Radiological health effects of the Chernobyl accident: estimation of radiation risks

Transcription

1 Radiological health effects of the Chernobyl accident: estimation of radiation risks V.K. Ivanov, A.F. Tsyb, A.I. Gorski, S.E. Khait, M.A. Maksioutov, O.K. Vlasov Medical Radiological Research Center, Russian Academy of Medical Sciences, Obninsk, Russia Abstract. Following the Chernobyl accident of April 6, 1986, two population groups were primarily exposed to radiation: emergency workers involved in mitigation of the accident consequences (liquidators) and residents of contaminated territories. A dramatic increase in thyroid cancer incidence among residents exposed as children has been apparent since 199. However, there remains considerable uncertainty about the existence of radiationrelated health effects in liquidators (the average dose of external radiation about 100 mgy). We studied a cohort of 71,870 Russian-resident males engaged in recovery operations within the 30-km zone between for whom individual external radiation dose estimates are available (the average dose 107 mgy). A total of 58 morphologically verified leukemia cases (ICD9 0-08) were identified in this cohort between 1986 and Of these, 16 cases of chronic lymphocytic leukemia (CLL) were excluded. Even though leukemia is known to be an early consequence of acute radiation exposure, various studies indicate that CLL rates do not increase with radiation exposure. We consider the relationship between leukemia incidence rate and absorbed radiation dose in analysis based on both external and internal comparisons. Both the external (SIR=.5, P<0.001) and internal comparisons (RR=., P=0.03) indicate significantly elevated risks in the mgy dose group. The areas worst affected by the Chernobyl accident include the Bryansk, Kaluga, Tula and Oryol oblasts of Russia. According to the Russian Commission on Radiation Safety the thyroid doses due to exposure to incorporated 131 I in the children population of the South-West areas of the Bryansk oblast is mgy. The paper presents results of the radiogenic thyroid cancer risk analysis in the Bryansk region among girls of 0-17 years at the time of exposure. In this group a total of 1 cases of thyroid cancer were diagnosed from 1991 to 001, the mean thyroid dose being 80 mgy, the number of person-years in the period from 1991 to The excess relative risk per unit dose of 1 Gy (ERR 1Gy ) with 95% confidence interval has been estimated to be 9.1 (3.0, 3.3). The highest risk of developing thyroid cancer was established among girls of 0- years at the time of exposure living in the Bryansk oblast: for them the ERR 1Gy is 5.3 (5., ). The results of epidemiological analysis obtained in the Russian National Medical and Dosimetric Registry is of major practical significance for optimizing the health care services to the population exposed to radiation as a result of the Chernobyl accident. Following the Chernobyl accident of April 6, 1986, two main population groups were exposed to radiation: emergency workers involved in mitigation of the accident consequences (liquidators) and residents of contaminated territories. A dramatic increase in thyroid cancer incidence among residents exposed as children has been apparent since 199 [1, ]. However, there remains considerable uncertainty about the existence of radiation-related health effects in liquidators (average dose of external radiation about 100 mgy) [3]. Our data demonstrates that there has been a significant increase in the incidence of leukemia other than chronic lymphocytic leukemia (CLL) among liquidators with external radiation dose estimates of 150 to 300 mgy. We studied a cohort of 71,870 Russian-resident males who were engaged in recovery operations within the 30-km zone between and for whom individual external radiation dose estimates were available (average dose 107 mgy) []. A total of 58 morphologically verified leukemia cases (ICD9 0-08) were diagnosed in this cohort between 1986 and 1998, from which we excluded 16 cases of CLL. While leukemia is a well-known early consequence of acute radiation exposure [5], various studies [3] indicate that CLL rates are not increased by radiation exposure. Present address: Medical Radiological Research Center, Russian Academy of Medical Sciences, Korolyov Street, Obninsk, 9036 Russia. 1

