The impact of adjuvant endocrine therapy on outcomes in estrogen receptor-low positive breast cancer patients.

Abstract

e12516 Background: Estrogen Receptor (ER)-low positive breast cancer, defined as ER expression of 1-10% by immunohistochemistry, is a rare subtype with molecular traits resembling ER-negative disease. While the 2020 American Society of Clinical Oncology (ASCO) guidelines recommend endocrine therapy for both ER-low and ER-high positive breast cancers, the benefit in the ER-low positive subgroup remains unclear. This study investigates the impact of adjuvant endocrine therapy on outcomes in Korean patients with ER-low positive, HER2-negative breast cancer. Methods: This retrospective cohort study patients with ER-low positive, HER2-negative breast cancer treated at Seoul National University Hospital between 2009 and 2023. Of 266 eligible patients, 247 received adjuvant endocrine therapy and 19 did not. Propensity score matching (PSM) was performed at a 1:1 ratio based on age, tumor stage, nodal stage, and chemotherapy administration. The primary endpoints were disease-free survival (DFS) and distant metastasis-free survival (DMFS). Multivariate analysis was conducted to identify variables affecting DFS. Results: The 5-year DFS was significantly higher in the endocrine therapy group compared to the non-endocrine therapy group (90.76% vs. 49.19%, p=0.00015), and this benefit persisted after PSM (p=0.026). Similarly, the 5-year DMFS was significantly improved in the endocrine therapy group both before (p=0.047) and after PSM (p=0.022). Multivariate analysis before PSM identified larger tumor size (HR 1.32, 95% CI 1.07-1.63, p=0.009), positive lymph node status (HR 2.71, 95% CI 1.07-6.87, p=0.036), and presence of lymphatic invasion (HR 3.62, 95% CI 1.67-7.85, p=0.001) as prognostic factors. After PSM, adjuvant endocrine therapy independently reduced the risk of recurrence (HR 0.0075, 95% CI 0.00008-0.71, p=0.035). Conclusions: This study demonstrates that endocrine therapy significantly improves survival outcomes in patients with ER-low positive, HER2-negative breast cancer. These findings support its clinical benefit in this distinct subtype and call for larger prospective studies to validate results and ultimately optimize treatment strategies.