Neoadjuvant Chemotherapy and Pathologic Complete Response in HR+/HER2− Breast Cancer: Impact of Tumor Ki67 and ER Status

<b><i>Introduction:</i></b> Neoadjuvant chemotherapy (NAC) is extensively employed in breast cancer (BC), primarily for aggressive subtypes like triple-negative and human epidermal growth factor receptor 2 (HER2)-positive BC and in estrogen receptor-positive (ER+)/HER2− BC with high-risk features. In ER+/HER2− BC, pathological complete rates are much lower (&lt;10%), while axillary dissection rates are higher. This study focuses on hormone receptor-positive (HR+)/HER2− BC patients undergoing NAC, examining its impact on pathological complete response (pCR) rates, with specific attention to tumor Ki67 and ER status. <b><i>Methods:</i></b> Retrospective data analysis from Kartal Dr. Lütfi Kırdar City Hospital included HR+/HER2− BC patients who received NAC. Clinicopathological factors, NAC response, and surgical outcomes were assessed. Statistical analyses evaluated the association between Ki67, ER status, and pCR. <b><i>Results:</i></b> Of 203 patients, 11.8% achieved pCR. Ki67 (<i>p</i> &lt; 0.001) and ER percentage (<i>p</i> &lt; 0.001) significantly correlated with pCR. Higher Ki67 was associated with increased pCR likelihood (HR: 1.03, 95% CI: 1.01–1.05). A Ki67-pCR probability curve revealed a cutoff of 23.5%. ER%-pCR analysis showed decreasing pCR rates with higher ER percentages. Multivariate analysis confirmed Ki67 (<i>p</i> = 0.003, HR: 1.02) and ER percentage (<i>p</i> = 0.019, HR: 0.97) as independent predictors of pCR probability. <b><i>Conclusion:</i></b> Consideration of Ki67 and ER percentage aids in NAC decisions for HR+/HER2− BC, identifying patients with high NAC response rates, facilitating axillary preservation, and potentially avoiding axillary dissection. The pCR rates in patients with Ki67 ≤24 are particularly low, especially in patients with a high ER percentage. In these cases, upfront surgery and adjuvant treatment should be considered instead of NAC.