Impact of estrogen receptor expression levels on chemo-responsiveness and prognosis of breast cancer patients treated with neoadjuvant chemotherapy

This study aimed to investigate the clinicopathological characteristics, response to chemotherapy, and clinical outcomes according to estrogen receptor (ER) expression levels in breast cancer (BC) patients treated with neoadjuvant chemotherapy (NAC). ER expression levels were categorized as ER-negative (expression in <1% of tumor cells), ER-low positive (1-10%), ER-intermediate positive (11-50%), and ER- high positive (>50%). Of the 1,365 cases, 647 (47.4%) were classified as ER-negative, 49 (3.6%) as ER- low positive, 48 (3.5%) as ER-intermediate positive, and 621 (45.5%) as ER-high positive BCs in pre-NAC biopsies. ER-intermediate positive tumors as well as ER-low positive tumors showed no differences in clinicopathological characteristics compared to ER-negative tumors with the exception of progesterone receptor positivity. While ER-low positive and ER-negative tumors showed similar chemo-responsiveness, ER-intermediate positive tumors were less responsive to NAC compared to ER-negative tumors. In patients with residual disease, pre- and post-NAC ER expression levels were found to be independent prognostic factors, but with no significant differences among ER-negative, ER-low positive, and ER-intermediate positive tumors. Our study indicates that ER-low positive BCs are similar to ER-negative BCs and that ER-intermediate positive BCs exhibit characteristics heterogeneous between ER-negative and ER-high positive BCs in terms of clinicopathological characteristics, chemo-responsiveness, and clinical outcomes.