Thyroid cancer incidence in the Ukraine after the Chernobyl accident: comparison with spontaneous incidences

The thyroid cancer incidence in the Ukraine among those born in the period 1968–1986 was analyzed with the aim to identify the enhancement due to the Chernobyl accident. Since any Ukrainian data referring to the time period before the accident are scarce and the variation of spontaneous incidences in other countries is immense, the Ukrainian incidences in the period 1986–1989 were used to estimate the baseline risk. Following 1990, the incidence in the southern part of the Ukraine increased by about 30%, independent of age. In the other parts the increase of the incidence depended on age at exposure. In the age group of 9-year-old children, the incidences in three regions defined as the `high-dose area', the northern, and the middle oblasts, increased by factors of 50, 20, and 6, respectively. These rates (1991–1995) are well above spontaneous rates in other countries. In the age group of 17-year-old juveniles, the incidence increased by a factor of 6 for the `high dose area' and in the three northern oblasts, whereas in the nine `middle' oblasts it was similar to the incidence of the `southern' Ukraine. These rates are within the range found in other countries.     

Thyroid cancer incidence in Ukraine: trends with reference to the Chernobyl accident

For the first time, a comparative analysis of thyroid cancer incidence in Ukraine after the Chernobyl accident was done in a cohort that is almost as large as the general population. On the basis of thyroid doses from radioactive iodine in individuals aged 1-18 years at the time of accident, geographic regions of Ukraine with low and high average accumulated thyroid doses were established and designated "low-exposure" and "high-exposure" territories, respectively. A significant difference of thyroid cancer incidence rates as a function of time between the two territories was found. That is, the increase in the incidence was higher in high-exposure regions than in low-exposure regions. The incidence rates varied substantially among the different attained age-groups, especially in the youngest one (up to 19 years old). The analysis that was adjusted for screening and technological effects also indicated that in the high-exposure regions, thyroid cancer incidence rates at the age of diagnosis of 5-9, 10-14 and 15-19 years were significantly higher in those born in 1982-1986 compared to those born in 1987-1991, while in the low-exposure regions, no significant difference was observed. The observed probable excess of radiation-induced thyroid cancer cases in adults exposed to radioactive iodine from the Chernobyl accident, especially in females, may be due to the high power of the present study. However, it should be noted that our investigation was not essentially free from ecological biases.     

Thyroid cancer incidence among liquidators of the Chernobyl accident

The increase of thyroid cancer incidence rate among children living in the Chernobyl contaminated territories of Belarus, Russia and Ukraine has widely been accepted. Our current work deals with thyroid cancer incidence in the cohort of liquidators (99024 persons) living in 6 regions of Russia: North-West, Volgo-Vyatsky, Central-Chernozemny, Povolzhsky, North-Caucasus and Urals. In the period 1986-1998, a total of 58 thyroid cancer cases were detected in this cohort. We found a statistically significant increase of the thyroid cancer incidence rate in liquidators as compared to the baseline (male population of Russia) level (SIR=4.33, 95% CI: 3.29; 5.60). It was demonstrated that there is no dependence of incidence rates due to external radiation exposure (ERR/Gy=-2.23, 95% CI: -4.67; 0.22).     

Thyroid dose reconstruction for the population of Belarus after the Chernobyl accident

An ¹³¹I environmental transfer model – adapted for Belorussian conditions – was applied to estimate thyroid doses for different population groups. For this purpose the available data were analysed and the important radioecological parameters assessed, i.e. (a) the elimination rate of ¹³¹I from grass due to weathering and growth dilution, (b) the initial interception of ¹³¹I by vegetation, (c) the transfer coefficient for ¹³¹I from grass to cow's milk, (d) the yield of pasture grass and (e) the milk consumption rate. Additionally, the influence of applied countermeasures has been taken into account, such as the interruption of locally produced milk consumption, and the appropriate correction factors have been estimated. As a result, the average age-dependent thyroid doses were assessed for the Belorussian population. The highest average doses in children (> 1 Gy) have been estimated for the Bragin, Khoiniki, Narovlia and Vetka raions of the Gomel oblast. The thyroid exposure tends to decrease from the southeastern (closest to the Chernobyl nuclear power plant areas) to the northwestern part of the republic. When comparing the assessed thyroid doses with dose estimates derived from direct ¹³¹I activity measurements in thyroids (for the locations with more than 15 direct measurements), the results agree fairly well. The model calculation may perhaps overestimate thyroid doses of the population residing in the settlements of the central and northern parts of Belarus, distant from the areas with direct measurements of ¹³¹I activities in soil, grass and milk. These thyroid dose estimates may serve as a basis for further epidemiological studies and risk analyses.     

