aBased on male atomic bomb survivors, aged 20–60 years at exposure, as presented by Muirhead and others (1999).
protection recommendations are based. Overall, they do not suggest that current radiation risk estimates for cancer at low levels of exposure are appreciably in error. Uncertainty concerning the exact size of this risk, remains, however, as indicated by the width of the confidence intervals presented.
WORKERS FROM THE MAYAK FACILITY
A cohort of about 21,000 Russian nuclear workers who worked at the Mayak plutonium production complex between 1948 and 1972 is under study. The Mayak complex, which is located in the Chelyabinsk region of the Russian Federation, includes three main plants: a reactor complex, a radiochemical separation plant, and a plutonium production plant. Workers at all three plants had the potential for exposure to external radiation, and workers at the radiochemical and plutonium production plants also had the potential for exposure to plutonium. Recently, data on workers at two auxiliary plants, who had much less potential for exposure, have been added to the cohort under study to expand the comparison group. As for other nuclear worker cohorts, estimates of annual external doses are available from individual film badge monitoring data. Some workers were also monitored for plutonium exposure; however, since routine testing based on large urine samples did not begin until about 1970, only about 40% of workers with the potential for such exposure have been monitored.
External exposures and exposures of Mayak workers to plutonium far exceed those of other nuclear worker cohorts discussed previously in this chapter. For example, for the nearly 11,000 monitored workers hired before 1959, the mean cumulative external dose was 1.2 Gy, more than an order of magnitude higher than any of the cohorts described in Table 8-2. Thus, the Mayak cohort offers a unique opportunity to obtain reasonably precise estimates of risks from medium- to high-dose protracted external exposure that can then be compared to estimates based on acute exposure, such as those obtained from A-bomb survivors.
The first estimates of risk from external exposure were reported by Shilnikova and colleagues (2003). Analyses focused on leukemia (excluding CLL); cancers of the lung, liver, and bone (analyzed as a group); and solid cancers excluding lung, liver, and bone (also analyzed as a group). Lung, liver, and bone are the organs that receive the largest doses from plutonium, and excess cancers in all three organs have been linked clearly to plutonium exposure among Mayak workers (Gilbert and others 2000; Koshurnikova and others 2000; Kreisheimer and others 2000). Analyses were adjusted for internal exposure to plutonium by using the estimated body burden for workers who had plutonium-moni-toring data and by using a plutonium surrogate variable for workers who were not monitored for plutonium. The plutonium surrogate variable was developed recently from detailed work histories.
For leukemia, the estimated ERR/Gy was 6.9 (90% CI 2.9, 15) for the period 3–5 years after exposure and 0.5 (90% CI 0.1, 1.1) for the period 5 or more years after exposure. The estimate based on the entire period was 1.0 (90% CI 0.5, 2.0). There was no statistically significant departure from linearity and no evidence of modification by sex or age at hire.
Estimates and confidence intervals for the solid cancer end points are shown in Table 8-8. For these end points, linear-quadratic functions provided significantly better fits than linear functions with a “downturn” in the dose-response at high doses. This may have resulted from overestimation of doses of certain workers in early years due to inadequacies in early film dosimeters. If this is the case, estimates of the linear term from the fitted linear-quadratic function may be more reliable. The estimates for cancers of the lung, liver, and bone were higher than those for other organs, possibly because the adjustment for plutonium exposure was less adequate for these cancers. There was no evidence of modification of the dose-response by sex, age at hire, or time since exposure.
TABLE 8-8 Estimated ERR/Gy for Solid Cancers Among Mayak Workers