Characterization of weakly hormone receptor (HR)-positive, HER- negative breast cancer (BC) and current treatment strategies

e12621 Background: HR and HER2 status is critical for determining initial management of BC. Current guidelines define estrogen receptor (ER) and progesterone receptor (PR)+ as ≥1%. Previous reports of small cohorts with low HR+/HER2- disease showed similar rates of pathologic complete response (pCR) following neoadjuvant chemotherapy (NAC) as triple negative BC (TNBC). This study aims to further characterize this group of patients (pts), focusing on modern management and breast/axillary pCR rates following NAC. Methods: Pts with newly diagnosed stage I-III, HR+/HER2- BC from 1/1/2009–4/1/2019 were found using the University of Wisconsin Hospital and Clinics Cancer Registry. Medical records were reviewed for demographics, tumor characteristics with quantification level of ER and PR ( < 33%), treatments including NAC and adjuvant chemotherapy (chemo), endocrine therapy, surgery and radiation, and follow-up clinical data. Results: Data reviewed from 2,905 pts: 2,604 (89.6%) were HR+/HER2-, 282 (9.7%) were ER+/PR-, and 19 (0.7%) were ER-/ PR+. A total of 64 pts [median age 54 (22-87), 100% female] met inclusion criteria. At diagnosis, 23 (36%) were anatomic stage 1, 30 (47%) stage II, and 11 (17%) stage III; 18 (28%) had biopsy (bx) proven nodal disease. 57 (89%) had invasive ductal carcinoma; 9 (14%) were ER+/PR+, 43 (67%) ER+/PR-, and 12 (19%) ER-/ PR+. The majority [51 (80%)] received chemo, ~ half with NAC 30 (48%), 1 pt chemo plan was unknown. NAC regimens included an anthracycline (A) and taxane (T) [26 (41%)], a sole A regimen [1 (1%)], and an A+T and platinum regimen [3 (5%)]. 4 pts received capecitabine after NAC. 50 (78%) pts had a sentinel lymph node bx, but 13 (20%) had an axillary node dissection (ALND). 10 (16%) pts did not receive any endocrine therapy, 28 (44%) got tamoxifen, 22 (34%) an aromatase inhibitor, and 4 (6%) unknown. Of 28 pts who had NAC followed by breast and axillary surgery, 12 (43%) had pCR (ypT0/Tis/ypN0). Of the 12 pts who had bx proven nodal disease at diagnosis and NAC, 7 (64%) pts had pCR at the axilla. One pt had progressive disease on NAC, 1 had local recurrence and 8 (13%) pts had distant recurrence. Median time to recurrence was 13.6 (5.6 – 48.7) months. Only 2 pts with pCR had distant recurrence. Conclusions: BC that are HER2- and weakly HR+ treated with NAC demonstrated an axillary and overall pCR rate more similar to TNBC than breast cancers with strong HR+. Neoadjuvant approaches may reduce need for ALND and pCR may provide important prognostic information. Clinical trials should be developed to focus on this unique patient cohort.