Long-term trend of thyroid cancer risk among Japanese atomic-bomb survivors: 60 years after exposure

Kyoji Furukawa, Dale Preston, Sachiyo Funamoto, Shuji Yonehara, Masahiro Ito, Shoji Tokuoka, Hiromi Sugiyama, Midori Soda, Kotaro Ozasa and Kiyohiko Mabuchi
Article first published online: 16 AUG 2012

DOI: 10.1002/ijc.27749


Thyroid cancer risk following exposure to ionizing radiation in childhood and adolescence is a topic of public concern. To characterize the long-term temporal trend and age-at-exposure variation in the radiation-induced risk of thyroid cancer, we analyzed thyroid cancer incidence data for the period from 1958 through 2005 among 105,401 members of the Life Span Study cohort of Japanese atomic-bomb survivors. During the follow-up period, 371 thyroid cancer cases (excluding those with microcarcinoma with a diameter <10 mm) were identified as a first primary among the eligible subjects. Using a linear dose–response model, the excess relative risk of thyroid cancer at 1 Gy of radiation exposure was estimated as 1.28 (95% confidence interval: 0.59–2.70) at age 60 after acute exposure at age 10. The risk decreased sharply with increasing age-at-exposure and there was little evidence of increased thyroid cancer rates for those exposed after age 20. About 36% of the thyroid cancer cases among those exposed before age 20 were estimated to be attributable to radiation exposure. While the magnitude of the excess risk has decreased with increasing attained age or time since exposure, the excess thyroid cancer risk associated with childhood exposure has persisted for >50 years after exposure.

Thyroid cancer is not frequent but is a focus of special attention in populations exposed to ionizing radiation, particularly when children are exposed. This is because the thyroid is known to be highly susceptible to the carcinogenic effect of radiation exposure in childhood or adolescence, with increased risks apparent from about 5 years after exposure.  Studies of Japanese atomic-bomb survivors and other populations have shown that the risk of thyroid cancer associated with radiation exposure tends to decrease with time since exposure or with increasing age-at-exposure. However, with little empirical data existing on the lifetime risk of radiation-related thyroid cancer, important questions remain about the duration and temporal pattern of the risk. Having extended the follow-up in the Life Span Study (LSS) cohort of atomic-bomb survivors through 2005, the current study analyzed the temporal patterns of thyroid cancer risk up to 60 years after exposure, which provides one of the longest thyroid cancer follow-ups of a general population following radiation exposure.