Abstract PS4-08-04: Impact of adjuvant chemotherapy in the survival outcomes of early-stage hormone receptor (HR)-positive HER2-negative lobular versus ductal carcinoma patients

Abstract Background: Invasive lobular carcinoma (ILC) represents 10-15% of all breast cancers (BC) and differs biologically and clinically from invasive ductal carcinoma (IDC). Conflicting data exist with regard to variation in prognosis and benefit of chemotherapy (CT) between these histologic subtypes. Here, we compare the impact of adjuvant CT on survival outcomes in patients (pts) with early-stage (ES) HR+/HER2- ILC and IDC. Methods: We queried the National Cancer Database for pts who received upfront surgical resection for ES HR+ HER2- invasive BC between 2010 and 2021, classifying cohorts into ILC or IDC subtypes. Cox proportional hazards models assessed the relationship between overall survival (OS) and histology type adjusting for potential confounders including age, race, stage, grade, and treatment variables. Subgroup analyses were performed based on CT receipt and Oncotype DX recurrence score (RS), a genomic assay used to predict response to CT, categorized as (low (L) 0-15, intermediate (I) 16-25, high (H) >=26). Results: Of the 1,101,920 pts included, 14.7% (n= 162,430) had ILC. Compared to pts with IDC, the ILC cohort was comprised of a greater proportion of Whites (87.7 vs 85.4%), intermediate-grade (63.7 vs 48.8%), clinical T2 (26.5 vs 22.1%) and N0 disease (89.1 vs 88.3%) (all p