Exposure to radioactive iodines in childhood is associated with an increased risk of thyroid cancer

Exposure to radioactive iodines, mainly iodine 131 (I-131), in childhood is associated with an increased risk of thyroid cancer. Importantly, both iodine deficiency and supplementation appear to modify this risk, according to a new study in the May 18 issue of the Journal of the National Cancer Institute.

The accident at the Chernobyl nuclear power plant in April 1986 resulted in widespread radioactive contamination, particularly in parts of Belarus, the Russian Federation, and the Ukraine. For people living in these areas, the main radiation dose was to the thyroid and came from exposure to I-131 from drinking contaminated milk. (The thyroid uses iodine to make thyroid hormone.)

It has been estimated that the thyroids of several thousand children in Belarus received I-131 doses of at least 2 Gray, a unit of absorbed radiation dose. (People are usually exposed to a background radiation from natural sources of only 1 to 2 mGy per year.) In addition, a very large increase in the incidence of thyroid cancer in young people was observed as early as 5 years after the accident in Belarus and slightly later in the Russian Federation and the Ukraine. However, important questions remained about the magnitude of the potential modifying effect of iodine deficiency, which was common in most of the affected areas at the time of the Chernobyl accident.

To evaluate the risk of thyroid cancer after exposure to radioactive iodine in childhood and investigate factors that might modify this risk, Elisabeth Cardis, Ph.D., of the International Agency for Research on Cancer in Lyon, France, and colleagues conducted a case-control study of 276 thyroid cancer patients and 1,300 control subjects in Belarus and the Russian Federation who had been age 15 years or younger at the time of the Chernobyl accident.

The researchers observed a strong dose-response relationship between radiation dose to the thyroid received during childhood and the risk of thyroid cancer. This risk was three times higher in iodine-deficient areas than in other areas. Potassium iodide supplementation was associated with one-third the risk of radiation-related thyroid cancer compared with no supplementation. Potassium iodide was used in the former Soviet Union for goiter prophylaxis and was distributed, mainly in Belarus, to children evacuated after the Chernobyl accident.

"Both iodine deficiency and iodine supplementation appear to be important and independent modifiers of the risk of thyroid cancer after exposure to I-131 in childhood. This result has important public health implications in the case of exposure to radioactive iodines in childhood that may occur after radiation accidents or during medical diagnostic and therapeutic procedures. Indeed, stable iodine supplementation in iodine-deficient populations may reduce the subsequent risk of radiation-related thyroid cancer in these situations," the authors write.

In an editorial, John D. Boice Jr., of the International Epidemiology Institute in Rockville, Md., and the Vanderbilt University School of Medicine in Nashville, Tenn., raises questions for future research on the association between thyroid cancer and exposure to I-131 in childhood. These new findings, he writes, provide "new and, if confirmed, provocative information on the risk of radiation-induced thyroid cancer and on the modifying role of diets deficient in stable iodine and of administering iodine supplements months after the exposure has occurred."