Hormone receptor (HR) and human epidermal growth factor receptor-2 (HER2) status is critical for determining management of breast cancer. Previous reports of small cohorts with weak HR-positive (HR+)/HER2-negative (HER2-) disease showed similar rates of pathologic complete response (pCR) following neoadjuvant chemotherapy (NAC) as triple negative breast cancer (TNBC). This study aims to further characterize this group, focusing on pCR rates following NAC.

Patients with stage I-III, HR+/HER2- breast cancer were identified using the University of Wisconsin Hospital Cancer Registry. Medical records were reviewed for demographics, tumor characteristics with quantification level of estrogen and progesterone receptor (≤33%), treatment, and follow-up data.

Data was reviewed from 2,900 patients and a total of 64 patients met inclusion criteria. Eighty percent received chemotherapy, about half with NAC (n = 30, 48%). Of 28 patients who received NAC followed by breast and axillary surgery, 12 (43%; 95% CI 25%-63%) had pCR (ypT0/Tis/ypN0). Of the 11 patients who had biopsyproven nodal disease at diagnosis and NAC followed by axillary surgery, 7 (64%, 95% CI 31%-89%) patients had pCR at the axilla. Only one patient with pCR developed recurrent disease. For those that recurred, median time to recurrence was 13.6 (5.6-48.7) months.

Breast cancers that are HER2- and weakly HR+ treated with NAC demonstrated pCR rate more similar to TNBC than breast cancers that are strong HR+. Neoadjuvant approaches may improve pCR rates, which provides important prognostic information. Clinical trials should be developed to focus on this unique patient cohort.