2 We considered the relationship between leukemia incidence rate and absorbed radiation dose based on both external and internal comparisons. For the external comparison we used age-specific Russian male population rates for this time period to compute dose category-specific standardized incidence ratios (SIRs). The internal comparisons were based on using estimates of both category-specific relative risk and excess relative risk (ERR) per Gy. Parameter estimates, confidence intervals and significance tests were computed using the Poisson regression methods [6]. The results are summarized in Table I. Both the external (SIR=.5, P<0.001) and internal comparisons (RR=., P=0.03) indicate significantly elevated risks in the mgy dose group. Table I. Results of analysis of leukemia incidence among Chernobyl accident liquidators Liquidators of Liquidators of Dose group (mgy) Mean dose Mean age at exposure (years) Cases PYR External comparison Expected cases SIR % CI (0.9,.6) (0., 1.9) (0.6,.1) (1.7, 3.) (0.6, 3.0) (0.1, 1.6) (0.6,.) (1.7, 3.5) Internal comparison Relative risk % CI - (0., 1.6) (0., 1.9) (1.0, 3.6) - (0.1, 1.6) (0.3,.6) (0.8, 5.0) Pooled comparison group % CI - (1.3, 3.7) - (1.,.3) ERR/Gy 6.7 (90% CI 0.8, 3.5) 10.7 (90% CI 1.3, 81.) It should be emphasized that similar results were obtained for the liquidators of who are provided with better than usual health care by the law of the Russian Federation (Table I). Our results suggest that liquidators with doses in excess of 150 mgy (about 30% of all Chernobyl liquidators) should be regarded as having elevated risks of developing non-cll leukemia. In the territories of Belarus, Russia and Ukraine contaminated after the Chernobyl accident a significant increase in thyroid cancer incidence rates was observed after Among those exposed as children and adolescents in 1986 the excess above the spontaneous incidence level was tenfold and even more. Exposure of thyroid to radioiodines can be one of possible reasons of the increased incidence rate. At the same time, it may not be excluded that the elevation of incidence rate and improved registration level is explained by increased attention to this issue after the accident (screening effect). Thyroid cancer not being fatal, the indicated effect may play an important role and therefore is discussed among other things in the paper. The analysis of radiation risks covered the follow-up period The year 1991 was taken as the beginning of the follow-up time with allowance for the minimal latent period of 5 years in induction of radiogenic thyroid cancer. The subjects of the study were persons exposed as children and adolescents in 1986 (age 0-17). Since thyroid dose from radioiodines was accumulating during the first two months after the Chernobyl accident in 1986 and accurate demographic data were not available for specific rayons and population points (rayon is a territorial and administrative unit of oblast, there are 8 rayons in the Bryansk oblast), the age structure at the rayon level in 1986 was defined using results of the federal census of Given this assumption, the study population in 1986 consisted of people residing in the Bryansk oblast. The census data contained only the number of people living in population points and the age structure there was assumed to be the same as in rayons they belong to. Since only children and adolescents were studied, the analysis of radiation risks did not take into account changes in the cohort size due to mortality.

