Chornobyl Update: Chornobyl Accident: Estimation of the Thyroid Doses
The accident that took place on 26 April 1986 at the Chornobyl nuclear power plant located in Ukraine, about 12 km south of the border with Belarus, was the most severe ever to have occurred in the nuclear industry.
The accident caused the deaths of 30 power plant employees and firemen within a few days or weeks (including 28 deaths that were due to radiation exposure). In addition, about 240,000 clean-up workers (also called "liquidators" or "recovery operation workers") were called upon in 1986 and 1987 to take part in major mitigation activities at the reactor and within the 30-km zone surrounding the reactor. Residual mitigation activities continued on a relatively large scale until 1990.
The massive releases of radioactive materials into the atmosphere that resulted from the accident brought about the evacuation of about 116,000 people from areas surrounding the reactor during 1986. In addition, during the following years, about 220,000 people from what are at this time three independent countries (Belarus, the Russian Federation, and Ukraine) were relocated. Vast territories of those three countries were contaminated. During the first month after the accident, the radiation exposures of members of the public resulting from the Chornobyl accident were due mainly to the consumption of milk contaminated with I-131 (radioactive iodine isotope). The highest thyroid doses were received by young children.
The Radiation Epidemiology Branch of the Division of Cancer Epidemiology and Genetics of the U.S. National Cancer Institute (NCI) is involved in two epidemiological studies of thyroid diseases that are related to the consequences of the Chornobyl accident.
The two studies are conducted in parallel among the residents of Belarus and of Ukraine since 1996 who were children at the time of the accident. Because of the large number of children who received relatively high thyroid doses resulting from intakes of I-131, these studies represent the best opportunity to estimate how the risk of thyroid cancer varies as a function of radiation dose and age at exposure.
All cohort subjects - approximately 13,000 in Ukraine and 12,000 in Belarus - were sampled among the large number of children who had their thyroids monitored for gamma radiation within a few weeks after the accident. This measurement led to the determination of the thyroidal content of I-131 at the time of the measurement. The thyroid dose due to the I-131 intake was then derived from the thyroidal content of I-131 at that point in time, using personal information on residence history and dietary habits obtained during interviews and models simulating the behavior of I-131 in the environment and in the body.
NCI also is involved in an epidemiologic study of leukemia, lymphoma, and other blood diseases among Ukrainian cleanup workers. Ukrainian and other foreign dosimetrists, have been asked to estimate the individual bone-marrow doses received by the approximately 100 cases and 500 controls. This study provides an opportunity to add to current knowledge about the possible health consequences of exposure to relatively low doses of ionizing radiation received gradually over a period of several months. In addition, the NCI has a limited program of work concerning clean-up workers from Estonia, Latvia, and Lithuania.
We should also indicate that we are in the process of extending our thyroid study in Ukraine to include subjects who were exposed in utero.
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