Four years after the Chernobyl accident on 26 April 1986, an increased incidence of thyroid carcinoma (cancer) was noticed in children in Belarus and the Ukraine. The increase has continued, with well over 500 cases reported from the areas around Chernobyl. Background Paper 2 considers the size of the increase, with account taken of ascertainment, of the evidence that the increase is related to radiation exposure due to the Chernobyl accident, of the isotope or isotopes responsible, and of the likely future effects.

External radiation is known to cause thyroid cancer, but radioactive iodine has been used for many years in the treatment of thyrotoxicosis without any increase in thyroid carcinoma. The majority of cases of thyroid carcinoma following exposure to X rays were papillary in type, the latent period was about 5-10 years, sensitivity decreased with increasing age at exposure, and the increase in the risk of induction of thyroid cancer persisted for decades.

Over 400 cases of thyroid cancer have occurred in children under the age of 15 in Belarus between 1990 and 1995 inclusive. The increase is greatest in the Gomel oblast, which had relatively high deposition densities. The rate in Gomel is about 200 times that seen in England and Wales, for example, for this normally rare tumour in children. About 150 cases have occurred in the `contaminated' northern oblasts of the Ukraine: an incidence about 20 times higher than in England and Wales, and 7 times higher than in the southern Ukraine. The diagnoses of thyroid cancer have been confirmed in over 90% of the cases available for study (over half of all cases are available); increased ascertainment of cases from screening has been excluded as a major contributory factor. Virtually all cases were papillary in type. The situation in the Russian Federation is less clear. In the oblasts most affected by radiation, there appears to have been an increase in childhood thyroid carcinoma, but probably to a lesser extent than in Belarus or in the Ukraine.

There is strong evidence that the increased incidence of childhood thyroid cancer is due to radiation exposure as a result of the Chernobyl accident, based on the geographical and temporal distribution of the cases. In both Belarus and the Ukraine they are commonest in the oblasts close to Chernobyl that had high levels of contamination. The incidence in children born more than 6 months after the accident drops dramatically to the low levels expected in unexposed populations.

The evidence that the increase is due to radioactive isotopes of iodine is at present circumstantial. It is based on the fact that the increase in cancer in the exposed population is limited to thyroid tumours. The relatively high thyroid doses are due to the thyroid's ability to concentrate radioiodine, which was deposited at high levels in areas close to the Chernobyl plant. Other contributory factors cannot be excluded, but appear unlikely to have played a major role. The likelihood of developing thyroid carcinoma is much greater in those who were very young at exposure, and drops rapidly with age. This is consistent with the known high uptake of radioiodine in young children and the higher sensitivity of young children to the carcinogenic effect of X radiation to the thyroid. ¹³¹I was the major cause of radiation exposure of the thyroid. Short lived isotopes of iodine could have played a role in the immediate vicinity of the plant. A reported increase in incidence of hypothyroidism and of thyroid nodules in Gomel oblast is also consistent with radioiodine exposure.

Future effects are difficult to predict, because of a lack of experience of exposure at this level of a large population. The cohort made up of those who were very young at the time of the accident and were exposed at high levels is likely to carry an increased risk of developing thyroid cancer for many years. The level of risk could be high; further studies are needed to establish a dose-response relationship and to improve the prediction of likely future incidence. Thyroid cancer, if properly treated, does not carry a high mortality and the observations suggest that targeted screening of those exposed at high levels at a young age may be effective.