Vladimir Drozdovitch

Cancer consequences of the Chernobyl accident: 20 years on

26 April 2006 marks the 20th anniversary of the Chernobyl accident. On this occasion, the World Health Organization (WHO), within the UN Chernobyl Forum initiative, convened an Expert Group to evaluate the health impacts of Chernobyl. This paper summarises the findings relating to cancer. A dramatic increase in the incidence of thyroid cancer has been observed among those exposed to radioactive iodines in childhood and adolescence in the most contaminated territories. Iodine deficiency may have increased the risk of developing thyroid cancer following exposure to radioactive iodines, while prolonged stable iodine supplementation in the years after exposure may reduce this risk. Although increases in rates of other cancers have been reported, much of these increases appear to be due to other factors, including improvements in registration, reporting and diagnosis. Studies are few, however, and have methodological limitations. Further, because most radiation-related solid cancers continue to occur decades after exposure and because only 20 years have passed since the accident, it is too early to evaluate the full radiological impact of the accident. Apart from the large increase in thyroid cancer incidence in young people, there are at present no clearly demonstrated radiation-related increases in cancer risk. This should not, however, be interpreted to mean that no increase has in fact occurred: based on the experience of other populations exposed to ionising radiation, a small increase in the relative risk of cancer is expected, even at the low to moderate doses received. Although it is expected that epidemiological studies will have difficulty identifying such a risk, it may nevertheless translate into a substantial number of radiation-related cancer cases in the future, given the very large number of individuals exposed.

Chernobyl-related thyroid cancer in children of Belarus: a case-control study.

The accident at the Chernobyl nuclear power plant on April 26, 1986, released approximately 2 EBq of 131I and other radioiodine isotopes that heavily contaminated southern Belarus. An increase in thyroid cancer reported in 1992 and attributed to the Chernobyl accident was challenged as possibly the result of intensive screening. We began a case-control study to test the hypothesis that the Chernobyl accident caused the increase in thyroid cancer. Records of childhood thyroid cancer in the national therapy centers in Minsk in 1992 yielded 107 individuals with confirmed pathology diagnoses and available for interview. Pathways to diagnosis were (1) routine endocrinological screening in 63, (2) presentation with enlarged or nodular thyroid in 25 and (3) an incidental finding in 19. Two sets of controls were chosen, one matched on pathway to diagnosis, the other representing the area of heavy fallout, both matched on age, sex and rural/urban residence in 1986. The 131I dose to the thyroid was estimated from ground deposition of 137Cs, ground deposition of 131I, a data bank of 1986 thyroid radiation measurements, questionnaires and interviews. Highly significant differences were observed between cases and controls (both sets) with respect to dose. The differences persisted within pathway to diagnosis, gender, age and year of diagnosis, and level of iodine in the soil, and were most marked in the southern portion of the Gomel region. The case-control comparisons indicate a strong relationship between thyroid cancer and estimated radiation dose from the Chernobyl accident.

Thyroid cancer risk in Belarus among children and adolescents exposed to radioiodine after the Chornobyl accident

Background:Previous studies showed an increased risk of thyroid cancer among children and adolescents exposed to radioactive iodines released after the Chornobyl (Chernobyl) accident, but the effects of screening, iodine deficiency, age at exposure and other factors on the dose–response are poorly understood.Methods:We screened 11 970 individuals in Belarus aged 18 years or younger at the time of the accident who had estimated 131I thyroid doses based on individual thyroid activity measurements and dosimetric data from questionnaires. The excess odds ratio per gray (EOR/Gy) was modelled using linear and linear–exponential functions.Results:For thyroid doses <5 Gy, the dose–response was linear (n=85; EOR/Gy=2.15, 95% confidence interval: 0.81–5.47), but at higher doses the excess risk fell. The EOR/Gy was significantly increased among those with prior or screening-detected diffuse goiter, and larger for men than women, and for persons exposed before age 5 than those exposed between 5 and 18 years, although not statistically significant. A somewhat higher EOR/Gy was estimated for validated pre-screening cases.Conclusion:10–15 years after the Chornobyl accident, thyroid cancer risk was significantly increased among individuals exposed to fallout as children or adolescents, but the risk appeared to be lower than in other Chornobyl studies and studies of childhood external irradiation.

