Outcomes according to treatment received for small node-negative HER2+breast tumors in the Surveillance, Epidemiology, and End Results (SEER) database, 2010-2019.

517 Background: The magnitude of systemic therapy (tx) benefit for small, node-negative HER2+ breast tumors is unknown, with uncertainty about how benefit varies across stage I tumor sizes. We analyzed treatment patterns and outcomes for patients (pts) with stage IA (pT1N0) HER2+ breast cancer in the national SEER database. Methods: Pts with one lifetime diagnosis of pT1N0 HER2+ breast cancer between 2010-2019 were included. Receipt of chemotherapy (chemo) was categorized as yes vs no/unknown; receipt of endocrine tx is not available in SEER. All systemic tx was administered in the adjuvant setting. The primary outcome variable was breast cancer-specific survival (BCSS; recurrence events are not available in SEER), which was compared between treatment groups using a multivariate Cox model adjusted for age at diagnosis (dx), race, tumor grade/histology/size, hormone receptor (HR) status, radiation, marital status, income, rurality. Chemo use predictors were evaluated by multivariable logistic regression. Results: We identified 12,861 pts with pT1N0 HER2+ breast cancer; 9,513 (74%) HR-positive and 3,348 (26%) HR-. Median age at dx was 59 yrs. Median follow-up was 46 months. In the overall cohort, BCSS at 3, 5, and 7 yrs was: 99.3% (with chemo) vs 99.1% (without chemo); 98.6% (with chemo) vs 97.4% (without chemo); and 97.5% (with chemo) vs 96.4% (without chemo), respectively. Adjusted hazard ratio (HzR) for BCSS events in the chemo vs no chemo group of the overall cohort was 0.63 (p=0.004;). BCSS events by tumor size and HR status are shown; p values are not shown where event rates were too low to run an adjusted model. Chemo use increased significantly between 2010-2019 within each tumor size category (all p for trend < 0.04), except for T1a HR+/HER2+ and T1mi HR-/HER2+. Pts were less likely to receive chemo if they were older, Hispanic (vs non-Hispanic White), separated/divorced/widowed (vs single), or had lower median household income (vs >$75K). Pts were more likely to receive chemo if they had larger tumor size, HR- tumor, higher tumor grade, were married (vs single), or had later yr of dx. Conclusions: Receipt of chemo was associated with higher BCSS in the overall stage IA cohort and among pts with HR+/HER2+ stage IA tumors. Tumors ≤ 1cm had excellent outcomes with or without chemo. Sociodemographic factors including income, race, and marital status impacted chemo receipt. [Table: see text]