Chapter 1

 

MEDICAL CONSEQUENCES OF THE ACCIDENT AT THE CHERNOBYL NPP:
FORECAST AND COMPARISON WITH ACTUAL DATA IN THE NATIONAL REGISTRY

 

 

Introduction

 

More than 14 years have passed since the accident at the Chernobyl nuclear power plant. What can be said today regarding its medical consequences? If one turns to the “Medline” International Medico-Informational System currently available on the Internet, one will see that this problem has been addressed in over 2,000 scientific articles. Yet, a lot of issues associated with an unbiased assessment of medical effects of the Chernobyl accident remain unresolved and invite further exploration.

In May 2000, the 49th UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) Session was held in Vienna. This authoritative international organization has always paid significant attention to evaluating medical effects of the Chernobyl accident. Publications by leading experts in radiation epidemiology on the issues related with predicted and actual data have been analyzed. The scientific studies performed by the National Radiation and Epidemiological Registry (NRER) have been recognized as those having one of the highest UNSCEAR citation indices. The NRER was created pursuant to a Resolution of the Russian Federation Government and is based on the Medical Radiological Research Center of the Russian Academy of Medical Sciences (in the town of Obninsk). The Chernobyl Registry is headed by leading two Russian experts in radiation epidemiology, the RAMS Academician A.F. Tsyb and Professor V.K. Ivanov. Below, they give their current evaluations of the medical effects of the Chernobyl accident, as well as predictions of potential delayed effects resulting from radiation exposure.

 

Delayed effects from radiation impact:
lessons learned from
Hiroshima and Nagasaki

 

Major conclusions of the large-scale radioepidemiological studies carried out in Hiroshima and Nagasaki after the atomic bombing of these cities in 1945 are of great significance for making predictions of the medical effects of the Chernobyl accident. That is because the models developed up to now for predicting delayed radiation effects that are presently accepted by the International Commission on Radiological Protection (ICRP) are based chiefly on these Japanese data.

The decision to embark upon epidemiological research in Hiroshima and Nagasaki was made by the United States Government in 1948. At present, these studies are performed by the Radiation Effects Research Foundation (RERF) with the financial support from the USA and Japan.

What major results have been obtained in Japan? There are two major results directly related with the rate of radiation-induced cancer diseases among hibakusha’s (atomic bomb survivors). An epidemiological observation of a cohort of 86.5 thousand people in 1950-1990 has shown that (Table 1):

·   there is a definite excess of the cancer morbidity and mortality over respective spontaneous levels;

·   there exists a dependence of cancer morbidity and mortality on dose in the range of the so-called medium and high exposure doses (above 0.3 Sv); the radiation risk of leukemia is 5-7 times higher than the radiation risk for solid cancers.

The average doses of exposure to the population and the liquidators of the Chernobyl accident were several times lower than those for hibakusha’s. This leads to an important question: is it possible to use the models based upon Japanese data for predicting delayed medical effects of the Chernobyl accident?

Table 1

Results of oncoepidemiological research in Hiroshima and Nagasaki
(a cohort of 86,572 survivors followed up between 1950 and 1990)

(Preston et al., 1996)

 

 

Mortality

Observed

Expected

Î - Å

Solid cancers

7578

7244

334

Leukemias

249

162

87

 

 

Thus, up to now, the results of more than 45 years of follow-up of the 86-thousand cohort of survivors after the atomic bombing of Hiroshima and Nagasaki, whose exposure doses were no higher than 6 Sv, have allowed detecting 420 additional deaths from radiation induced cancers. The total number of the deaths from spontaneous cancers within the above period was 7,406.

Because of the much larger exposure and consequent much larger doses in the Japanese cohort as compared to Chernobyl doses, the detecting of exposure effects from the Chernobyl accident turned out to be a challenge.

 

National Chernobyl Registry of Russia:
objectives, stages of development, current status

 

The resolution to establish an all-Union registry of persons who suffered from the radiation impact resulting from the Chernobyl accident was adopted in June 1986 (USSR Ministry of Health resolution No. 833 of June 23, 1986). What goals were pursued by the Government when establishing the Registry? Two are worth noting as major goals:

·       informational support to clinical examination of the population and the liquidators who suffered from radiation exposure, for the purpose of developing an optimal strategy for mitigation of medical effects of the accident at the Chernobyl NPP;

·       large-scale radioepidemiological studies for objective assessment of dependence of response on dose and of radiation risks.

