Improved Prognosis in HER2-Low Breast Cancer Patients with Reduced Ki67 Index After Neoadjuvant Chemotherapy: A Multi-Center Retrospective Study.

Background: HER2-low breast cancer represents a distinct subgroup with unique clinical characteristics and treatment challenges. Nevertheless, it remains uncertain whether there exists a distinction in δKi67 between patients with HER2-low and HER2-zero statuses, and whether the prognosis varies among patients with differing HER2 statuses and δKi67. Methods: We conducted a multi-center retrospective study to investigate the correlation between alterations in Ki67 index following NAC and the prognosis among patients with HER2-low or HER2-zero breast cancer. 3 distinct cohorts comprising patients with HER2-negative breast cancer who underwent NAC were included. Comprehensive clinicopathological data were documented, with particular emphasis on evaluating changes in Ki67 index from baseline to post-NAC. These changes were then correlated with disease-free survival (DFS) through rigorous analysis. Results: Three cohorts, comprising 403, 315, and 72 patients respectively, were finally included. The study found that δKi67 did not show significant associations with other variables and were not identified as independent risk factors for survival. Nevertheless, across the three cohorts, following NAC, HER2-low breast cancer patients with δKi67 below the cut-off value demonstrated a better prognosis compared to those with δKi67 above the cut-off value. Additionally, their prognosis was also superior to that of HER2-zero breast cancer patients with δKi67 below the cut-off value. Conclusion: Our study demonstrates that HER2-low breast cancer patients with δKi67 values below the cut-off point after NAC are associated with improved prognosis. Monitoring δKi67 index and HER2 status may help identify patients who are likely to benefit from NAC and guide personalized treatment strategies.