Emerging cancer incidence trends in younger generations: age-specific trends and birth cohort effects from 25 years of Korean Cancer Registry data.

BACKGROUND: Using national cancer statistics from 1999 to 2023, this study aimed to identify the most common cancer types and temporal incidence trends among adults < 50 and ≥ 50. We further sought to estimate age-specific annual percentage changes (APCs) in cancer incidence and to examine birth cohort effects for major cancers. METHODS: Age-specific APCs were estimated and formally tested against the overall APC. Birth cohort effects were quantified as incidence rate ratios adjusted for age and calendar period, with the 1959 birth cohort as the reference. RESULTS: In 2023, thyroid cancer was the most common cancer in individuals younger than 50 years, whereas breast cancer in women and prostate cancer in men predominated among those ≥ 50 years. Age-specific APCs in cancer incidence did not align with the overall APC, indicating that incidence trends should not be summarized across all age groups. Notably, colorectal, thyroid, and kidney cancers exhibited greater annual increases in younger adults than in older adults. Birth cohort analyses indicated that, compared with the 1959 birth cohort, more recent cohorts exhibited higher risks of colorectal, thyroid, and kidney cancers in both sexes, as well as breast cancer in women. CONCLUSIONS: These findings indicate a shifting cancer burden toward younger populations in Korea, with notable birth cohort effects. Further studies are needed to identify underlying risk factors and inform age-appropriate prevention strategies.