In December 1991 (the time of the disintegration of the Soviet Union), the Registry’s database contained personal medico-dosimetric information on 659,292 individuals, including 284,919 participants of recovery work following the Chernobyl accident (so-called liquidators). All the republics of the former Soviet Union had been involved in the development of the Registry, as well as numerous scientific and practical institutions (Ivanov et al., 1999).

On September 22, 1993, the Russian Federation Government adopted Resolution No. 948 “On State Registration of Citizens Who Have Suffered from the Radiation Impact and Radiation Exposure As a Result of Chernobyl and Other Radiation Catastrophes and Incidents”. As of January 1, 2000, the Russian National Medical and Dosimetric Registry (RNMDR) contained personal medical and dosimetric data on 550,076 individuals who had suffered from the radiation impact as a result of the Chernobyl accident and who resided in the territory of the Russian Federation, including 179,923 participants in the elimination of the consequences of the Chernobyl accident  (Figure 1).

To date, approximately 7,000,000 diagnoses of diseases detected among the liquidators and the population in Russian territories contaminated with radionuclides have been accumulated in the RNMDR central database.

 

Figure 1. The distribution of primary registration groups in the registered contingent of 550,000 persons.

 

 

The annual collection of individual medical and dosimetric data is performed by the Registry by means of 15 regional centers and five departmental registries (RF Ministry for Atomic Energy, Ministry of Defense, Ministry of Internal Affairs, Ministry of Railways, Federal Security Service). The Bryansk, Kaluga, Oryol and Tula regions transfer information directly to the Federal level of the Registry in Obninsk.

 

Prediction of delayed health effects of the Chernobyl accident

 

To solve the problem of predicting radiation effects that are remote in time, the following must be present:

·       a prediction model that is adequate in the range of low exposure doses that were characteristic of the Chernobyl accident;

·       estimates of individual exposure doses to persons who suffered from the impact of the radiation.

As noted above, a number of models for the prediction of remote radiation effects have been developed in Japan and verified through direct epidemiological investigations for medium and high exposure dose ranges. The International Commission on Radiological Protection (ICRP) has suggested a linear extrapolation of the dose-response relationship extend to the range of low exposure doses (A) (Figure 2). At the same time, in the absence of direct epidemiological corroboration in the scientific literature of radiation effects at low exposure doses, the so-called "threshold model" has received some recognition as well (B). Within the framework of this model, it is assumed that there are no radiation effects at exposure doses that are below a certain threshold value. In what follows we will use the linear model A.   

Exposure doses. The issue of exposure doses to the liquidators has been addressed many times in the scientific literature and mass media. When doing so, conflicting opinions were often voiced. In their meeting in Vienna in May 2000, UNSCEAR performed a comprehensive consideration of the issue and confirmed the adequacy of the dosimetric data about liquidators in the Registry.

Thus, based on the ICRP’s linear non-threshold model and data on doses of external exposure to liquidators, a prediction regarding delayed radiation effects related with cancer diseases can be made.

Below, some prognostic estimates for liquidators of 1986 who received the highest external exposure doses (average dose is 16 cGy) are considered (Figure 3).

 

Figure 2. Models for radiation risks.

 

 

Figure 3. Radiation doses to liquidators.

 

 

In 2000, solid cancers (all types of malignant neoplasms, with the exception of leukemias) could be detected in 385 per 100,000 liquidators in 1986. The principal question arises: for how many liquidators was cancer induced by radiation? Based on the Registry’s personal individual dosimetric data and up-to-date non-threshold radiation risk prediction models, it can be estimated that out of the 385 cancer cases, only 18 (4.7%) were induced by the dose of external radiation absorbed while performing recovery work at the Chernobyl NPP (Table 2). In this table the linear No-Threshold Model (line A in Figure 2) of ICRP is used to calculate the “expected” numbers, with the coefficients detailed in Pierce et al. (1996) divided by a factor of 2 to account for a for low dose rate compared to the Japanese high dose rate.

As for the leukemia incidence among liquidators of 1986, 11 cases per 100,000 liquidators are expected in 2000, out of which 4 (36.4%) will be induced by the radiation factor. When comparing this rate with that for solid cancers (4.7%), it may be derived that the radiation risk of leukemias is 7.5 times higher than that for solid cancers. At the same time, the rate of this disease is lower than that of solid cancers by a factor of about 35.