3 The risk of radiogenic thyroid cancer is known to depend on sex, age at exposure, thyroid dose and attained age. In turn, thyroid dose due to radioiodines is also a function of age at exposure and increases with a decrease in age [7-15]. Therefore, for analysis of the age dependence of incidence rate the study population was divided into categories by age at exposure: children of 0-, 5-9, 10-1 years, and adolescents of years plus two categories by sex. The main demographic characteristics of the study cohort are shown in Table II. Table II. Key characteristics of the study cohort of persons exposed as children and adolescents (0-17 years), follow-up period Females Males Population size Number of cases 1 55 Mean dose for members of the study population (Gy) Mean dose for cases (Gy) In the first stage of the study focus was placed on thyroid cancer incidence among those exposed as children and adolescents (age 0-17 years). Results of estimating the dynamics of the standardized incidence ratio SIR are shown in Fig. 1. As can be seen from the figure, SIR>1, i.e. it exceeds the control which is the incidence rate in Russia in general and the value is relatively stable in time. SIR Females (0-17 at exposure) Population Control Calendar years SIR Calendar years Males (0-17 at exposure) Population Control FIG. 1. Dynamics of SIR. The value of SIR over the whole follow-up period is.9 (., % CI) for girls and 8.8 (6.6, % CI) for boys. This excess in SIR can be explained either by exposure to radiation, or regional differences in the thyroid cancer spontaneous incidence rates and registration levels. Main characteristics of this cohort and radiation risk coefficients are presented in Table III. As indicated, the estimates of excess relative risk per unit dose were derived using internal and external controls. It follows from Table III that the relative risk increases with increasing dose, both for girls and boys. The value of the excess relative risk per 1 Gy is positive and statistically significant within 95% confidence intervals. It should be noted that the risk coefficients obtained with different control groups (internal and external) appear to be close for girls (a relatively large number of cases) and different for boys, the number of cases among boys being one third. The subsequent discussion will show that the estimates derived with both control groups are basically close, in other words asymptotic convergence of risk estimates can be seen in both models. It is, therefore, the authors opinion that in case of a limited number of cases, estimates based on using external control are more reliable. To study the age dependence of radiation risk all cases were divided into three groups by age a exposure: 0-, 5-9, 10-1 and years. Results of calculations are included in Table IV. 3

4 Table III. Estimated radiation risk of thyroid cancer among persons exposed as children and adolescents (0-17 years), follow-up period. Dose groups PYR Mean dose (Gy) Observed Expected a RR (95% CI) Girls (0.9,.3) (1.,.6) (1.7, 5.7) Total Internal control ERR per 1 Gy=9.1 (3.0, 3.3) External control ERR per 1 Gy=11. (5.,.6); SIR=3.1 (.,.1) b Boys (0.3,.8) (0.83, 3.1) (1.9, 1.0) Total Internal control ERR per 1 Gy=1.6 (6.0, 151.3) External control ERR per 1 Gy=3.5 (1.8, 60.8); SIR=.5 (0.3, 5.1) a Spontaneous incidence in nonexposed cohort; b SIR for nonexposed members of the cohort. Table IV. Estimated radiation risks of thyroid cancer in girls ( follow-up period). Dose groups PYR Mean dose (Gy) Observed Expected a RR (95% CI) Girls 0- years at exposure (0., 3.5) (0.9, 9.5) (1.9, 16.1) Total ERR per 1 Gy=5.3 (5., 9953) (internal control) ERR per 1 Gy=8.8 (.3, 38) (external control); SIR=1.7 (NA c, 6.0) c Girls 5-9 years at exposure (0.,,9) (1., 6.) (NA, 6.1) Total Internal control ERR per 1 Gy=10.1 (-0.1, 8.7) External control ERR per 1 Gy=10. (0.1, 75.5); SIR=3.3 (0.9, 6.1) Girls 10-1 years at exposure (0., 1.39) (0.1, 1.1) (0.8, 1.61) Total Internal control ERR per 1 Gy=1.0 (-5.3, 15.0) External control ERR per 1 Gy=1. (-5., 15.); SIR=. (.8, 6.5) Girls years at exposure (0.6, 3.3) (0.7, 5.6) (0.3,.0) Total Internal control ERR per 1 Gy<0 External control ERR per 1 Gy<0; SIR=.0 (.3, 6.1) c NA Confidence interval is not defined.

