The Chernobyl accident
	The nuclear reactor number 4 at the Chernobyl nuclear power plant in Ukraine, exploded on 26 April 1986 releasing 2.6x1011 MBq of radioiodine. The most heavily contaminated areas were southern Belarus, northern Ukraine and the Bryansk and Kaluga regions of Russia. Immediate protective actions were not taken and thyroid doses of 2-5 Gy in the most heavily contaminated areas were found. Since the estimates were made in May-June 1986, radiation from short- lived isotopes, mostly 132I and 133I, could not be taken into account. A few years after the accident increased risks of thyroid cancer among children were reported and the increase has continued (33,34). In Belarus, 390 thyroid cancers among children have been diagnosed between 1986 and mid 1995, compared to 7 thyroid cancers in the 9 years preceding the accident giving a nearly 56-fold increase in incidence (35). The cancers were considered highly aggressive, infiltrating surrounding tissue and showing a high rate of lymph node metastases. Similar findings were reported from Ukraine, 211 juvenile thyroid cancers were diagnosed 1986-1994 compared to 25 for the period 1981-1985(36). The increase started in 1990 and was calculated to be 7-10 times that of the period preceding the accident. For a long period of time no increased risk of childhood thyroid cancer was found in Russia, not even in the regions closest to Belarus and Ukraine. In 1991 a "prophylactic systematic medical examination" of populations living in the south west district of the Bryansk region, closest to the Gomel area, was started. A total of 62 thyroid cancers among children and adolescents at time of accident has been found, 19 of these being exposed in utero. A case- control study is under way. Conflicting results have been reported for the so-called clean-up workers. Thyroid examinations, including ultrasound and selectively fine needle biopsies, were carried out in 1,984 Estonian clean-up workers (38). A total of 201 workers (10.2%) revealed thyroid nodules at ultrasound and 2 papillary thyroid cancers was found. It was concluded that no apparent effect related to the accident was found. In a recent study of thyroid cancer risk in 119,000 Russian clean-up workers, 47 thyroid cancers were found giving an ERR per GY close to what has been seen in children after external exposure, 5.31 (95% CI 0.04-10.58)(39). A peculiar finding was that the highest risk was not noted for those working in April-May 1986 but in June the same year. This might indicate that other factors, such as external exposure, may play a role in the increased thyroid cancer incidence. Today very few question the etiology of the thyroid tumors found among the children in the Chernobyl area even though the contribution of other risk factors than ionizing radiation is not clear. However, even though the Chernobyl accident will give the scientific community the opportunity to study the carcinogenic effect of radioiodines there is still a lot to be done. So far most of the studies have been aggregate or ecological studies, where thyroid cancer incidence was related to time periods or geographical areas without individual doses or even residency taken into consideration. In order to address influencing factors cohort or case-control studies have to be undertaken. The number of individuals and the dose range must be sufficient since these are the factors influencing the statistical power.