Time trends of thyroid cancer incidence in Belarus after the Chernobyl accident.

The rates of childhood thyroid cancer incidence observed in Belarus during the period 1986 to 1995 are described as a function of time after exposure, age at exposure, and sex. Conclusions are drawn for the excess absolute risk function. After a minimum latent period of about 3 years after exposure, this risk function has a linear increase with time for at least 6 years. After correction for the dependence of average doses on age, the radiation-induced absolute thyroid risk in Gomel is about a factor of 3 higher for children up to age 10 at exposure compared to older ones; this may be due in part to different case-collection quality. In addition, in the group up to 10 years at exposure, the thyroid of girls is more sensitive to radiation by a factor of about 1.5 than the thyroid of boys on an absolute scale. Risk estimates from external exposure are consistent with risk estimates from Gomel assuming that the increase in excess cases reaches a plateau soon.     

Thyroid cancer risk in areas of Ukraine and Belarus affected by the Chernobyl accident

The purpose of the present study was to analyze the thyroid cancer incidence risk after the Chernobyl accident and its degree of dependence on time and age. Data were analyzed for 1034 settlements in Ukraine and Belarus, in which more than 10 measurements of the (131)I content in human thyroids had been performed in May/June 1986. Thyroid doses due to the Chernobyl accident were assessed for the birth years 1968-1985 and related to thyroid cancers that were surgically removed during the period 1990-2001. The central estimate for the linear coefficient of the EAR dose response was 2.66 (95% CI: 2.19; 3.13) cases per 10(4) PY-Gy; for the quadratic coefficient, it was -0.145 (95% CI: -0.171; -0.119) cases per 10(4) PY-Gy(2). The EAR was found to be higher for females than for males by a factor of 1.4. It decreased with age at exposure and increased with age attained. The central estimate for the linear coefficient of the ERR dose response was 18.9 (95% CI: 11.1; 26.7) Gy(-1); for the quadratic coefficient, it was -1.03 (95% CI: -1.46; -0.60) Gy(-2). The ERR was found to be smaller for females than for males by a factor of 3.8 and decreased strongly with age at exposure. Both EAR and ERR were higher in the Belarusian settlements than in the Ukrainian settlements. In contrast to ERR, EAR increases with time after exposure. At the end of the observation period, excess risk estimates were found to be close to those observed in a major pooled analysis of seven studies of childhood thyroid cancer after external exposures.     

Nuclear-power-plant accidents: thyroid cancer incidence and radiation-related health effects from the Chernobyl accident

Following the Chernobyl accident, enormous amounts of radioisotopes were released in the atmosphere and have contaminated surrounding populations in the absence of rapid protective countermeasures. The highest radiation doses were delivered to the thyroid gland, and the only direct consequence of radiation exposure observed among contaminated population is the increased incidence of thyroid cancers among subjects who were children in 1986 and who lived at that time in Belarus, Ukraine or Russia.     

The time trends of cancer incidence in the most contaminated regions of the Ukraine before and after the Chernobyl accident

Cancer incidence rates in the Ukraine and in the most contaminated regions of the Ukraine are presented for the period before and after the Chernobyl accident.     

Thyroid cancer risk in Belarus after the Chernobyl accident: Comparison with external exposures

Within the time period 1990–1993, childhood thyroid cancer incidence due to the Chernobyl accident increased dramatically in Belarus, especially with regard to the birth cohort January 1, 1971, to May 31, 1986. This rise subsequently slowed down, i.e. during the period 1994–1996. The respective data were analysed and compared with the results of an analysis on the time dependence of thyroid cancer incidence in a pooled cohort of persons who had been exposed during childhood to external radiation with high dose rates. Concerning the period of 5–10 years following exposure, the excess absolute cancer risk per unit thyroid dose in the latter (external) exposure group was found to exceed the one in the Belarus group by a factor of two. This difference, however, is not statistically significant. The age-adjusted average excess absolute risk per unit thyroid dose for the period of 5–50 years following external childhood exposure was found to be 8 female and 14 male cases per 10⁴ person-year · Gy, which is a factor about 2.5 times higher than for the non-adjusted risk in the pooled cohort, as reported by Ron et al. in 1995. Assessments of future excess thyroid cancer cases due to the Chernobyl accident were done on the basis of the time dependence of thyroid cancer risk following external exposure. The thyroid cancer incidence among the birth cohort considered in Belarus and for a period starting from the cessation of the available observation data (1 January 1997) and extending to 50 years after the Chernobyl accident has been estimated to be about 15,000 cases, with an uncertainty range of 5000–45,000 cases. According to our calculations, 80% of these cases exceed the baseline risk under enhanced thyroid surveillance. Received: 8 June 1999 / Accepted in revised form: 20 November 1999     