Estimates of the cancer burden in Europe from radioactive fallout from the Chernobyl accident

The Chernobyl accident, which occurred April 26, 1986, resulted in a large release of radionuclides, which were deposited over a very wide area, particularly in Europe. Although an increased risk of thyroid cancer in exposed children has been clearly demonstrated in the most contaminated regions, the impact of the accident on the risk of other cancers as well as elsewhere in Europe is less clear. The objective of the present study was to evaluate the human cancer burden in Europe as a whole from radioactive fallout from the accident. Average country‐ and region‐specific whole‐body and thyroid doses from Chernobyl were estimated using new dosimetric models and radiological data. Numbers of cancer cases and deaths possibly attributable to radiation from Chernobyl were estimated, applying state‐of‐the‐art risk models derived from studies of other irradiated populations. Simultaneously, trends in cancer incidence and mortality were examined over time and by dose level. The risk projections suggest that by now Chernobyl may have caused about 1,000 cases of thyroid cancer and 4,000 cases of other cancers in Europe, representing about 0.01% of all incident cancers since the accident. Models predict that by 2065 about 16,000 (95% UI 3,400–72,000) cases of thyroid cancer and 25,000 (95% UI 11,000–59,000) cases of other cancers may be expected due to radiation from the accident, whereas several hundred million cancer cases are expected from other causes. Although these estimates are subject to considerable uncertainty, they provide an indication of the order of magnitude of the possible impact of the Chernobyl accident. It is unlikely that the cancer burden from the largest radiological accident to date could be detected by monitoring national cancer statistics. Indeed, results of analyses of time trends in cancer incidence and mortality in Europe do not, at present, indicate any increase in cancer rates—other than of thyroid cancer in the most contaminated regions—that can be clearly attributed to radiation from the Chernobyl accident.

Breast cancer in Belarus and Ukraine after the Chernobyl accident

An increase in breast cancer incidence has been reported in areas of Belarus and Ukraine contaminated by the Chernobyl accident and has become an issue of public concern. The authors carried out an ecological epidemiological study to describe the spatial and temporal trends in breast cancer incidence in the most contaminated regions of Belarus and Ukraine, and to evaluate whether increases seen since 1986 correlate to radiation exposure from the Chernobyl accident. The authors investigated the trends through age‐cohort‐period‐region analyses of district‐specific incidence rates of breast cancer for Gomel and Mogilev regions of Belarus and Chernigiv, Kyiv and Zhytomir regions of Ukraine. Dose‐response analyses were based on Poisson regression, using average district‐specific whole body doses accumulated since the accident from external exposure and ingestion of long‐lived radionuclides. The study demonstrated increases in breast cancer incidence in all areas following the Chernobyl accident, reflecting improvements in cancer diagnosis and registration. In addition, a significant 2‐fold increase in risk was observed, during the period 1997–2001, in the most contaminated districts (average cumulative dose of 40.0 mSv or more) compared with the least contaminated districts (relative risk [RR] in Belarus 2.24, 95% confidence interval [CI] 1.51–3.32 and in Ukraine 1.78, 95% CI = 1.08–2.93). The increase, though based on a relatively small number of cases, appeared approximately 10 years after the accident, was highest among women who were younger at the time of exposure and was observed for both localised and metastatic diseases. It is unlikely that this excess could be entirely due to the increased diagnostic activity in these areas.