 

 

 

Table 2

Forecast of effects from radiation exposure to participants in recovery operations at the Chernobyl NPP

 

Year of arrival
in 30-km zone around ChNPP

Cancer diseases
per 100,000

Expected in 2000

Risk for life

Total

Number and % induced by radiation

Total

Number and %  induced by radiation

1986

Solid cancers

385

18 (4.7%)

21638

1089 (5.3%)

Leukemias

11

4 (36.4%)

485

113 (23.1%)

1987

Solid cancers

366

10 (2.7%)

19922

581 (2.9%)

Leukemias

9

2 (22.2%)

430

59 (13.7%)

1988-1990

Solid cancers

328

4 (1.2%)

19532

228 (1.2%)

Leukemias

8

1 (12.5%)

392

22 (5.6%)

 

As pointed out above, the Registry currently incorporates individual medical and dosimetric data on 179,923 participants in the work on elimination of Chernobyl accident consequences. Out of these, 37,109 cases of solid cancers are expected for lifetime, and these diseases will be induced by radiation for 1,340 persons (3.6%). For the above-mentioned liquidator cohort numbering 179,923 individuals, 809 cases of leukemias are expected for lifetime, of which 138 (17%) will be induced by radiation.

Below, some results of prognostic assessment on radiation-related cancer induction among residents in Russian territories contaminated by radionuclides are given. The radiation impact was most pronounced for inhabitants of the Bryansk oblast, particularly the southwestern districts. The calculational results are evidence that the radiation-related excess in solid cancers for all the Bryansk oblast (region) residents will amount to 0.3%, and for leukemias, 1.2%. The most unfavorable prediction refers to induction of radiation-related thyroid cancer in children (at the time of the Chernobyl accident) of the Bryansk oblast (region). Possibly, 30 percent of all the thyroid cancer cases in this population group will be induced by the radiation factor (incorporated 131I).

These are some prognostic estimates of the delayed radiation effects as based on the data now in the National Registry.

Actual data in the National Registry: morbidity, disability, mortality
(observation period: 1986-2000)

 

To what extent are prognostic estimates in compliance with actual data of the National Registry? Can any reasonable conclusions on the medical effects of the Chernobyl accident be made as early as at the current stage (the Japanese registry has been functioning for around 50 years)?

First, let us address the issue of the rate of leukemias among liquidators. As noted above, Japanese scientists in Hiroshima and Nagasaki have assigned the highest radiation risk to leukemias.

Among liquidators, 145 leukemias have been detected, out of which 50 cases were induced by radiation.

An analysis of the rate of leukemia incidence in liquidators for the past years has shown a number of very important conclusions.

Firstly, for the first 2-3 years after the Chernobyl accident, no difference was observed in the rate of leukemias between liquidators and the expected rate. About 5-7 cases per 100,000 were registered annually, which is within the range of statistical data on cancer diseases among the male population of the country, normalized by age.

Secondly, between 1992-1995 (after the latent period of radiogenic leukemia induction), the registered rate of leukemia among liquidators was approximately twice the expected (spontaneous) level. It is worth noting that a similar peak of the rate of leukemia incidence among liquidators between 1992-1995 was registered independently by the national Chernobyl registries of Belorussia and Ukraine.

Thirdly, in the most recent years of tracing (1996-2000), the leukemia incidence among liquidators has been permanently decreasing, approaching the expected (spontaneous) level.

Thus, the main conclusion can be made, the prediction regarding the radiation induced leukemia incidence among liquidators is well corroborated by the factual data available in the Registry. A dose dependence of radiation-related leukemia cases for the range of low exposure doses was brought to light for the first time.

What can be said about the actual data of the Registry describing the rate of solid cancers among liquidators? As one would expect from the longer latent period appropriate to sold cancers, the excess of the solid cancer incidence among liquidators over the spontaneous level has yet to be proved in an unambiguous fashion. And it is no wonder. Firstly, according to our prediction, the expected morbidity rise cannot exceed 3-4 percent (this figure is close to a statistical scatter typical of epidemiological studies). Secondly, the latent period of solid cancer induction is about 10 years, starting from the moment of the radiation impact, and the increase should not yet be evident.

Among liquidators, 55 thyroid cancers have been detected, out of which 12 cases were induced by radiation.

Disability rate of among liquidators. In recent years, a significant rise in the number of disabled liquidators has been registered (Figure 4).

 

*

 

 

Figure 4. Temporal variations of disability rates among liquidators.

 

 

For the time being, 27% of liquidators have the status of disabled. It is a very high percentage if one takes into account that the average age of the liquidators is currently 48-49 years. At the same time, no dose dependence has been detected for the disability rate among liquidators. This, in essence, implies that there is a high priority to understanding the social factor in the dynamics of the disability rate.

