Increased Risk Of Thyroid Diseases Linked To Exposure At Chernobyl, Study Shows

The Chernobyl accident in 1986 was the largest nuclear accident ever and exposed many individuals to radioactive iodines. Thyroid cancer is one of the most radiosensitive tumors when exposure occurs at young ages. Previous studies showed that the risk of thyroid cancer increased with radiation dose from radioactive iodines, but the effects of radiation on benign thyroid diseases has been largely unknown.

Researchers selected a random sample of 32,385 persons from a database of more than 75,000 records of thyroid radioactivity measurements taken within two months after the incident in those under the age of 18 who resided in three heavily contaminated areas in Ukraine. Various methods were used to trace these subjects and invite them for screening which consisted of an examination of the thyroid gland and sonogram, blood and urine tests, a detailed questionnaire, and an independent clinical examination by an endocrinologist.

Those with suspicious findings were further referred for fine-needle biopsy and surgery as needed. The scientists reported a significant three-fold increase in risk for those exposed to the standard measure of 1 gray of radiation compared to those with zero dose. The study further indicates that women had a notably higher risk of follicular adenoma compared with men.

"The Chernobyl accident presented an unparalleled opportunity to study the association between radioactive iodines and a spectrum of thyroid diseases," said Lydia B. Zablotska, MD, PhD, principal investigator of the project and assistant professor of Epidemiology at the Mailman School of Public Health. "While there have been several analytical epidemiologic studies of thyroid cancer following the Chernobyl accident, none had evaluated follicular adenoma in particular. This paper presents risk estimates of follicular adenoma in relation to individual thyroid doses as well as the effects of gender, age at exposure, iodine deficiency, and other possible effect-modifying factors."

The researchers note that the radioactive iodines accumulated in the human thyroid gland via the consumption of contaminated milk and other food items. Various earlier studies have shown that the populations of Ukraine, Belarus, and the Russian Federation experienced large increases of thyroid cancer in those exposed to the fallout before the age of 18 years. This increase became apparent 4--5 years after the accident and confirmed previous suggestions.

Also among the major strengths of the Mailman School study are the availability of individual thyroid doses based on thyroid radioactivity measurements made within two months after the accident and the data collected during screening. While the researchers caution that these results are based on a small number of cases, "our findings are further strengthened by the high fine-needle biopsy compliance rate of 93 percent. Because all those referred for surgery following fine-needle aspiration biopsy have been operated upon, it is unlikely that follicular adenoma was underdiagnosed," Dr. Zablotska said.

Ukraine is known to have a mild to moderate iodine deficiency previously shown to affect radiation-related risks of thyroid cancer. In this study several indicators of iodine deficiency such as place of residence, history of thyroid diseases, and current urinary iodine excretion did not modify the risk of follicular adenoma.

"Our findings confirm that follicular adenoma is strongly related to exposure from radioactive iodines, and while we found that the risk is modified by gender, age at exposure, place of residence, and personal history of thyroid diseases do not affect its subsequent development," said Dr. Zablotska.

The paper is published in the February 2008 issue of the American Journal of Epidemiology. The research was supported by funding from the National Cancer Institute.