A reanalysis of cancer mortality in Canadian nuclear workers (1956–1994) based on revised exposure and cohort data

Abstract

Background:A 15-country study of nuclear workers reported significantly increased radiation-related risks of all cancers excluding leukaemia, with Canadian data a major factor behind the pooled results. We analysed mortality (1956–1994) in the updated Canadian cohort and provided revised risk estimates.Methods:Employment records were searched to verify and revise exposure data and to restore missing socioeconomic status. Excess relative risks per sievert (ERR/Sv) of recorded radiation dose and 95% confidence intervals (CIs) were estimated using Poisson regression.Results:A significant heterogeneity of the dose–response for solid cancer was identified (P=0.02), with 3088 early (1956–1964) Atomic Energy of Canada Limited (AECL) workers having a significant increase (ERR/Sv=7.87, 95% CI: 1.88, 19.5), and no evidence of radiation risk for 42 228 workers employed by three nuclear power plant companies and post-1964 AECL (ERR/Sv=−1.20, 95% CI: <−1.47, 2.39). Radiation risks of leukaemia were negative in early AECL workers and non-significantly increased in other workers. In analyses with separate terms for tritium and gamma doses, there was no evidence of increased risk from tritium exposure. All workers had mortality lower than the general population.Conclusion:Significantly increased risks for early AECL workers are most likely due to incomplete transfer of AECL dose records to the National Dose Registry. Analyses of the remainder of the Canadian nuclear workers (93.2%) provided no evidence of increased risk, but the risk estimate was compatible with estimates that form the basis of radiation protection standards. Study findings suggest that the revised Canadian cohort, with the exclusion of early AECL workers, would likely have an important effect on the 15-country pooled risk estimate of radiation-related risks of all cancer excluding leukaemia by substantially reducing the size of the point estimate and its significance.

Answers to Common Questions About the Use and Safety of CT Scans

“…A statistically significant increase in cancer mortality was originally found in this group, but subsequent analyses excluding data from Canada (which were determined to be flawed) eliminated the statistical significance of the study. 30,31 A high-quality study 32 from the United Kingdom focusing on occupational exposures did report a small but statistically significant increase in cancer risk, albeit at dose levels much higher (200-500 mSv) than the dose levels typically associated with diagnostic imaging (<1-25 mSv).…”

Radiation doses from radiological imaging do not increase the risk of cancer.

“…9 Without the Canadian data, the 15-country study would not show an increased risk of cancer from low-dose radiation. 10 A third data set Brenner used was the Oxford study 11 of childhood cancers, a case-control study that showed increased risk of childhood cancers following prenatal radiation. Major deficiencies have been identified in this study, and publications have raised doubts about a causal link between prenatal radiation and childhood leukaemias observed in such studies, since cohort studies involving much higher radiation doses in atomic bomb survivors have failed to show the increased risk of childhood leukaemias.…”

Designing exposure registries for improved tracking of occupational exposure and disease

“…The NDR has also been an important source of data for several epidemiological investigations of exposure and disease among different occupational groups in Canada. [12][13][14] The Ontario Asbestos Workers Registry, also a mandatory registry, has similar goals in terms of prospectively monitoring exposure and notifying workers who reach a particular duration of exposure, but has not to date been used for any surveillance or epidemiological investigations, though these activities could be supported. 15 Though none of the Canadian registries did so, exposure registries can also be used as a basis for medical screening in individuals.…”