Study of plutonium workers at the Mayak complex inRussia

Abstract

In the September 2000 issue of Radiation Research, Koshurnikova et al(2000 Radiat. Res. 154 237-45) and Gilbert et al (2000 Radiat. Res 154 246-52)report their preliminary findings of bone cancer and liver cancermortality, respectively, among radiation workers who started work at theMayak nuclear complex in Chelyabinsk, Russia, during 1948-58. Mayak isthe site where, from 1948, plutonium was produced for the weaponsprogramme of the USSR, and workers were exposed to relatively highlevels of the element, particularly in the 1940s and 1950s. It is knownthat once plutonium enters the bloodstream it accumulates in the liverand bone, and animal experiments have demonstrated that plutonium cancause liver and bone cancers. It is also known from groups of peopleexposed to other alpha-particle-emitting radionuclides that bone cancercan be induced by radium and that liver cancer can be induced bythorium. However, apart from a single bone cancer in a group of 26 LosAlamos Manhattan Project workers, epidemiological studies of workforcesexposed to plutonium in the USA and the UK have failed to provideevidence of an excess risk of these cancers. This is almost certainlydue to lack of statistical power, the highest assessed plutonium bodyburden in the Sellafield workforce, the most exposed group of workerspreviously studied, being about 7 kBq. This compares with a mean bodyburden in the Mayak workers of 8.4 kBq and a maximum burden of 173 kBq.Therefore, the heavily exposed Mayak workers provide an opportunity toobtain direct epidemiological evidence for exposure-related excess risksof bone and liver cancers. The two papers report the results ofpreliminary studies of bone and liver cancer mortality among11 000workers who started employment at Mayak during 1948-58. The study of the Mayak workers is not straightforward because of theabsence of relevant background cancer mortality rates for Russia exceptfor recent years, and because of the current limitations of theplutonium exposure data based on the analysis of routine urine samples.Also, the cumulative external dose must be taken into account whenassessing the effects of plutonium exposure. The authors found 23cancers occurring in the bone or soft tissue directly adjacent to bonesurfaces, and 60 liver cancers. Despite the preliminary nature of theanalyses reported in these papers, due to the present uncertainties inthe data, compelling evidence is presented for a plutonium-relatedexcess risk of these cancers in the Mayak workers under study. Analysesthat compare risks between various groups within the Mayak workforceshow that the excess risks (adjusted for external dose) are highest inthose workers with body burdens estimated to be in excess of 7.4 kBq(bone cancer relative risk 7.9, 95% confidence interval 1.6 to 32; livercancer relative risk 17, 95% confidence interval 8.0 to 36), and amongplutonium workers without routine monitoring data. In addition, thereare clear trends of increasing risks with increasing body burden. Thelimitations of the data currently preclude the quantification of excessrisks in terms of organ dose, but the evidence presented encourages theview that risk coefficients might eventually be obtained from the studyof the Mayak workforce. These papers follow a paper earlier this year (Kreisheimer etal 2000 Radiat. Res. 154 3-11) which compared lung cancer mortalityamong 1669male Mayak workers exposed to plutonium with that among 2172 maleMayak workers exposed to external radiation only. Inhalation ofplutonium led to high doses to the lung: a mean cumulative dose ofaround 0.5 Gy was assessed to have been received by the most heavilyexposed group of 610 plutonium production workers, among whom onequarter of deaths were due to lung cancer. The influence of cigarettesmoking is always a problem in such studies, but the authors concludethat, of the 117 lung cancers among the plutonium workers, 75 were dueto inhalation of plutonium. The estimated excess relative risk per unitlung dose is 12.1 per Gy at age 60 (95% confidence interval 7.8 to 20).Further work, including the examination of lung cancer mortality amongfemale Mayak workers (who were predominantly non-smokers, butexperienced higher plutonium exposures than the men), could lead to arefinement of risk estimates, but a plutonium-related excess risk oflung cancer among the Mayak workers is clear.