Risk of hematological malignancies among Chernobyl liquidators

Abstract Kesminiene, A., Evrard, A-S., Ivanov, V. K., Malakhova, I. V., Kurtinaitis, J., Stengrevics, A., Tekkel, M., Anspaugh, L. R., Bouville, A., Chekin, S., Chumak, V. V., Drozdovitch, V., Gapanovich, V., Golovanov, I., Hubert, P., Illichev, S. V., Khait, S. E., Kryuchkov, V. P., Maceika, E., Maksyoutov, M., Mirkhaidarov, A. K., Polyakov, S., Shchukina, N., Tenet, V., Tserakhovich, T. I., Tsykalo, A., Tukov, A. R. and Cardis, E. Risk of Hematological Malignancies among Chernobyl Liquidators. Radiat. Res. 170, 721–735 (2008). A case-control study of hematological malignancies was conducted among Chernobyl liquidators (accident recovery workers) from Belarus, Russia and Baltic countries to assess the effect of low- to medium-dose protracted radiation exposures on the relative risk of these diseases. The study was nested within cohorts of liquidators who had worked around the Chernobyl plant in 1986–1987. A total of 117 cases [69 leukemia, 34 non-Hodgkin lymphoma (NHL) and 14 other malignancies of lymphoid and hematopoietic tissue] and 481 matched controls were included in the study. Individual dose to the bone marrow and uncertainties were estimated for each subject. The main analyses were restricted to 70 cases (40 leukemia, 20 NHL and 10 other) and their 287 matched controls with reliable information on work in the Chernobyl area. Most subjects received very low doses (median 13 mGy). For all diagnoses combined, a significantly elevated OR was seen at doses of 200 mGy and above. The excess relative risk (ERR) per 100 mGy was 0.60 [90% confidence interval (CI) −0.02, 2.35]. The corresponding estimate for leukemia excluding chronic lymphoid leukemia (CLL) was 0.50 (90% CI −0.38, 5.7). It is slightly higher than but statistically compatible with those estimated from A-bomb survivors and recent low-dose-rate studies. Although sensitivity analyses showed generally similar results, we cannot rule out the possibility that biases and uncertainties could have led to over- or underestimation of the risk in this study.

Radiation and the risk of chronic lymphocytic and other leukemias among chornobyl cleanup workers

Background: Risks of most types of leukemia from exposure to acute high doses of ionizing radiation are well known, but risks associated with protracted exposures, as well as associations between radiation and chronic lymphocytic leukemia (CLL), are not clear. Objectives: We estimated relative risks of CLL and non-CLL from protracted exposures to low-dose ionizing radiation. Methods: A nested case–control study was conducted in a cohort of 110,645 Ukrainian cleanup workers of the 1986 Chornobyl nuclear power plant accident. Cases of incident leukemia diagnosed in 1986–2006 were confirmed by a panel of expert hematologists/hematopathologists. Controls were matched to cases on place of residence and year of birth. We estimated individual bone marrow radiation doses by the Realistic Analytical Dose Reconstruction with Uncertainty Estimation (RADRUE) method. We then used a conditional logistic regression model to estimate excess relative risk of leukemia per gray (ERR/Gy) of radiation dose. Results: We found a significant linear dose response for all leukemia [137 cases, ERR/Gy = 1.26 (95% CI: 0.03, 3.58]. There were nonsignificant positive dose responses for both CLL and non-CLL (ERR/Gy = 0.76 and 1.87, respectively). In our primary analysis excluding 20 cases with direct in-person interviews < 2 years from start of chemotherapy with an anomalous finding of ERR/Gy = –0.47 (95% CI: < –0.47, 1.02), the ERR/Gy for the remaining 117 cases was 2.38 (95% CI: 0.49, 5.87). For CLL, the ERR/Gy was 2.58 (95% CI: 0.02, 8.43), and for non-CLL, ERR/Gy was 2.21 (95% CI: 0.05, 7.61). Altogether, 16% of leukemia cases (18% of CLL, 15% of non-CLL) were attributed to radiation exposure. Conclusions: Exposure to low doses and to low dose-rates of radiation from post-Chornobyl cleanup work was associated with a significant increase in risk of leukemia, which was statistically consistent with estimates for the Japanese atomic bomb survivors. Based on the primary analysis, we conclude that CLL and non-CLL are both radiosensitive.

Individual thyroid dose estimation for a case-control study of Chernobyl-related thyroid cancer among children of Belarus-part I: 131I, short-lived radioiodines (132I, 133I, 135I), and short-lived radiotelluriums (131MTe and 132Te).

