Family History of Cancer and Risks of Concordant and Discordant Cancers: A Nationwide Cohort Study of Korean Women.

Evidence on discordant familial cancer associations and their relevance in non-Western populations remains limited. This study assessed concordant and discordant cancer risks across major cancer sites in Korean women by comparing individuals with and without a family history of cancer. In total, 5,015,485 cancer-free women aged ≥ 40 years who participated in the national breast cancer screening program were included. Participants reported a family history of stomach, colorectal, liver, breast, cervical, or other cancers at screening. The risks of overall, site-specific, concordant (same cancer site as the family history), and discordant cancers (different cancer sites from the family history) were evaluated. Overall, a family history of any cancer was associated with a higher hazard of cancer (HR, 1.11; 95% CI, 1.10-1.12), with similar associations for cancers diagnosed before age 50 years (HR, 1.12; 95% CI, 1.09-1.14) and those diagnosed at age 50 years or older (HR, 1.13; 95% CI, 1.12-1.14). Elevated hazards associated with a family history were observed for most cancer sites, except for cervical, kidney, and brain cancers. Concordant associations were observed for gastric, colorectal, breast, and liver cancers (HRs, 1.36-2.43), whereas cervical cancer showed no significant concordant association. Modest but statistically significant discordant associations were observed across the five family history cancer types (HRs, 1.05-1.13). These associations were generally consistent across age groups. A family history of cancer was associated with both overall and site-specific cancer risk, with stronger associations for concordant cancers and more modest associations for discordant cancers.