Canadian National Dose Registry of Radiation Workers 1951-2007: Overview of Research

The National Dose Registry (NDR) of Canada is a unique resource for direct estimation of health risks associated with low radiation doses. This is the largest national occupational radiation exposure database, as it includes about 600,000 nuclear, industrial, medical and dental workers exposed to the average cumulative dose of several mSv. Analyses of the NDR data based on a cohort of about 200,000 workers first exposed before 1984 and followed through 1987 and 1988 for mortality and cancer incidence respectively, showed that mortality from most causes of death considered, and cancer incidence were lower than those in the general population, which is typical of occupational cohorts. There was a significant increase over the general population in the incidence of thyroid cancer and melanoma (however, this was not certain to be related to radiation exposure). Significant dose-response was found for mortality from all causes, all cancers, lung cancer, circulatory diseases, accidents, for incidence of all cancers, cancers of the rectum and lung, leukaemia, all cancers except lung and all cancers except leukaemia. In addition, in male workers significant dose-response was found for the incidence of colon, pancreatic and testicular cancers. Estimates of cancer risks based on mortality and incidence were higher than those in most other occupational cohorts and in the Atomic bomb survivors studies. Biologically-based analysis of lung cancer incidence in the NDR showed that for protracted exposure to low radiation doses there was a significant radiation effect on promotion and malignant conversion, but not on the initiation stage of carcinogenesis. This is in contrast to the findings for high-dose acute exposures in Atomic bomb survivors, where initiation and possibly promotion was found to be affected by radiation exposure. An inverse-dose-rate effect, i.e. increase in risk with protraction of a given dose, was found in the NDR cohort.