A cohort study was conducted to investigate the relation between cancer incidence and occupational
exposure to ionizing radiation. Records containing dose information from 1951 to 1988 for 191,333 persons were
extracted from the National Dose Registry of Canada. The records were linked to the Canadian Cancer Data
Base, with incidence data from 1969 to 1988. Standardized incidence ratios were calculated using Canadian
cancer incidence rates stratified by age, sex, and calendar year. Excess relative risks were obtained from
internally based dose-response analyses. The following significant results were found for males and females
combined: a deficit in the standardized incidence ratio for all cancers combined; elevated standardized incidence
ratios for thyroid cancer and melanoma; and elevated excess relative risks for rectum, leukemia, lung, all cancers
combined, all except lung, and all except leukemia. For males, cancers of the colon, pancreas, and testis also
showed significantly elevated excess relative risks. The specific cancer types listed above have been implicated
in previous studies on occupational exposure to ionizing radiation, except for testis, colon, and melanoma, while
the findings on thyroid cancer from previous studies are inconclusive. The thyroid standardized incidence ratios
in this study are highly significant, but further investigation is needed to assess the possibility of association with
occupational radiation exposure. Am J Epidemiol 2001;153:309–18.