Mortality among liquidators. In the last 2-3 years, mass media have published a number of articles containing some data on an increase in mortality among liquidators. It was pointed out that more than 10,000 liquidators who lived in Russia died in these years. Lamentably, it is true. It is no secret that at the same time the demographic situation in Russia is getting catastrophic. The mortality among the male population of the age group of 40 to 50 years old (which is the age group to which the majority of liquidators belong) has risen by at least 70% (Figure 5). Therefore, according to the Registry’s data, the mortality rate among liquidators for all causes, including cancer diseases, does not exceed a similar index for male population of the country.

 

 

Figure 5. Lifetime of Russian male population.

 

 

Let us consider the actual data of the National Registry of residents in Russian territories contaminated with radionuclides. As mentioned above (Japanese data and our prognostic estimates), the leukemia incidence should be treated as a definite indicator of the role of radiation factor. A comparison of the leukemia incidence rate among residents of the 7 most contaminated districts of the Bryansk oblast and the population of the country as a whole has not allowed the detection of any significant difference.

 

Figure 6. Rate of leukemia incidence among Bryansk oblast population compared to the population of all of Russia).

 

We deduced from the data of figure 6 that there are no reasons for associating the radiation risks with induction of blood diseases.

 

When predicting delayed effects of radiation in the general population, we have noted earlier that an unfavorable situation with thyroid cancer in children (as of the time of the Chernobyl accident) of the Bryansk oblast cannot be ruled out. What do we have in reality? Unfortunately, the prognostic estimates have been fully corroborated by the actual data from the Registry. As of the time of the accident at the Chernobyl nuclear power plant, among the children in the Bryansk oblast, about 30% of detected thyroid cancer cases were induced by the radiation impact from incorporated 131I.

Among the children (at the time of the Chernobyl accident) of the Bryansk oblast, 170 thyroid cancer cases have been detected, out of which 55 cases are related to radiation (Ivanov et al., 2001).

These are the actual data of the National Registry as of 14 years after the Chernobyl accident.

 

Problems and prospects of the National Registry

 

Up to now, we have been discussing the issues of direct radiation effects as applied to medical consequences of the Chernobyl accident, that is induction of radiation-related cancer among liquidators and population in Russian territories contaminated by radionuclides.

For the first time, large-scale epidemiological investigations based on the National Registry have made it possible to uncover a relationship between leukemia incidence among liquidators exposed to low doses (no higher than 0.2 Sv) and the dose.

An extremely unfavorable situation with thyroid cancer incidence among children, primarily in the Bryansk oblast, arises. This is a very definite result.

Are medical effects of the Chernobyl accident limited to the above problems (that is, those of cancer diseases)? Probably, not. An unbiased evaluation should be given for risk factors, potentially not associated directly with radiation impact.

In May 2000, Professor I. Shigematsu, who had been in the head of the Japanese registry on hibakusha’s for many years and had been awarded the prestigious Sievert Prize for this work, presented a detailed report at the 10th International Congress on Radiation Protection. In the presentation, a number of important problems were addressed. Additionally, some results of the epidemiological studies on assessment of major risk factors influencing the lifetime after radiation impact (completed in 1999) were presented for the first time. At first glance, the results do not allow for any obvious conclusions. Along with radiation, such human characteristics as elevated blood pressure, smoking, unbalanced nutrition, excess weight etc. were considered to be significant risk factors. In what way is it related to Chernobyl? Apparently, the relationship is straightforward. Taking into account the social problems of the country after the accident at the Chernobyl nuclear power plant, as well as the often unjustified exaggeration of stress situations, by mass media as well, extremely inadequate level of aid to liquidators and residents in Russian regions contaminated with radionuclides, one may well expect an unfavorable forecast regarding the remaining lifetime of the persons involved those factors which have no direct relation to absorbed exposure doses. This is corroborated by the medical data accumulated in the National Registry. Along with further studies of cancer incidence (the latent period in induction of radiogenic solid cancers is about 10 years; it came to its end in 1996), of great significance would be a large-scale epidemiological research into medical non-cancer effects of the Chernobyl accident. Only then, we may be able to develop an optimal strategy for mitigation of the delayed health effects of the accident at the Chernobyl nuclear plant.

 

References

 

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. - 1999. - Vol. 19, No 4. - P. 305-318.

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/Editors G.N.Souchkevitch, M.N.Repacholi. - Geneva: World Health Organization, 2001.

Pierce, D.A., Shimuzu, Y., Preston, D.L., Vaeth, M., and Mabuchi, K. Studies of the mortality of atomic bomb survivors, Report 12, Part 1: Cancer 1950-1990. Radiation Research – 1996 - 146:1-27.