (26.4%) ER-negative patients. In the HER2-negative subgroup, ER-low patients had significantly worse breast cancer-free survival (BCFS) (P = 0.016) and a higher risk of locoregional recurrence (P < 0.001) compared to ER-high patients, while no significant differences were observed in prognosis between ER-low and ER-negative patients. In the HER2-positive subgroup, there were no significant differences in prognosis among patients with different ER expression levels. ET could significantly reduce the risk of locoregional recurrence and distant metastasis and significantly improve BCFS for ER-low patients, especially in the HER2-negative subgroup. However, in the HER2-positive subgroup, ET did not improve BCFS or overall survival for ER-low patients.

[CONCLUSION] HER2-negative/ER-low breast cancer has a similar clinical behavior to triple-negative breast cancer, and ET could significantly reduce the risk of recurrence and metastasis for this subgroup. However, in HER2-positive patients, ER-low breast cancer lacks a predictive role in prognosis and derives no clear benefit from ET.