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

Breast cancer incidence has historically been higher in Western countries; however, rates have increased substantially in East Asia, with variation by age and generation. Direct comparisons between native Asians and their US counterparts remain limited. We examined the effects of age-period-cohort (APC) on breast cancer incidence among native Japanese women, with comparisons with Japanese Americans, native Korean women, Korean Americans, and US White and Black women.

We analyzed population-based cancer registry data from Japan (1985-2019), Korea (1993-2017), Hawaii (1988-2017), Los Angeles County (1988-2017), and Surveillance, Epidemiology, and End Results 8 (1985-2019). Women 25 to 84 years of age were grouped into 5-year age groups and calendar periods, and APC analyses were conducted using the NCI APC web-based tool.

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 US Whites, with incidence increasing 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%), whereas US Whites were stable or declining and US Blacks showed modest increases.

These findings indicate an ongoing epidemiologic transition in East Asia and demonstrate how migration and generational change shape breast cancer risk across populations.

Direct comparison of native and US Asian populations reveals migration-related divergence in breast cancer risk.