Abstract PS3-11-18: Neoadjuvant Response in HER2+ Early Breast Cancer: A Comparative Study Between HER2-Enriched and Luminal HER2 Subtypes

Abstract Introduction: HER2-positive breast cancer represents approximately 15–20% of all breast tumors and is characterized by amplification of the ERBB2 gene, associated with aggressive biological behavior. However, this group includes biologically distinct subtypes based on hormone receptor (HR) status. Tumors lacking HR expression (HER2-enriched) typically show increased sensitivity to neoadjuvant therapy (NAT), while HR-positive tumors (Luminal HER2) often exhibit lower rates of pathological complete response (pCR), despite favorable long-term outcomes in some cases. We aimed to compare pathological complete response (pCR) and survival outcomes between Luminal HER2 (HER2+/HR+) and HER2-enriched (HER2+/HR−) breast cancer subtypes following neoadjuvant therapy (NAT) in the real world setting. Methods: All HER2-positive breast cancer treated with NAT and surgery from 2007 to 2018, who received at least 1 dose of neoadjuvant trastuzumab, in a single center were analyzed. Tumors were classified as Luminal HER2 or HER2-enriched based on immunohistochemistry. Endpoints included pCR, defined as absence of invasive disease in breast and axilla (ypT0/is ypN0), relapse-free survival (RFS) and overall survival (OS). Associations between categorical variables were assessed using chi-square and Fisher’s exact tests. Survival was estimated using Kaplan-Meier curves and factors associated with RFS were evaluated in a multivariate analysis. Bilateral p-values