Male breast cancer incidence among atomic bomb survivors.

To learn more about the role of ionizing radiation in the development of male breast cancer, we evaluated male breast cancer incidence among 45 880 male members of the Life Span Study cohort of Japanese atomic bomb survivors. Male breast cancers, diagnosed between January 1, 1958, and December 31, 1998, were identified through the Hiroshima and Nagasaki Tumor Registries. Nine male breast cancers were diagnosed among exposed Life Span Study members (crude rate = 1.8 per 100,000 person-years), and three were diagnosed among nonexposed cohort members (crude rate = 0.5 per 100,000 person-years). A statistically significant dose-response relation was observed (excess relative risk per sievert = 8, 95% confidence interval = 0.8 to 48; P = .01). Our finding of a statistically significant association between ionizing radiation and male breast cancer incidence adds to the very limited information that shows an association between radiation exposure and an increased risk of male breast cancer.