Trends in Female Breast Cancer Incidence among Japanese, Korean, and U.S. Populations: An Age-Period-Cohort Analysis.

BACKGROUND: Breast cancer incidence has historically been higher in Western countries; however, rates have risen substantially in East Asia, with variation by age and generation. Direct comparisons between native Asians and their U.S. counterparts remain limited. Therefore, the age-period-cohort (APC) effects of breast cancer incidence were examined among native Japanese women, with comparisons with Japanese Americans, native Korean women, Korean Americans, and U.S. White and Black women. METHODS: We analyzed population-based cancer registry data from Japan (1985-2019), Korea (1993-2017), Hawaii (1988-2017), Los Angeles County (1988-2017), and SEER 8 (1985-2019). Women aged 25-84 years were grouped into 5-year age groups and calendar periods, and APC analyses were conducted using the NCI APC web-based tool. RESULTS: Incidence increased with age in all populations; however, the patterns differed. Native Japanese and Koreans had the lowest overall rates with an early-midlife peak, whereas Japanese Americans resembled U.S. Whites, with incidence rising into older ages. Period and cohort effects were strongest among native Japanese and Koreans, with sustained increases among women born after 1950. Annual incidence increases were highest among native Koreans (5.3%) and native Japanese (3.8%), followed by Korean Americans (2.8%) and Japanese Americans (1.6%), while U.S. Whites were stable or declining and U.S. Blacks showed modest increases. CONCLUSIONS: These findings indicate an ongoing epidemiologic transition in East Asia and demonstrate how migration and generational change shape breast cancer risk across populations. IMPACT: Direct comparison of native and U.S. Asian populations reveals migration-related divergence in breast cancer risk.