problems pose significant difficulties for epidemiology: the inability to classify workers accurately by level of internal radiation exposure may lead to confounding of the analyses of association between external low-LET radiation dose and cancer risk.

Lung cancer has been the primary outcome of interest in studies of workers in fuel enrichment and production facilities. Lung cancer mortality was found to be significantly elevated, compared to national rates, among workers in nuclear fuel processing facilities in three reports (Loomis and Wolf 1997; Checkoway and others 1988; Frome and others 1990), but not in others (Brown and Bloom 1987; Dupree and others 1987, 1995; Ritz and others 1999b). An association between external low-LET radiation dose and lung cancer mortality was observed in two cohorts in the United States (Fernald and Y-12; Checkoway and others 1988; Ritz and others 1999a), and an association with lung cancer incidence (using a 20-year lag) was observed in one study in the United Kingdom (McGeoghegan and Binks 2000a). No association was found in other papers on the U.S. (Hadjimichael and others 1983; Ritz and others 2000) and U.K. (McGeoghegan and Binks 2000b) cohorts. No information on dose to the lung from internal contamination was available for analysis in these studies. In studies where estimation of dose to the lung from internal contamination was carried out, an association was observed at Y-12, but not at Rocketdyne (Ritz and others 1999a). In contrast, a U.S. multifacility case-control study of lung cancer among workers exposed to uranium dust at TEC, Y-12, Fernald, and Mallinckrodt found no such association; there was a suggestion, however, of positive associations among workers hired over age 45 (Dupree and others 1995). Therefore, risk estimates for low-LET radiation-induced lung cancer risk in these cohorts should be treated with caution.

Following the observation of increased prostate cancer mortality related to cumulative external radiation dose in the AEA (Beral and others 1988; Rooney and others 1993) a nested case-control study was conducted of prostate cancer risk among employees of that facility. The study showed that exposure to five radionuclides (tritium, chromium-51, iron-59, cobalt-60, and zinc-65), evaluated separately, was associated with an increased risk of prostate cancer. Analyses of the association between external radiation dose and prostate cancer risk were carried out both for workers with probable exposure to these radionuclides and for those who had no such exposure. The association between external dose and prostate cancer was restricted to those with radionuclide exposure.

In the Combined UK Industrial Workforce study, Carpenter and colleagues (1998) carried out analyses of cancer mortality in relation to external radiation dose in two groups—those who had been monitored for internal radionuclide contamination and those who had not. A positive association was seen in both groups of workers, although it was statistically significant only among those who had been monitored for internal contamination.

Cancer mortality and incidence was studied among Sellafield workers in relation to exposure to plutonium and to external low-LET radiation (Omar and others 1999). A significant association between mortality from leukemia excluding CLL (13 deaths) was seen in relation to external radiation dose using a 2-year lag, as had been seen in the previous follow-up of this cohort (Douglas and others 1994). When analyses were restricted to plutonium workers and took into account both external low-LET radiation dose and estimated plutonium dose, the association was no longer statistically significant, based on six deaths.

Other Occupational Exposures

Wing and colleagues (1993) evaluated the effect of potential exposure to beryllium, lead, and mercury in the ORNL cohort by identifying workers with potential for these exposures from employment records. Adjustment for these potential exposures had little effect on the radiation risk estimates. The interpretation of these results is limited by the absence of individual exposure estimates for the chemicals considered.

Rinsky and colleagues (1981) considered exposure to a number of workplace carcinogens in a case-control study of lung cancer among civilian employees of the Portsmouth naval shipyard. Asbestos and welding by-products were found to confound the association between radiation exposure and lung cancer risk in this population, where radiation workers appear to be more heavily exposed to asbestos and welding fumes than other workers. The unadjusted lung cancer odds ratio for workers with a cumulative dose of 10–49.99 mSv was 1.8 (95% CI 1.1, 3.1) compared to workers with no history of radiation exposure; adjustment for asbestos and welding fumes reduced it slightly to 1.7 (95% CI 1.0, 2.9).

Modifiers of Radiation Risk

Several authors have reported an association between age at exposure and/or attained age and the risk of radiation-induced cancer. This has been reported in the Hanford, ORNL, and Rocketdyne cohorts (Gilbert and others 1993a; Stewart and Kneale 1996; Richardson and Wing 1999a; Ritz and others 1999b), but not in five other cohorts in which it was considered—Rocky Flats, AECL, AEA, AWE, and Sellafield (Cardis and others 1995; IARC 1995).

The three-country and the NRRW studies (Cardis and others 1995; Muirhead and others 1999) of nuclear industry workers currently provide the most comprehensive and precise direct estimates of the effects of protracted exposures to low levels of low-LET radiation. Although the estimates are lower than the linear estimates obtained from studies of atomic bomb survivors, as seen in Table 8-7, they are compatible with a range of possibilities, from a reduction of risk at low doses, to risks twice those on which current radiation



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