Omitting adjuvant endocrine therapy (ET) in patients with estrogen receptor (ER)-low (1%-10% by IHC) early-stage breast cancer may be associated with increased mortality, according to a retrospective analysis of 7018 women enrolled in the National Cancer Database. Most patients had aggressive tumor features, including grade 3 disease (74%), PR-negativity (73%), and HER2-negativity (65%). All underwent surgery and chemotherapy, with a 42% ET omission rate one year postoperatively.
Multivariable analysis showed ET omission was more likely in patients with PR-negative, HER2-negative, high-grade tumors, elevated Ki-67, and node-negative disease. Among those receiving neoadjuvant chemotherapy, omission was higher in patients with residual disease than in those achieving pathologic complete response (pCR). ET omission was also more frequent in patients diagnosed during 2020, possibly reflecting pandemic-related treatment changes.
At a median follow-up of 3 years, ET omission correlated with worse overall survival (OS) (HR, 1.23; P = .02). This risk was significant particularly among patients with residual disease post-neoadjuvant chemotherapy (HR, 1.26; P = .046) and those with ER expression of 6%-10% (HR, 1.42; P = .048). These findings support the continued use of ET in ER-low breast cancer until prospective evidence clarifies optimal management.