Screening effects in risk studies of thyroid cancer after the Chernobyl accident

In this article scenarios have been developed, which simulate screening effects in ecological and cohort studies of thyroid cancer incidence among Ukrainians, whose thyroids have been exposed to ¹³¹I in the aftermath of the Chernobyl accident. If possible, the scenarios were based on directly observed data, such as the population size, dose distributions and thyroid cancer cases. Two scenarios were considered where the screening effect on baseline cases is either equal to or larger than that of radiation-related thyroid cancer cases. For ecological studies in settlements with more than ten measurements of the ¹³¹I activity in the human thyroid in May–June 1986, the screening bias appeared small (<19%) for all risk quantities. In the cohort studies, the excess absolute risk per dose was larger by a factor of 4 than in the general population. For an equal screening effect on baseline and radiation-related cancer (Scenario 1) the excess relative risk was about the same as in the general population. However, a differential screening effect (Scenario 2) produced a risk smaller by a factor of 2.5. A comparison with first results of the Ukrainian–US-American cohort study did not give any indication that a differential screening effect has a marked influence on the risk estimates. The differences in the risk estimates from ecological studies and cohort studies were explained by the different screening patterns in the general population and in the much smaller cohort. The present investigations are characterized by dose estimates for many settlements which are very weakly correlated with screening, the confounding variable. The results show that under these conditions ecological studies may provide risk estimates with an acceptable bias.     

Leukaemia and thyroid cancer in emergency workers of the Chernobyl accident:

This work focuses on the direct epidemiological assessment of the risks of radiation-induced leukaemia and thyroid cancer in emergency workers (EW) after the Chernobyl accident. The Russian National Medical Dosimetric Registry (RNMDR) contains data for 168 000 EW as of January 1, 1996. The analysis relates to 48 leukaemias and 47 thyroid cancers, diagnosed and verified. Radiation risks are estimated by comparing the EW data with national data for a male population of the same age distribution. For leukaemia, an excess relative risk per Gy (ERR/Gy) of 4.30 (95% CI: 0.83, 7.75) is obtained, while the excess absolute risk per 10⁴ person-years (PY) Gy (EAR/10⁴ PY Gy) is found to be 1.31 (95% CI: 0.23, 2.39); for thyroid cancer an ERR/Gy of 5.31 (95% CI: 0.04, 10.58) is obtained, and an EAR/10⁴ PY Gy of 1.15 (95% CI: 0.08, 2.22).     

Thyroid dose assessment for the Chernigov region (Ukraine): Estimation based on131I thyroid measurements and extrapolation of the results to districts without monitoring

Based on the results of¹³¹I thyroid activity measurements in three districts of the Chernigov region (Ukraine), individual doses were calculated and an approach of the age dependence of the average thyroid exposure was derived. Using the relationships between the thyroid doses and the¹³⁷Cs deposition as well as the location relative to the Chernobyl Nuclear Power Plant (NPP), age-dependent average thyroid doses were extrapolated also for those settlements of this region where no monitoring measurements have been carried out. The highest doses were found in the west of the region with the lowest distance to the Chernobyl NPP. In this part, the highest mean of the thyroid dose in a settlement was 3.3 Gy for infants and 0.5 Gy for adults. The collective thyroid dose was 31000 and 27 000 person-Gy for children and adults, respectively. Based on this assessment, 140 and 21 excess thyroid cancer cases are predicted for children and adults, respectively. In the years 1989 to 1991, in the whole contaminated territory of the Ukraine 0.4–1.2 cases per 100000 children were observed. Although the absolute numbers are very small, this indicates the possibility of an increase in thyroid cancer morbidity among children. The same trend also seems to be indicated in the Chernigov region. A careful epidemiological study in the future is necessary to enable a final evaluation of radioinduced cancers in this region.     

