Supplementary Material for: Proliferation-Guided Risk Stratification in HER2-Low vs. HER2-Positive Hormone Receptor-Positive Breast Cancer: A Multicenter Retrospective Analysis

Background: HER2-low breast cancer is a newly characterized subgroup with unclear prognostic implications. This study investigates the prognostic role of Ki67 in hormone receptor-positive (HR+) HER2-low and HER2-positive breast cancer. Methods: We retrospectively analyzed 224 HR+ breast cancer patients from four tertiary centers (96 HER2-low, 128 HER2-positive). Patients were stratified into Stage I–III (n=156) and Stage IV (n=68). Survival outcomes were assessed according to HER2 and Ki67 status using Kaplan–Meier analysis and Cox regression. Results: HER2-low patients were older (≥65 years: 65% vs. 28%, p