Low ER plus Breast Cancer Is This a Distinct Group?

Objectives: Estrogen receptor (ER) level can be semiquantified by immunohistochemistry (IHC) using the H-score. The score, given as the sum of the percent staining multiplied by the intensity level, ranges from 0 to 300. Methods: Forty-nine ER+/HER2- invasive tumors with low ER expression (H-scores of 1-100, representing approximately 5% of all tumors) were studied for various morphologic parameters, progesterone receptor (PR), and Ki-67 IHC. Results: Eighteen of 49 patients received neoadjuvant chemotherapy. The morphologic analysis showed that these tumors are often grade 3 and frequently demonstrate a sheet-like growth pattern, an intratumoral lymphocytic inflammatory infiltrate, and necrosis. Eighty percent of tumors showed a Ki-67 proliferation index of more than 50%, and 94% were PR-. Of the 18 patients who received neoadjuvant chemotherapy, six (33%) achieved pathologic complete response. Conclusions: The low ER+/HER2- cases have morphologic features and a response to the chemotherapy rate that are more similar to triple-negative tumors than the usual type of ER+ tumors.