Fallout from the Chernobyl accident and overall cancer incidence in Finland

Aim: We studied whether incidence of all cancer sites combined was associated with the radiation exposure due to fallout from the Chernobyl accident in Finland. An emphasis was on the first decade after the accident to assess the suggested “promotion effect”. Methods: The segment of Finnish population with a stable residence in the first post-Chernobyl year (2 million people) was studied. The analyses were based on a 250 m × 250 m grid squares covering all of Finland and all cancer cases except cancers of the breast, prostate and lung. Cancer incidence in four exposure areas (based on first-year dose due to external exposure <0.1 mSv, 0.1–1.3, 0.3–0.5, or ≥0.5 mSv) was compared before the Chernobyl accident (1981–1985) and after it (1988–2007) taking into account cancer incidence trends for a longer period prior to the accident (since 1966). Results: There were no systematic differences in the cancer incidence in relation to radiation exposure in any calendar period, or any subgroup by sex or age at accident. Conclusion: The current large and comprehensive cohort analysis of the relatively low levels of the Chernobyl fallout in Finland did not observe a cancer promotion effect.     

Transfer factor of131I from the fallout to human thyroid dose equivalent after the Chernobyl accident

Summary A similar pattern of variation with time in observed maxima of daily dose equivalent rates in human thyroids (TD - µSv·d^–1) and of daily fallout radioactivities (FR - kBq·m^–2) has been found after the Chernobyl accident. An estimate of the time-lag between the maxima in TD lines and the preceding FR peaks was made of about seven days for adult and nine days for juveniles. Applying this time-lag it was possible to estimate transfer factors from the fall-out to thyroid dose equivalent: the highest estimated values were 221 µSv/kBq·m^–2 for adult and 641 µSv/kBq·m^–2 for juvenile thyroids. These values differ from those published by UNSCEAR (United Nations 1988), which have been calculated for various regions of Czechoslovakia, from ingestion and inhalation intake estimates. A broad variation of transfer factor values could be expected to result from such transfer calculations using ingestion and inhalation estimates. The findings also support the concept of a need for prolonged iodine prophylaxy after emissions of radioiodine into the environment.Abbreviations TD dose equivalent rates in thyroids [µSv·d^–1] - FR fall-out radioactivity (-ies) [kBq·m^–2]     

Twenty years after the Chernobyl accident: solid cancer incidence in various groups of the Ukrainian population

Several major international studies such as those performed on the A-bomb survivors, have shown a clear linkage between the exposure to ionizing radiation and the occurrence of various cancer types including leukemia. While these studies are mostly characterized by high dose rates, studies on populations exposed after the Chernobyl accident are in most cases characterized by low dose rates which are typical for occupational radiation protection. Here, data on more than 60,000 Ukrainian workers who participated in recovery operation works in Chernobyl in 1986–1987, more than 50,000 evacuees from the city of Prypyat and the 30 km zone, and about 360,000 residents of most contaminated territories are presented, which cover a period of observation from 1980 to 2004. For all cancers combined, statistically significant higher incidence rates than the national rates were found only for the recovery workers (standardized incidence ratio (SIR) 117.2%, 95% confidence interval: 114.1–120.3), while those for the other investigated groups were lower. In all groups under study a significant increase of thyroid cancer incidence rates has been registered. This increase appears to be associated, at least partly, with the fallout of radioiodine, and it was found not only in children, but also in adolescents and adults. For example, the most significant excess was found for male recovery workers corresponding to a factor of 8.0. It is important to keep in mind, however, that the contribution of confounding factors such as an intensified thyroid screening after the Chernobyl accident could not be quantified, in the present study. For female recovery workers there was also an excess of breast cancer over the national rates (SIR 190.6%; 95% confidence interval: 163.6–217.7%). An analysis of the two other groups (evacuees and residents of contaminated territories) gave controversial results: relative to the local standard there was a statistically significant excess, while comparison with the national level did not substantiate this conclusion.     

A radioecological model for thyroid dose reconstruction of the Belarus population following the Chernobyl accident

A radioecological model was developed to estimate thyroid exposures of the Belarus population following the Chernobyl accident. The input of the model includes an extensive data set of the ¹³⁷Cs activity per unit area deposited during the Chernobyl accident, the rainfall data for different regions of Belarus, the ¹³¹I/¹³⁷Cs ratio in the deposit and the start of the grazing period in Belarus in April/May 1986. The output of the model is the age-dependent thyroid exposure due to the intake of ¹³¹I with fresh milk. Age-dependent average thyroid doses were assessed for selected regions of Belarus. The maximum thyroid doses were estimated for the inhabitants of Gomel oblast where the highest deposition was observed among the regions considered here. The lowest doses were estimated for Vitebsk oblast with the lowest level of depositions. The mean exposures for the oblasts of Grodno, Minsk, Mogilev and Brest were very similar. The results were compared with estimations of thyroid exposure that were based on ¹³¹I measurements in human thyroids, and they are in good agreement. The model may be used for the assessment of thyroid doses in Belarus for areas where no ¹³¹I measurements are available.     

