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

Abstract

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.

Figure 1

Fitted dose–response functions for thyroid cancer incidence in the LSS cohort. The solid line is the fitted linear ERR dose response, and the dashed curve is the fitted ERR based on linear-exponential dose–response model. The points are non-parametric estimates of the ERR in dose categories with 95% CIs. The line and points are all gender-averaged estimates at age 60 after exposure at age 10.

Figure 2

Fitted temporal patterns and age-at-exposure variation in the radiation-associated risk for thyroid cancer in the LSS cohort. The upper panel presents the ERR at 1 Gy and the lower panel the fitted EAR at 100,000 person-years-Gy, with attained age for ages at exposure of 5 (“0–9” for categorical estimates), 15 (“10–19”), 25 (“20–29”) and 41 (“30–”) years. All curves and points are gender-averaged estimates.