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Background/Objectives: Ki-67 is widely used as an immunohistochemical marker of tumor proliferation in hormone receptor-positive (HR-positive), HER2-negative breast cancer, but its interpretation is limited by variability and uncertain concordance with genomic assays. The Oncotype DX® Recurrence Score (RS) is a validated multigene assay with established prognostic and predictive utility. This study evaluated the relationship between Ki-67 and RS in clinical practice. Methods: We retrospectively analyzed women in Greece with early-stage estrogen receptor-positive, HER2-negative breast cancer without distant metastasis (pM0) who underwent Oncotype DX testing between 2020 and 2023. Eligible patients were node-negative or postmenopausal with node-positive disease. RS was categorized as low (0–25) or high (>25). Ki-67 was assessed using binary (5–<30%, ≥30%) classifications. Associations were analyzed using correlation, concordance, and non-parametric methods. Results: Among 2967 patients, the median RS was 16, with similar distributions across nodal subgroups. Ki-67 and RS demonstrated a modest positive correlation as continuous variables (R = 0.30, p < 0.001). After stratification, associations with RS were observed only in tumors with high Ki-67 expression, whereas no correlation was detected in low or intermediate groups. RS distributions differed significantly across Ki-67 strata. Overall concordance between Ki-67-based proliferation categories and RS-based genomic risk was 56.2%, with discordant cases in both directions. Conclusions: Ki-67 shows a modest association with Oncotype DX RS, but substantial discordance indicates Ki-67 should not substitute genomic testing in HR-positive/HER2-negative early breast cancer.
Dragoumis et al. (Tue,) studied this question.