5 As might be expected, radiation risk increases with a decrease in age at exposure (the highest for girls 0- years at exposure). Statistically significant risk is observed for those exposed as children of 0-1 years, but does not exist for adolescents of years at exposure. The observed absence of statistical significance for girls of 10-1 years in case of using the internal control can be explained by a limited number of cases. References 1. Josefson, D., Childhood thyroid cancers rise 10-fold in the Ukraine. BMJ, 319:15, (1999).. Kazakov, V.S., Demidchik, E.P., Astakhova, L.N., Thyroid cancer after Chernobyl. Nature, 359:11, (199). 3. United Nations Scientific Committee on the Effects of Atomic Radiation. UNSCEAR, 000 report, E.00.IX., United Nations, New York (000).. Ivanov, V.K., Tsyb, A.F., Ivanov, S.I., Souchkevitch, G.N., Low doses of ionizing radiation: health effects and assessment of radiation risks for emergency workers of the Chernobyl accident, edited by G.N. Souchkevitch, M.N. Repacholi, World Health Organization, Geneva (001). 5. Preston, D., Kusumi, S., Tomonaga, M., Izumi, S., Ron, E., Kuramoto, A., Kamada, N., Dohy, H., Metsui, T., Nonaka, H., Thompson, D., Soda, M., Mabuchi, K., Cancer incidence in atomic bomb survivors. Part III: Leukemia and Multiple Myeloma, Radiation Research, 137(Suppl.):S68-S97, (199). 6. Breslow, N., Day, N., Statistical methods in cancer research, IARC, Lyon (1987). 7. Likhtarev, I.A., Sobolev, B.G., Kairo, I.A., et al., Thyroid cancer in the Ukraine. Nature, 365:365, (1995). 8. Jacob, J., Goulko, G., Heidenreich, W.F., et al., Thyroid cancer risk to children calculated. Nature, 39: 31-31, (1998). 9. Thomas, G., Karaoglou, A., Willliams, E.D. (Eds.), Radiation and Thyroid Cancer, Proceeding of an International Seminar on Radiation and Thyroid Cancer, World Scientific Publishing, Brussels- Luxembourg (1999). 10. Ivanov, V.K., Gorski, A.I., Tsyb, A.F., Maksioutov, M.A., Rastopchin, E.M., Dynamics of thyroid cancer incidence in Russia following the Chernobyl accident. J. Radiol. Prot., 19(): , (1999). 11. Ivanov, V.K., Gorski, A.I., Pitkevitch, V.A., Tsyb, A.F., Risk of radiogenic thyroid cancer in Russia following the Chernobyl accident, in Proceeding of an International Seminar on Radiation and Thyroid Cancer, edited by Thomas, G., Karaoglou, A., Willliams, E.D., World Scientific Publishing, Brussels-Luxembourg (1999), p Heidenreich, W.F., Kenigsberg, Y., Jacob, P., Buglova, E., Gulko, G., Paretzke, H.G., Demidchik, E.P., Golovneva, A., Time trends of thyroid cancer incidence in Belarus after Chernobyl accident. Radiation Research, 151:617-65, (1999). 13. Jacob, P., Kenigsberg, Y., Zvonova, I., Gulko, G., Buglova, E., Heidenreich, W.F., Golovneva, A, Bratilova, A.A., Drozdovitch, V., Kruk, J., Pochtennaja, G.T., Balonov, M., Demidchik, E.P., Paretzke, H.G., Childhood exposure due to the Chernobyl accident and thyroid cancer risk in contaminated areas of Belarus and Russia. British J. of Cancer, 80(9): , (1999). 1. Likhtarev, I.A., Kairo, I.A., Shpak, V.M., et al., Radiation-induced and background thyroid cancer of Ukranian children (dosimetric approach). Int. J. Radiat. Med., 3-:51-66, (1999). 15. Ivanov, V.K., Gorski, A.I., Tsyb, A.F., Maksioutov, M.A., Vlasov, O.K., Godko, A.M., Risk of Radiogenic Cancer in the Population of the Bryansk and Oryol Regions of Russia after the Chernobyl Accident, Chernobyl: Message for the 1st Century, in Proceeding of the Sixth Chernobyl Sasakava Medical Cooperation Symposium, Moscow, Russia May 001, Elsevier, Moscow (00), p