Title: Contemporary Trends in Breast Cancer in Females Under the Age of Fifty: An NCDB Study

Introduction Breast cancer among patients under 50 years old accounts for 18% of new cases. Few studies have reported current trends in clinical-pathologic features and treatment patterns for young patients. We evaluated these trends in a modern cohort of breast cancer patients under 50. Methods We identified women with breast cancer from the National Cancer Database from 2004-2017. Patients were grouped into 18-29, 30-39, 40-49, and ≥ 50-year cohorts. Proportions and temporal comparisons between demographic, clinicopathologic features, and treatment types were evaluated. Temporal trends across sequential periods were performed. Results Of the 2,387,902 patients selected, 554,941 (23.3%) were younger than 50. During 2004-2017, the proportions remained stable in the 18-29 (0.5-0.6%) and 30-39 (4.5-5%) age groups, while decreasing in the 40–49 group (absolute difference: -4.8%, p<0.001). Overall, in those younger than 50, early-stage breast cancer (clinical stage 0-II) increased by 3.9%, while stages III and IV decreased by 2.7% and 1.3% (p<0.001), respectively. Mastectomy rates and neoadjuvant systemic therapy use increased by 10.4% and 9.8%, respectively (p<0.001) in all groups under 50. Conclusions Despite stable proportions in the youngest age groups (18-29 and 30-39), a noteworthy decrease in the 40-49 age group was observed, suggesting potential shifts in disease detection. The rise in early-stage disease and neoadjuvant systemic therapies should theoretically translate into an increase in the number of breast-conserving candidates. However, the increase in mastectomies highlights the need to better understand the factors influencing treatment decisions in this population.