Global trends and regional disparities in young-onset breast cancer: Age-specific patterns from 1990 to 2021.

568 Background: Young-onset breast cancer (YOBC), defined as breast cancer in women under 40, is a growing global health concern with unique biological and clinical implications. Analyzing its patterns and distribution is essential for developing effective healthcare strategies and ensuring efficient resource allocation. Methods: Data on incidence, prevalence, mortality, and morbidity indicators (DALYs, YLD, and YLL rates per 100,000) were obtained from the Global Burden of Disease (GBD) 2021 database for women under 40 from 1990 to 2021. The dataset, covering 204 countries, was stratified into five age groups: 35–39, 30–34, 25–29, 20–24, and < 20 years. Linear regression was used to calculate average annual percent change (AAPC) and 95% confidence intervals (CI) for each indicator and age group. Statistical significance was set at p < 0.05. Results: Between 1990 and 2021, YOBC accounted for 16,783,674 deaths globally. Incidence increased across all age groups, with the largest increases in women under 30. AAPC for incidence was 2.27 (95% CI: 2.21–2.33) in those under 20, 2.18 (95% CI: 2.11–2.24) for ages 20–24, and 1.60 (95% CI: 1.51–1.70) for ages 25–29 (all p < 0.0001). Prevalence showed similar increases: 2.35 (95% CI: 2.29–2.41), 2.24 (95% CI: 2.18–2.30), and 1.64 (95% CI: 1.54–1.73), respectively (all p < 0.0001). Mortality AAPCs were 1.54 (95% CI: 1.46–1.61) for under 20, 1.35 (95% CI: 1.23–1.46) for ages 20–24, and 0.68 (95% CI: 0.59–0.76) for ages 25–29, while older cohorts saw declines, including -0.38 (95% CI: -0.53 to -0.23, p < 0.0001) for ages 35–39. DALYs also increased in younger groups, with AAPCs of 1.56 (95% CI: 1.49–1.63) for under 20 and 1.37 (95% CI: 1.26–1.48) for ages 20–24 but declined in older cohorts. Regionally, Turkey had the highest AAPCs across metrics, Malawi had the highest incidence and YLL for those under 20, and Zimbabwe, Yemen, and Lesotho reported the highest incidence, prevalence, YLL, and DALY. In contrast, Ukraine, Mariana Island and Armenia experienced the greatest declines in incidence, prevalence, YLL, and DALY. Conclusions: The global burden of YOBC has risen sharply, especially in the youngest cohorts (< 20 and 20–24 years), with significant increases in low- and middle-income regions like Turkey, Sub-Saharan Africa, and the Middle East. Age-specific interventions, early detection, and preventive measures are needed to address the growing YOBC rates.