Thyroid cancer after exposure to radioactive 131I.

The thyroid gland is susceptible to radiation carcinogenesis, and the thyroid cancer risk decreases with increasing age at exposure, with a low risk above 20 years of age at exposure. The risk is best described be a linear dose-response relationship down to 0.1 Gy. Epidemiological studies of patients have not observed any increased risk for thyroid cancer after 131I exposure, but the statistical power to detect risks in children is limited. The Chernobyl accident led to substantial 131I exposure in Belarus, the Russian Federation and Ukraine. About 4000 cases of thyroid cancer have been diagnosed among those who were children and adolescents in 1986, including about 3000 in the age group 0-14 years. The risk per Gy from 131I in young subjects may be less than that seen after external low-LET radiation. A recent case-control study found a threefold risk for thyroid cancer among children from severely iodine-deficient areas, as compared with those living in lesser iodine-deficient areas. A threefold risk reduction was observed among those children receiving stable iodine compared with those not receiving iodine.