Thyroid doses in Ukraine due to 131I intake after the Chornobyl accident. Report II: dose estimates for the Ukrainian population

This paper describes the revision of the thyroid dosimetry system in Ukraine using new, recently available data on (i) revised ¹³¹I thyroid activities derived from direct thyroid measurements done in May and June 1986 in 146,425 individuals; (ii) revised estimates of ¹³¹I ground deposition density in each Ukrainian settlement; and (iii) estimates of age- and gender-specific thyroid masses for the Ukrainian population. The revised dosimetry system estimates the thyroid doses for the residents of the settlements divided into three levels depending on the availability of measurements of ¹³¹I thyroid activity among their residents. Thyroid doses due to ¹³¹I intake were estimated in this study for different age and gender groups of residents of 30,353 settlements in 24 oblasts of Ukraine, Autonomous Republic Krym, and cities of Kyiv and Sevastopol. Among them, dose estimates for 835 settlements were based on ¹³¹I thyroid activities measured in more than ten residents (the first level), for 690 settlements based on such measurements done in neighboring settlements (the second level), and for 28,828 settlements based on a purely empirical relationship between the thyroid doses due to ¹³¹I intake and the cumulative ¹³¹I ground deposition densities in settlements (the third level). The arithmetic mean of the thyroid doses due to ¹³¹I intake among 146,425 measured individuals was 0.23 Gy (median of 0.094 Gy); about 99.8% of them received doses less than 5 Gy. The highest oblast-average population-weighted thyroid doses were estimated for residents of Chernihiv (0.15 Gy for arithmetic mean and 0.060 Gy for geometric mean), Kyiv (0.13 and 0.051 Gy) and Zhytomyr (0.12 and 0.049 Gy) Oblasts followed by Rivne (0.10 and 0.039 Gy) and Cherkasy (0.088 and 0.032 Gy) Oblasts, and Kyiv City (0.076 and 0.031 Gy). The geometric mean of thyroid doses estimated in this study for the entire Ukraine essentially did not change in comparison with a previous estimate, 0.020 vs. 0.021 Gy, respectively. The ratio of geometric mean of oblast-specific thyroid doses estimated in the present study to previously calculated doses varied from 0.51 to 3.9. The highest increase in thyroid doses was found in areas remote from the Chornobyl nuclear power plant with a low level of radioactive contamination: by 3.9 times for Zakarpatska Oblast, 3.5 times for Luhansk Oblasts and 2.9 times for Ivano-Frankivsk Oblast. The developed thyroid dosimetry system is being used to revise the thyroid doses due to ¹³¹I intake for the individuals of post-Chornobyl radiation epidemiological studies: the Ukrainian-American cohort of individuals exposed during childhood and adolescence, the Ukrainian in utero cohort, and the Chornobyl Tissue Bank.

     

Thyroid cancer incidence among Chernobyl emergency workers: follow-up period 1986-2003

A vast amount of research articles devoted to the increase in childhood thyroid cancer incidence in the most contaminated by radionuclides territories of Belarus, Russia and Ukraine affected by the Chernobyl accident were published recent years. However, the amount of research studies of thyroid cancer incidence among the Chernobyl emergency workers (liquidators) is quite scanty. In the article results of the study of thyroid cancer incidence in the cohort of the Chernobyl liquidators (103427 persons) residing in 6 administrative regions of Russia (North-West, Volgo-Vyatsky, Central-Chernozemny, Povolzhsky, North-Caucasus and Urals) are described and discussed. For the period 1986-2003 eighty seven cases of thyroid cancer were detected in the cohort. Statistically significant excess of the incidence among liquidators over baseline incidence, SIR = 3.39 (95% CI: 2.73; 4.16), among men of Russia has been found. The highest thyroid cancer incidence (SIR = 6.49) was registered among liquidators who had been involved in mitigation works during April-July 1986. At the same time no statistically significant relationship between the incidence rate and external radiation dose, ERR = 1.68 (95% CI: -0.95; 6.46), was detected.