Abstract A076: Using a counterfactual framework to understand the underlying causes of Black-White racial disparities in breast cancer mortality in the US

Abstract Background: Black women have the highest breast cancer (BC) mortality rate of any race group in the US. The disparity is often attributed to a higher prevalence of poor prognostic factors, such as estrogen receptor (ER)−negative tumors. However, racial disparities in BC survival are worse for patients with ER−positive tumors. Determining whether the BC mortality disparity is driven by the higher prevalence of poor prognostic factors or worse post-diagnosis survival can guide needed interventions. Methods: Incident case counts and survival data for non-Hispanic Black (NHB) and non-Hispanic White (NHW) patients diagnosed with invasive BC between 31 to 84 years old during 2010 to 2019 were obtained from the US Surveillance, Epidemiology, and End Results (SEER)-17 cancer registries. We calculated age-standardized incidence-based mortality (IBM) rates for both NHB and NHW women in 2019 using patient data and the 2019 population-at-risk data from SEER. We used a counterfactual framework to estimate the Black-White ratios of IBM (IBMBW) assuming equal distributions of ER status or equal hazard rates of BC death (overall and by ER status) among NHB and NHW women. Results: We included 50,922 NHB and 305,642 NHW BC cases in our analysis. Compared with NHW patients, NHB patients were more likely to have ER−negative tumors (28% vs. 15%). The estimated IBM rate per 100,000 for NHB women was 41.4 compared with 24.8 for NHW women (IBMBW = 1.67). When assuming equal distributions of ER status, IBMBW decreased from 1.67 to 1.43. When assuming equal hazard rates among NHB and NHW patients, regardless of ER status, IBMBW decreased from 1.67 to 0.87. Conclusion: Our preliminary results suggest the racial disparity in BC mortality can be eliminated when survival rates, but not subtype distribution, are matched among NHB and NHW patients, underscoring the importance of post-diagnosis survival for mitigating racial disparities. Citation Format: Maret L. Maliniak, Jeffrey M. Switchenko, Leah Moubadder, Rebecca Nash, Lindsay J. Collin, Lauren E. McCullough. Using a counterfactual framework to understand the underlying causes of Black-White racial disparities in breast cancer mortality in the US abstract. In: Proceedings of the 18th AACR Conference on the Science of Cancer Health Disparities; 2025 Sep 18-21; Baltimore, MD. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2025;34(9 Suppl):Abstract nr A076.