Abstract Introduction: Breast cancer (BC) in adolescents and young adults (AYAs), defined as individuals aged 15-39 years, poses distinct clinical and public health challenges as underrepresentation in guidelines or screening exclusion. It is the most common solid tumor in young women, contributing substantially to morbidity and mortality. AYAs are more likely to experience diagnostic delays, aggressive subtypes, and worse outcomes due to unique biological and psychosocial vulnerabilities. In Brazil, where ethnic and socioeconomic disparities are pronounced, understanding the drivers of BC mortality in young women treated in the public healthcare system is essential. This study investigates factors associated with mortality in a nationwide cohort of Brazilian women aged 15-39 years diagnosed with BC over a 23-year period. Methods: A secondary database study was conducted, including women aged 15 to 39 years who were diagnosed and treated for BC (ICD-10 C50) within Brazil’s public healthcare system, as recorded in the national Hospital Cancer Registry between 2000 and 2023. Non-analytic cases were excluded. The primary end point was death occurring after diagnosis and during the initial course of proposed treatment. Sociodemographic, clinical, and tumor-related variables were collected. Descriptive analysis was performed, and multivariable logistic regression using the Stepwise Forward method was applied to identify factors associated with the outcome. Variables with a p-value 0.05 were retained in the final model. Results: Of 66,726 eligible women, most were aged 30-39 (87.3%), self-identified as Black or mixed-race (55.4%), had ≥9 years of education (59.0%), and lived with a partner (54.3%). The majority were from the Southeast (44.8%) and Northeast (26.9%). Invasive carcinoma of no special type was the most common subtype (86.8%). At diagnosis, 40.3% were stage II and 36.7% stage III. Treatment initiation exceeded 60 days in 44.6% of cases. Neoadjuvant chemotherapy was administered to 39.5%, surgery to 49.3%, radiotherapy to 8.3%, and endocrine therapy to 2.0%. A total of 2,337 women (3.5%) died before or during the initial course of treatment. In adjusted analyses, younger age was associated with higher mortality, with a 2% reduction in odds of death per year of age (OR 0.98; 95% CI: 0.96-0.99; p = 0.003). Black or mixed-race women had a 40% higher risk of death (OR 1.40; 95% CI: 1.23-1.61), and those with lower education, a 36% increase (OR 1.36; 95% CI: 1.21-1.52). Living in the North, Northeast, or Central-West was associated with 30% higher mortality (OR 1.30; 95% CI: 1.14-1.47), and absence of a partner increased the risk by 23% (OR 1.23; 95% CI: 1.10-1.38). Advanced stage (III/IV) was the strongest predictor of death (OR 7.54; 95% CI: 6.37-8.93; p 0.001). Conclusion: This nationwide study demonstrates a high burden of early mortality among young Brazilian women with breast cancer, particularly those from marginalized racial, educational, and geographic backgrounds. Non-white ethnicity, low educational attainment, absence of a partner, and residence in underserved regions were independently associated with increased risk of death. Late-stage diagnosis was the most significant predictor. These findings underscore the urgent need for targeted public health interventions to promote earlier diagnosis, reduce treatment delays, and improve equity in breast cancer care across Brazil. Citation Format: A. Pires, A. Gonçalves, C. Resende, L. Thuler, M. Bello, A. Bergmann, G. Bretas. Early Mortality in Young Women with Breast Cancer: Insights from 66,000 Cases in the Brazilian Public Health System (2000-2023) abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-06-27.
Pires et al. (Tue,) studied this question.