Current status and epidemiological research needs for achieving a better understanding of the consequences of the Chernobyl accident

Twenty years after the Chernobyl accident, there is no clearly demonstrated increase in the incidence of cancers in the most affected populations that can be attributed to radiation from the accident, except for the dramatic increase in thyroid cancer incidence among those exposed in childhood and adolescence. Increases in the incidence of cancers and other diseases have been reported in Belarus, the Russian Federation, and Ukraine, but much of the increase appears to be due to other factors, including improvements in diagnosis, reporting, and registration. Recent findings indicate a possible doubling of leukemia risk among Chernobyl liquidators and a small increase in the incidence of premenopausal breast cancer in the very most contaminated districts. Increased risks of cardiovascular diseases and cataracts have also been reported. These findings, however, need confirmation in well-designed analytical epidemiological studies with careful individual dose reconstruction. The absence of demonstrated increases in cancer risk--apart from thyroid cancer--is not the proof that no increase has in fact occurred. Based on the experience of atomic bomb survivors, and assuming that there is a linear, no-threshold dose-response relationship between exposure to ionizing radiation and the development of cancer in humans, a small increase in the relative risk of cancer is expected, even at the low to moderate doses received. Given the large number of individuals exposed, the absolute number of cancer cases caused could be substantial, particularly in the future. It is therefore essential to continue to use population registries to monitor trends in disease morbidity and mortality in the most contaminated areas, as well as among liquidators, in order to assess the public health impact of the accident. Studies of selected populations and diseases are also essential in order to study the real effect of the accident and compare it to predictions. Careful studies may in particular provide important information on the effect of exposure rate and exposure type in the low to medium dose range and on factors that may modify radiation effects. As such, they may have important consequences for the radiation protection of patients and of the general population in case of further nuclear emergencies.