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 cancer mortality, respectively, among radiation workers who started work at the Mayak nuclear complex in Chelyabinsk, Russia, during 1948-58. Mayak is the site where, from 1948, plutonium was produced for the weapons programme of the USSR, and workers were exposed to relatively high levels of the element, particularly in the 1940s and 1950s. It is known that once plutonium enters the bloodstream it accumulates in the liver and bone, and animal experiments have demonstrated that plutonium can cause liver and bone cancers. It is also known from groups of people exposed to other alpha-particle-emitting radionuclides that bone cancer can be induced by radium and that liver cancer can be induced by thorium. However, apart from a single bone cancer in a group of 26 Los Alamos Manhattan Project workers, epidemiological studies of workforces exposed to plutonium in the USA and the UK have failed to provide evidence of an excess risk of these cancers. This is almost certainly due to lack of statistical power, the highest assessed plutonium body burden in the Sellafield workforce, the most exposed group of workers previously studied, being about 7 kBq. This compares with a mean body burden in the Mayak workers of 8.4 kBq and a maximum burden of 173 kBq. Therefore, the heavily exposed Mayak workers provide an opportunity to obtain direct epidemiological evidence for exposure-related excess risks of bone and liver cancers. The two papers report the results of preliminary studies of bone and liver cancer mortality among 11 000 workers who started employment at Mayak during 1948-58.

The study of the Mayak workers is not straightforward because of the absence of relevant background cancer mortality rates for Russia except for recent years, and because of the current limitations of the plutonium exposure data based on the analysis of routine urine samples. Also, the cumulative external dose must be taken into account when assessing the effects of plutonium exposure. The authors found 23 cancers occurring in the bone or soft tissue directly adjacent to bone surfaces, and 60 liver cancers. Despite the preliminary nature of the analyses reported in these papers, due to the present uncertainties in the data, compelling evidence is presented for a plutonium-related excess risk of these cancers in the Mayak workers under study. Analyses that compare risks between various groups within the Mayak workforce show that the excess risks (adjusted for external dose) are highest in those 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; liver cancer relative risk 17, 95% confidence interval 8.0 to 36), and among plutonium workers without routine monitoring data. In addition, there are clear trends of increasing risks with increasing body burden. The limitations of the data currently preclude the quantification of excess risks in terms of organ dose, but the evidence presented encourages the view that risk coefficients might eventually be obtained from the study of the Mayak workforce.

These papers follow a paper earlier this year (Kreisheimer et al 2000 Radiat. Res. 154 3-11) which compared lung cancer mortality among 1669 male Mayak workers exposed to plutonium with that among 2172 male Mayak workers exposed to external radiation only. Inhalation of plutonium led to high doses to the lung: a mean cumulative dose of around 0.5 Gy was assessed to have been received by the most heavily exposed group of 610 plutonium production workers, among whom one quarter of deaths were due to lung cancer. The influence of cigarette smoking is always a problem in such studies, but the authors conclude that, of the 117 lung cancers among the plutonium workers, 75 were due to inhalation of plutonium. The estimated excess relative risk per unit lung 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 among female Mayak workers (who were predominantly non-smokers, but experienced higher plutonium exposures than the men), could lead to a refinement of risk estimates, but a plutonium-related excess risk of lung cancer among the Mayak workers is clear.