This study presents an analysis of the thyroid cancer incidence in the population of the most contaminated territories of Bryansk, Kaluga, Oryol and Tula oblasts affected by the Chernobyl accident. The follow-up period is 1991-2008, and the cohort size is 309 130 people. For that period 978 thyroid cancer cases were detected. The excess relative risk per 1 Gy (ERR/Gy) is found to be statistically significant for children and adolescents (0-17 y of age) at the time of the Chernobyl accident (ERR/Gy=3.22; 95 % confidence interval (1.56; 5.81). In boys, the ERR/Gy was higher than in girls -6.54 and 2.24, respectively. A statistically significant decrease in ERR/Gy with time since exposure, by a factor of 0.37 per 10 y, was observed for the whole cohort and for boys separately, but not for girls. No radiation risks of a thyroid cancer among people of 18 y of age at exposure and older were found.
Of all potentially radiogenic cancers, leukemia, a type of cancer of the blood, has the highest risk attributable to ionizing radiation. Despite this, the quantitative estimation of radiation risk of a leukemia demands studying very large exposed cohorts, because of the very low level of this disease in unexposed populations and because of the tendency for its radiation risk to decrease with time. At present, the Japanese cohort of atomic bomb survivors is still the primary source of data that allows analysis of radiation-induced leukemia and the underlying dose-response relationship. The second large cohort that would allow to study radiation-induced leukemia is comprised of individuals who were exposed due to the accident of the Chernobyl nuclear power plant in 1986. The objective of the present study was to estimate radiation risks of leukemia incidence among the Russian cohort of Chernobyl emergency workers, for different time periods after the accident. Twenty-five years after the Chernobyl accident and based on the results of the present study, one can conclude that the radiation risk of leukemia incidence derived from the Russian cohort of Chernobyl emergency workers is similar to that derived from the cohort of atomic bomb survivors: The time-averaged excess relative risk per Gray (ERR Gy(-1)) equals 4.98 for the Russian cohort and 3.9 for the life span study (LSS) cohort; excess absolute risk decreases with time after exposure at an annual rate of 9% for the Russian cohort, and of 6.5% for the LSS cohort. Thus, the excess in risk of leukemia incidence in a population due to a single exposure is restricted in time after exposure by the period of about 15 years.
This paper presents the results of a retrospective cohort study of cancer incidence and mortality among emergency workers of the Chernobyl accident, for the follow-up period 1992-2009. The cohort selected for analysis consists of 67,568 emergency workers who worked in the Chernobyl exclusion zone in 1986-1987. External radiation whole-body absorbed dose varied from 0.0001 gray (Gy) to 1.24 Gy, with a median of 0.102 Gy. Over the follow-up period 1992-2009, a total of 4,002 solid cancers of different sites were identified as the result of annual compulsory health examination, and a total of 2,442 deaths from all solid cancers in the study cohort were reported. Poisson regression was applied for the analysis of cancer incidence and mortality. The analysis of the standardized incidence ratio (SIR) has shown a statistically significant increase in cancer incidence in the cohort as compared with baseline cancer incidence among males of Russia. The average excess over the entire follow-up period is 18 % [SIR = 1.18, 95 % confidence interval (CI) 1.15; 1.22]. In contrast, however, no increase in the mortality from all cancers among the emergency workers as compared to the baseline mortality in Russian men was found. Values of excess relative risk of cancer incidence and mortality per 1 Gy (ERR Gy(-1)) are 0.47 (95 % CI 0.03; 0.96, p value = 0.034) and 0.58 (95 % CI 0.002; 1.25, p value = 0.049), respectively. These values are statistically significant.
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