Abstract— Large amounts of radioiodines were released into the atmosphere during the accident at the Chernobyl nuclear power plant on 26 April 1986. In order to investigate whether the thyroid cancers observed among children in Belarus could have been caused by radiation exposures from the Chernobyl accident, a team of Belarusian, Russian, and American scientists conducted a case-control study to compare cases and controls according to estimated thyroid dose. The primary purpose of this paper is to present detailed information on the estimated thyroid doses, due to intakes of 131I, that were used in the case-control study. The range of the 131I thyroid doses among the 107 cases and the 214 controls was found to extend from 0.00002 to 4.3 Gy, with medians of approximately 0.2 Gy for the cases and 0.07 Gy for the controls. In addition, the thyroid doses resulting from the intakes of short-lived radioiodines (132I, 133I, and 135I) and radiotelluriums (131mTe and 132Te) were estimated and compared to the doses from 131I. The ratios of the estimated thyroid doses from the short-lived radionuclides and from 131I for the cases and the controls range from 0.003 to 0.1, with median values of approximately 0.02 for both cases and controls.

Risk of Thyroid Cancer among Chernobyl Liquidators

After the Chernobyl accident in 1986, the “liquidators” or clean-up workers were among those who received the highest radiation doses to the thyroid from external radiation. Some were also exposed to radioiodines through inhalation or ingestion. A collaborative case-control study nested within cohorts of Belarusian, Russian and Baltic liquidators was conducted to evaluate the radiation-induced risk of thyroid cancer. The study included 107 cases and 423 controls. Individual doses to the thyroid from external radiation and from iodine-131 (131I) were estimated for each subject. Most subjects received low doses (median 69 mGy). A statistically significant dose-response relationship was found with total thyroid dose. The Excess Relative Risk (ERR) per 100 mGy was 0.38 [95% confidence interval (CI): 0.10, 1.09]. The risk estimates were similar when doses from 131I and external radiation were considered separately, although for external radiation the ERR was not statistically significantly elevated. The ERR was similar for micro carcinomas and larger size tumors, and for tumors with and without lymph node involvement. Although recall bias and uncertainties in doses could have affected the magnitude of the risk estimates, the findings of this study contribute to a better characterization the risk of thyroid cancer after radiation exposure in adulthood.

Radrue method for reconstruction of external photon doses for Chernobyl liquidators in epidemiological studies.

Between 1986 and 1990, several hundred thousand workers, called “liquidators” or “clean-up workers,” took part in decontamination and recovery activities within the 30-km zone around the Chernobyl nuclear power plant in Ukraine, where a major accident occurred in April 1986. The Chernobyl liquidators were mainly exposed to external ionizing radiation levels that depended primarily on their work locations and the time after the accident when the work was performed. Because individual doses were often monitored inadequately or were not monitored at all for the majority of liquidators, a new method of photon (i.e., gamma and x rays) dose assessment, called “RADRUE” (Realistic Analytical Dose Reconstruction with Uncertainty Estimation), was developed to obtain unbiased and reasonably accurate estimates for use in three epidemiologic studies of hematological malignancies and thyroid cancer among liquidators. The RADRUE program implements a time-and-motion dose-reconstruction method that is flexible and conceptually easy to understand. It includes a large exposure rate database and interpolation and extrapolation techniques to calculate exposure rates at places where liquidators lived and worked within ∼70 km of the destroyed reactor. The RADRUE technique relies on data collected from subjects’ interviews conducted by trained interviewers, and on expert dosimetrists to interpret the information and provide supplementary information, when necessary, based upon their own Chernobyl experience. The RADRUE technique was used to estimate doses from external irradiation, as well as uncertainties, to the bone marrow for 929 subjects and to the thyroid gland for 530 subjects enrolled in epidemiologic studies. Individual bone marrow dose estimates were found to range from less than one &mgr;Gy to 3,300 mGy, with an arithmetic mean of 71 mGy. Individual thyroid dose estimates were lower and ranged from 20 &mgr;Gy to 507 mGy, with an arithmetic mean of 29 mGy. The uncertainties, expressed in terms of geometric standard deviations, ranged from 1.1 to 5.8, with an arithmetic mean of 1.9.