Several years after an initial agreement was reached between the United States and the USSR in 1988, to cooperate in the area of nuclear reactor safety, the National Cancer Institute (NCI) undertook to develop long-term studies of thyroid cancer among exposed children in Ukraine and Belarus. The incidence of thyroid cancer in children, ordinarily very rare, had begun to increase markedly following the accident, and a rigorous investigation was needed to assess the relationship between exposure to I-131 from Chernobyl and the occurrence of thyroid cancer and other thyroid disorders. Responsibility for the study resides in the Radiation Epidemiology Branch of NCI.
The first contacts between the U.S. and Belarusian governments concerning this project were made in the early 1990s. In the next few years, a study protocol was developed and approved and agreement for funding was signed in 1996. Thyroid cancer screening of study participants began in 1998.
The screening program carried out in all six oblasts of Belarus comprised three cycles and consisted of serial examinations of the thyroid glands of approximately 12,000 individuals who were exposed as children (0-18 years) and who had thyroid radioactivity measurements made in the weeks following the accident. The screening protocol was designed to determine whether a participant had thyroid cancer or a range of other thyroid diseases, such as benign nodules.
Most members of the cohort reside in Gomel, Minsk and Mogilev Oblasts. The screening program consists of the following: a medical history; palpation of the thyroid by an endocrinologist and an ultrasonographer; an ultrasound examination of the thyroid; and collection of a blood sample (for measurement of thyroid hormones and thyroid auto-antibodies) and a spot urine sample (to measure excreted iodine); a detailed dosimetry interview to obtain a residential history, data on consumption of contaminated foods, and use of iodine prophylaxis is also administered.
The radioiodine doses estimated for each cohort member have been based on the thyroid radioactivity measurements, ecologic data (for example, deposition patterns), and interview data on residential location and consumption of contaminated foods around the time of the accident.
Active screening was completed in 2008. Follow-up of the cohort continues through linkage with the Belarus Cancer Registry. The study center, originally based at the Research Institute of Radiation Medicine and Endocrinology in Minsk, was transferred in 2004 to the new Republican Research Center of Radiation Medicine and Human Ecology in Gomel.
The primary objective of this collaborative research is to determine the relationship between I-131 exposure from the Chernobyl accident and risk of thyroid cancer. The effect of dose level, age at the time of exposure and gender are of particular interest, along with effects of stable iodine status. Associations between I -131 and benign thyroid nodules and other thyroid diseases are also being evaluated. The effect of I-131 in inducing thyroid cancer will be compared with published data on external radiation to assess their relative potency.
1st Screening Cycle
The initial enrollment in the first round of screening was 11,918. To insure a high level of participation, the screening examinations were conducted both at the stationary center in Minsk (approximately 40%) and in mobile units (approximately 60%) that were sent out to parts of the study area some distance from Minsk. The first cycle was completed in 2001. As a result of the first screening, 85 cases with doses < 5 Gy were identified. The excess odds ratio per Gy for this group was 2.15 (95% CI: 0.81, 5.47) (Zablotska et al., 2011).
2nd - 3rd Screening Cycles
Seventy-one confirmed thyroid cancers cases were identified in the 2nd and 3rd cycles. Active screening has been terminated but future follow-up of the cancer experience of the cohort continues through linkage with the national Belarus Cancer Registry.
Benefits to Belarusians include the long-term screening and evaluation of the population at risk and continued monitoring through the Belarus Cancer Registry. The project also assists local health officials in developing appropriate health-care programs by providing pertinent, valuable data on a range of topics, including iodine nutrition. Technical and scientific training has been offered to collaborators and the general population has been educated on the importance of early detection and regular screening in preventing morbidity and death from thyroid cancer. Publications describing the findings are beginning to appear. Our research findings contribute to the data base that health care providers, health policy and nuclear safety experts will draw upon to formulate future plans for similar exposure situations. In addition, the results will add to our basic understanding of the carcinogenic and other adverse health effects of I-131.