Iodine-131 (131I) and other radioisotopes of iodine are contained in fallout from atmospheric nuclear weapons tests and are among the radionuclides most likely to be released in a nuclear reactor accident. In spite of nearly 50 years of experience the risk of thyroid disease, especially thyroid cancer, attributable to exposure to 131I remains unknown, although the carcinogenic potential of x-ray and gamma-ray exposure of the thyroid is reasonably well known. The available data also indicate that children face greater risks of radiation-induced thyroid cancer than do adults.

The nuclear power plant accident at Chernobyl on 26 April 1986 led to the release of large quantities of 131I and other radioisotopes of iodine into the atmosphere, contaminating thousands of square kilometers and exposing millions of people. Well-defined subsets of Belarusian and Ukrainian children aged 0-18 years or in utero at the time of the accident are being examined by well-trained specialists for thyroid disease at regular intervals. The screening program began in 1998. About 40,000 children who had their thyroids measured for radioactivity during the weeks immediately following the accident were identified in Belarus and about 70,000 were identified in Ukraine. Under a rigid research protocol a sample of approximately 12,000 of these children in each country receive complete diagnostic thyroid examinations, including palpation, ultrasound scanning, thyroid hormone and other laboratory tests, and fine-needle aspiration, as appropriate. Tissue diagnoses are determined by expert pathology examination. In addition to the analysis of thyroid radiation measurements made in May-June, 1986, efforts will be made to reconstruct each person's exposure and to estimate the radiation dose to the thyroid due to intakes of 131I and to irradiation from other radionuclides released during the accident. This will involve the reconstruction of deposition patterns and environmental pathways of the radioiodines, and of the location, dietary characteristics, and lifestyle of each person throughout the exposure period.

The data will be analyzed to evaluate the relationship between thyroid disease, particularly thyroid cancer, and the radiation dose to the thyroid among those exposed as children, with emphasis on the dose from 131I. The primary focus will be on dose-response analyses of person-year incidence data with stratification by gender, age at exposure, geographic area, time, and age at risk. The risk of thyroid cancer from exposure to radioiodines will be compared with the known risk from exposure to x-ray and gamma. Confounding factors, e.g., use of potassium iodide (KI) as a prophylactic measure, will be evaluated and controlled in the analysis, and the uncertainty of the dose estimates will be taken into account.

In addition to producing risk coefficients for thyroid cancer and other thyroid diseases in children as a function of gender and age at the time of exposure, it is expected that the analyses will contribute new knowledge of the carcinogenic effectiveness of 131I in comparison with that of x-ray and gamma radiation. This information will fill a major gap in the world's knowledge of radiation effects, and will provide guidance for radiation protection and public health policies wherever nuclear reactors are in operation.