Exposure to Ionizing Radiation in Adulthood and Thyroid Cancer Incidence

Background: Recent reviews of the radiation epidemiology literature have concluded that there is little evidence that exposure to ionizing radiation in adulthood causes thyroid cancer. This paper examines the association between radiation dose and thyroid cancer incidence among Japanese survivors who were adults at the time of the atomic bombings of Hiroshima and Nagasaki. Methods: Analyses were conducted using data on cancer incidence during the period 1958-1998 among 59,687 members of the Life Span Study of atomic bomb survivors who were 20 years or older at time of bombings of Hiroshima and Nagasaki. Poisson regression methods were used to derive estimates of associations between thyroid dose in gray (Gy) and thyroid cancer incidence by sex, age at exposure, and time-since-exposure. Results: The number of thyroid cancer cases among women (n = 241) was nearly 5-fold the number of cases among men (n = 55). Estimated thyroid dose was positively associated with thyroid cancer incidence among women A-bomb survivors (excess relative rate/Gy = 0.70; 90% confidence interval = 0.20-1.46). In contrast, a negative association was observed between thyroid dose and thyroid cancer among men (excess relative rate/Gy = -0.25; <0 to 90% confidence interval = 0.35). The association among women was positive for all time-since-exposure periods examined but tended to diminish in magnitude with time-since-exposure. Conclusions: Exposure to ionizing radiation in adulthood was positively associated with thyroid cancer among women A-bomb survivors, although the risk seems to be lower than for those exposed to radiation in childhood.