of studies of cancer frequency among exposed populations in these countries. Results from this follow-up may also be biased because participation in the annual examination may be related to illness and/or to level of exposure.
Means also exist in the affected countries to carry out “passive” follow-up of exposed persons and of the general population with the use of population registries—of mortality, cancer, and other diseases. In each country of the former USSR, population registration is carried out at the local level in the address bureaus (where the addresses of current residents are kept) and the ZAGS (buro zapicii akta grazhdanskovo sostoyania), which compiles all information about birth, marriage, divorce, and death of persons living in the administrative area. No centralized registry exists, however, and results of a pilot study (Cardis and Okeanov 1996) indicate that considerable time and effort may be needed to trace subjects who have moved from one area to another.
A computerized national Cancer Registry has been functioning in Belarus since the 1970s and registers all cases of malignant neoplasms. A comprehensive registry of hematological diseases also exists in Belarus, in the Institute of Haematology and Blood Transfusology. In Russia and the Ukraine, no centralized cancer registration system was in place at the time of the accident. Work has been carried out in both countries to set one up—at least in contaminated areas in Russia (Okeanov and others 1996; Storm and others 1996)—and quality control activities are continuing.
Information is also available systematically on the general (i.e., not only cancer) morbidity of the population of the three countries. In the countries of the former USSR, regional outpatient clinics systematically collect information on disease diagnoses on all the residents of the region they cover (not only those included in the Chernobyl Registry). This information is summarized locally and is sent on special statistical reporting forms at yearly intervals to the Ministry of Health. These forms contain information about the number of cases of acute and chronic diseases diagnosed in a given year in the population in all areas of the country. This information is not broken down by age or sex. No verification of completeness or duplicates is possible. This passive system of collecting morbidity data on the population contrasts with the active follow-up carried out, as described above, for persons included in the Chernobyl Registry. Comparisons of morbidity based on these sources must therefore be interpreted with caution.
The dosimetric information available for liquidators is subject to controversy because personal dosimeters in use in the early days after the accident were too few and generally too sensitive. A reasonable estimate of the average dose received by the group of 200,000 people who worked in 1986–1996 is 100 mSv (Ivanov and others 1996). Thus, the collective effective dose would be approximately 20,000 Sv. Some workers received their dose in a few minutes—for example working on the roof of the reactor—while others received it over months or even years, and the predominant radiation type and route of exposure varied according to the time and activity of liquidators.
Dose estimates have generally been derived in one of three ways:
individual dosimetry: the liquidator was given a personal dosimeter;
group dosimetry: an individual dosimeter was assigned to one member of a group of liquidators; or
itineraries: measurements of γ-ray levels were made at various points where liquidators worked, and an individual’s dose was estimated as a function of the points where he or she worked and the time spent in these places.
Liquidators are in principle included in the State Chernobyl Registries of Belarus, Russia, and Ukraine. Doses for a substantial proportion of them are missing from these registries. Liquidators who worked in the first year generally had higher recorded doses than those who worked in subsequent years. The level of dosimetric control and the adequacy of dose estimates vary between civilian liquidators (construction workers, logistic support), military liquidators (soldiers and officers who worked in decontamination, dosimetric control, and evacuation), and radiation specialists.
Increases (doubling or tripling) in the incidence of leukemia and thyroid cancer have been observed in most of the studies of liquidators from Belarus, Russia, and Ukraine. Increases in leukemia risk are not unexpected since predictions from risk estimates in atomic bomb survivors have shown that if the experience of the A-bomb survivors is applicable to the Chernobyl situation, a tripling of leukemia mortality could be expected in the first 10–12 years following exposure (Cardis and others 1996).
These results are difficult to interpret since, as indicated above, the follow-up of liquidators is much more active than that of the general population in the three countries. There are questions about the adequacy and completeness of the diagnostic information on liquidators in the Chernobyl Registry (Cardis and others 1996). For thyroid cancer in adults, the depth of screening to which the liquidators are subjected may greatly influence the observed incidence.
In a case-control study based on the limited dosimetric data of the Chernobyl Registry in Russia, no significant association was seen between the risk of leukemia and radiation dose (Ivanov and others 1997a, 1997b). A recent cohort study of Russian liquidators showed no association between external radiation dose and risk of thyroid cancer among 72,000 liquidators from six regions (Ivanov and others