Abstract C039: Nativity and breast cancer outcomes by breast cancer subtype in the ENCLAVE Study

Abstract Background: Associations of nativity and breast cancer (BC) mortality are mixed in studies using cancer registry data. We examined associations in diverse women with detailed individual-level data on BC treatment and subtype. Methods: The study population included 5,718 Asian (n=722, 71% foreign-born), Hispanic (n=631, 42% foreign-born), and non-Hispanic White (n=4,365, 9% foreign-born) women from the ENCLAVE Study diagnosed with invasive BC from 1996-2013 using pooled, harmonized data from two (of three) cohorts with data on BC subtype. Data on nativity, reproductive and behavioral factors were self-reported from questionnaires; data on disease severity, BC subtype (luminal A, luminal B, Her2 expressing, triple negative breast cancer), treatment, and mortality were obtained from cancer registries and electronic health records. Recurrences were identified by recurrence algorithms and medical record review. We fit Cox proportional hazards regression models to examine associations between self-reported nativity and recurrence, BC-specific mortality, and overall mortality. Covariates included age, sociodemographic factors, disease severity, and treatment, reproductive and behavioral factors. We also examined associations by race, ethnicity, and BC subtype. Results: Compared to US-born women, foreign-born women had lower total mortality (hazard ratio [HR]=0.75, 95% confidence interval [CI]: 0.64-0.89); lower risks were suggested for recurrence (HR=0.85, 95% CI: 0.68-1.06) and breast cancer-specific mortality (HR=0.87, 95% CI: 0.66-1.14). Stratified by BC subtype, foreign- (vs. US-) born women had a lower risk of overall mortality among those with luminal A (HR=0.72, 95% CI: 0.56-0.92) and luminal B (HR=0.66, 95% CI: 0.48-0.91) but not other subtypes. Among women with luminal B BC, these associations reached statistical significance for recurrence (HR=0.65, 95% CI: 0.45-0.94) and BC-specific mortality (HR=0.53, 95% CI: 0.33-0.84). We noted no associations in other BC subtypes though analyses of less common subtypes were underpowered. Associations did not differ by race or ethnicity. Conclusion: Foreign-born women with BC had lower risk of overall mortality than US-born women. Risks of recurrence and BC-specific mortality were lower in foreign-born vs. US-born women among those with the luminal B BC subtype. Associations were not explained by behavioral or reproductive factors. Citation Format: Candyce Kroenke, Rhonda Aoki, Stacey Alexeeff, Scarlett Gomez, Bette Caan, Brittany Morey, Jacqueline Torres, Larry Kushi. Nativity and breast cancer outcomes by breast cancer subtype in the ENCLAVE Study [abstract]. In: Proceedings of the 17th AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2024 Sep 21-24; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2024;33(9 Suppl):Abstract nr C039.