Low Estrogen Receptor ( ER)-Positive Breast Cancer and Neoadjuvant Systemic Chemotherapy Is Response Similar to Typical ER-Positive or ER-Negative Disease?

Objectives: Pathologic complete response (pCR) rate after neoadjuvant chemotherapy was compared between 141 estrogen receptor (ER)-negative (43%), 41 low ER+ (13%), 47 moderate ER+ (14%), and 98 high ER+ (30%) tumors. Methods: Human epidermal growth factor receptor 2-positive cases, cases without semiquantitative ER score, and patients treated with neoadjuvant endocrine therapy alone were excluded. Results: The pCR rate of low ER+ tumors was similar to the pCR rate of ER-tumors (37% and 26% for low ER and ER-respectively, P=.1722) but significantly different from the pCR rate of moderately ER+ (11%, P=.0049) and high ER+ tumors (4%, P<.0001). Patients with pCR had an excellent prognosis regardless of the ER status. In patients with residual disease (no pCR), the recurrence and death rate were higher in ER-and low ER+ cases compared with moderate and high ER+ cases. Conclusions: Low ER+ breast cancers are biologically similar to ER-tumors. Semiquantitative ER H-score is an important determinant of response to neoadjuvant chemotherapy.