Racial disparities in cancer-specific survival of estrogen receptor–negative, progesterone receptor–positive breast cancer: A focus on the American Indian population.

e12708 Background: Estrogen receptor–negative, progesterone receptor–positive (ER−/PR+) breast cancer is a raresubtype (2–8% of cases) that remains biologically incompletely characterized. Prior studiessuggest aggressive behavior and shared molecular and morphologic features with triple-negativebreast cancer, yet racial and ethnic disparities in outcomes, particularly among AmericanIndian/Alaska Native (AI/AN) patients, are poorly defined. We used population-based data tocompare cancer-specific survival (CSS) between ER−/PR+ and ER+/PR+ breast cancer acrossracial and ethnic groups. Methods: Female patients with first primary invasive, HER2-negative, PR-positive breast cancer diagnosedbetween 2010 and 2022 were identified from the SEER 17 database. ER status was classified aspositive or negative; PR-negative, borderline, and unknown cases were excluded. Patientsidentified by death certificate only or autopsy only were excluded from survival analyses. CSSwas estimated using Kaplan–Meier methods. Cox proportional hazards models compared ER-negative versus ER-positive disease, adjusting for age, stage, and year of diagnosis, and werestratified by race/ethnicity. Analyses were conducted using SEER*Stat v9.0.42.0 and SAS v9.4. Results: Across the cohort, ER-negative status was associated with inferior CSS compared with ER-positive disease (adjusted HR 3.3, 95% CI 3.1–3.6; p<0.001). This association was observedacross all racial and ethnic groups. Adjusted HRs were 3.5 for non-Hispanic Whites, 2.4 for non-Hispanic Blacks, and 2.5 for AI/AN patients, though estimates for AI/AN patients were limitedby small sample size and wider confidence intervals. Notably, ER-positive disease demonstratedexcellent five-year CSS among AI/AN patients, comparable to that observed in White patients. Conclusions: In this population-based analysis of PR-positive breast cancer, ER-negative status was associatedwith worse cancer-specific survival across racial and ethnic groups. ER-positive disease wasassociated with favorable outcomes among AI/AN patients compared to outcomes observed inWhites, challenging assumptions of uniformly poor prognosis in this population. These findingshighlight heterogeneity within PR-positive breast cancer and underscore the need for furtherstudy in understudied populations. Adjusted Cox proportional hazards models for breast cancer–specific survival comparing ER-negative versus ER-positive disease within progesterone receptor–positive breast cancer stratified by race/ethnicity. Group HR (95% CI) P-value Overall (adjusted) 3.3 (3.1–3.6) <0.001 Non-Hispanic White 3.5 (3.2–3.9) <0.001 Non-Hispanic Black 2.4 (2.0–2.8) <0.001 American Indian/Alaska Native 2.5 (1.1–5.9) 0.032 Asian/Pacific Islander 2.4 (1.7–3.3) <0.001 Hispanic (all races) 3.5 (3.0–4.2) <0.001