787 RADIATION RESEARCH 159, 787–798 (2003) 0033-7587/ 03 $5.00  2003 by Radiation Research Society. All rights of reproduction in any form reserved. Cancer Mortality Risk among Workers at the Mayak Nuclear Complex N. S. Shilnikova, a D. L. Preston, b,1 E. Ron, c E. S. Gilbert, c E. K. Vassilenko, d S. A. Romanov, a I. S. Kuznetsova, a M. E. Sokolnikov, a P. V. Okatenko, a V. V. Kreslov a and N. A. Koshurnikova a a Southern Urals Biophysics Institute, Ozyorsk, Russia; b Department of Statistics, Radiation Effects Research Foundation, Hiroshima, Japan; c Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, Maryland; and d Department of Radiation Safety, Mayak Production Association, Ozyorsk, Russia Shilnikova, N. S., Preston, D. L., Ron, E., Gilbert, E. S., Vassilenko, E. K., Romanov, S. A., Kuznetsova, I. S., Sokol- nikov, M. E., Okatenko, P. V., Kreslov, V. V. and Koshurni- kova, N. A. Cancer Mortality Risk among Workers at the Mayak Nuclear Complex. Radiat. Res. 159, 787–798 (2003). At present, direct data on risk from protracted or fraction- ated radiation exposure at low dose rates have been limited largely to studies of populations exposed to low cumulative doses with resulting low statistical power. We evaluated the cancer risks associated with protracted exposure to external whole-body  radiation at high cumulative doses (the average dose is 0.8 Gy and the highest doses exceed 10 Gy) in Russian nuclear workers. Cancer deaths in a cohort of about 21,500 nuclear workers who began working at the Mayak complex between 1948 and 1972 were ascertained from death certifi- cates and autopsy reports with follow-up through December 1997. Excess relative risk models were used to estimate solid cancer and leukemia risks associated with external -radia- tion dose with adjustment for effects of plutonium exposures. Both solid cancer and leukemia death rates increased signifi- cantly with increasing -ray dose (P  0.001). Under a linear dose–response model, the excess relative risk for lung, liver and skeletal cancers as a group (668 deaths) adjusted for plu- tonium exposure is 0.30 per gray (P  0.001) and 0.08 per gray (P  0.001) for all other solid cancers (1062 deaths). The solid cancer dose–response functions appear to be nonlinear, with the excess risk estimates at doses of less than 3 Gy being about twice those predicted by the linear model. Plutonium exposure was associated with increased risks both for lung, liver and skeletal cancers (the sites of primary plutonium de- position) and for other solid cancers as a group. A significant dose response, with no indication of plutonium exposure ef- fects, was found for leukemia. Excess risks for leukemia ex- hibited a significant dependence on the time since the dose was received. For doses received within 3 to 5 years of death the excess relative risk per gray was estimated to be about 7 (P  0.001), but this risk was only 0.45 (P  0.02) for doses received 5 to 45 years prior to death. External -ray expo- sures significantly increased risks of both solid cancers and leukemia in this large cohort of men and women with occu- 1 Address for correspondence: Department of Statistics, 5-2 Hijiyama Koen, Minami ku, Hiroshima 730-0045, Japan; e-mail: preston@rerf.or. jp. pational radiation exposures. Risks at doses of less than 1 Gy may be slightly lower than those seen for doses arising from acute exposures in the atomic bomb survivors. As dose esti- mates for the Mayak workers are improved, it should be pos- sible to obtain more precise estimates of solid cancer and leu- kemia risks from protracted external radiation exposure in this cohort.  2003 by Radiation Research Society INTRODUCTION At present, radiation risk estimates used as a basis for radiation protection standards are derived from data on per- sons exposed externally at high doses and high dose rates, including the atomic bomb survivors in Hiroshima and Na- gasaki and many medically exposed cohorts (1–4). There naturally are concerns about the relevance of these data for estimating risks from exposure at low doses and dose rates, which are of primary interest for radiation protection. Di- rect data on risk from protracted or fractionated exposure at low dose rates mainly have been limited to studies of populations (such as nuclear workers) exposed to low cu- mulative doses with resulting low statistical power and high potential for confounding. The Mayak Production Association, which is located in the Southern Urals in the Russian Federation, about 100 km from the city of Chelyabinsk, began operations in 1948 as the first and largest nuclear weapons facility in the for- mer Soviet Union. A substantial number of workers at the Mayak facility, especially those employed in the first de- cade of operation, received cumulative doses that far ex- ceed those of nuclear workers in other countries. Thus these workers offer an unusual opportunity to study the effects of protracted whole-body exposure at cumulative doses that are sufficiently large to estimate risks with some degree of precision. The Mayak worker cohort is unique among co- horts used to study radiation effects because it includes a large proportion of women (24%) whose average doses are similar to those for men. The cohort has been described in several publications (e.g. refs. 5–7 ). Quantitative estimates of cancer mortality risks associated with chronic low-dose- rate exposure at Mayak can be an important complement to estimates of the effects of acute exposures derived from