Abstract PS4-08-01: Overall Survival and Breast Cancer-Specific Survival in HER2-Low versus HER2-Zero Breast Cancer: A Single-Center Cohort Study

Abstract Background Human epidermal growth factor receptor 2 (HER2)-low breast cancer, defined as immunohistochemistry (IHC) 1+ or 2+ with negative in situ hybridization (ISH), represents approximately 40-50% of all breast cancers. Despite growing therapeutic interest, the prognostic significance compared to HER2-zero (IHC 0) tumors remains controversial. This study aimed to explore the clinicopathologic data and compare survival outcomes between HER2-low and HER2-zero breast cancers. Methods This retrospective cohort study analyzed data from breast cancer patients treated at the National Cancer Center in South Korea. From 13,785 patients diagnosed with breast cancer between 2005 and 2023, a total of 7,281 patients were included after excluding those with no surgery, distant metastasis at presentation, ductal carcinoma in situ (DCIS), follow-up duration