BACKGROUND: In ER + /HER2- early breast cancer (BC), 21-Gene Recurrence Score (RS) >
25 indicates high-risk of distant-recurrence and predicts benefit from adjuvant chemotherapy (aCT) regardless of tumor-size. However, T1a/b (≤ 1 cm) node-negative (N0) tumors, regarded as of low risk of recurrence, were under-represented in the RS trials. We therefore aimed to investigate the benefit of aCT in patients with T1a/bN0 BC, RS >
25, where clinical and genomic risk indicators are discordant. METHODS: This retrospective observational cohort study utilized Israel's national Oncotest database to identify Clalit Health Services (CHS) members, diagnosed with T1a/bN0 HR + /HER2- BC, who underwent RS testing between February 2006, and December 2019. Patients with RS >
25 who received aCT were matched 1:1 by propensity-scoring to similar patients receiving no aCT. Invasive disease-free survival (iDFS) and distant recurrence were the study endpoints. Patient demographic and clinical data were obtained from CHS's centralized database. Kaplan--Meier analysis with log-rank testing was used for comparing outcomes. RESULTS: During the study period, high-risk RS result (>
25) was identified in 156/9858 patients of the study cohort. aCT was administered to 74 (47.4%) and median follow-up was 121 months. Within the 148 matched-cases, eighteen iDFS-events occurred, nine (12.1%) in each group (χ CONCLUSIONS: In this study cohort, patients with T1a/bN0 BC, RS >
25 that received aCT, did not have improved outcomes and the 21-Gene RS >
25 was not found to be predictive, possibly due to the low number of events observed.