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  7.   What is the reason for the increase in thyroid cancer, especially in children, after the Chernobyl accident?

During the few days and weeks after the accident, the most danger of radioactive exposure in the territories around from the Chernobyl Nuclear Power Plant was the presence of Iodine-131, Iodine-132, and Iodine-133 radionuclides, which selectively concentrate in the thyroid gland. Only in the central areas of Russia, about 12% of the total content of iodine J-131 that was discharged into the atmosphere fell out. This isotope entered into the body through inhaled air, contaminated foods (primarily milk products), and through the skin.

The thyroid gland (the main organ of radioactive Iodine-131 accumulation) was the primary organ irradiated. A clearly defined relationship between the thyroid irradiation dose and human age was established, which was explained by age differences in thyroid mass as well as differences in nutrition (chiefly, milk products). The maximal doses of radiation were seen in groups of children under 3 years, and the minimal doses, in adults.

Fig. 13: Thyroid dose distribution among children with thyroid measurements after the Cherynobyl (Ukraine) accident

Figure 13

Source: A. Bouville, US National Cancer Institute, Epidmiol Rev Vol 19, No. 2, 1997

Examination of the results of 10,600 direct measurements of iodine-131 content in the thyroid gland of children and adults from a 30-km wide zone allowed the estimation of the irradiation dose range and determination of the organs most irradiated. It was found that the percentage of children under 3 years exposed to doses of radiation exceeding 2000 mGy was 40-60%, the percentage of children from age 4 to 7 that were exposed to this level of radiation was 20%. At the same time, the percentage of adults exposed to irradiation doses exceeding 2000 mGy was more than 3%. The annual risk of thyroid cancer at age 18 following exposure was 25 cases per 10,000 persons. Sensitivity to radiation among adults was two times less than among children and teenagers.

Sensitivity to radiation among females was two times higher than among males. It should be noted that the existence of a prolonged latent period (minimum 5 years) before appearance of thyroid cancer symptoms could shift children's cancer to teenage and adult groups. Unfortunately, the risk of thyroid cancer as a result of radioactive contamination can remain for 40 to 50 years, though the risk generally diminishes with time.

It was found that the Chernobyl accident was followed by a significant increase in thyroid cancer frequency in the contaminated territories. Ten years after the accident, (compared to year 1985), thyroid cancer increased 6.5 to 10 times, especially in children under age 15 (see Diagrams 1 and 2).

This information is now considered a manifestation of the impact of radiation on children and teenagers due to the accident at the Chernobyl Nuclear Power Plant. This impact may have resulted from the immediate influence of radioactive iodine on thyroid tissue as well as from indirect influences on other systems, particularly from the immune system.

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