Circulatory Disease Mortality in a Canadian Occupational Cohort with Low Dose Exposure to Ionizing Radiation

Exposure to high doses of ionizing radiation, such as those received during radiotherapy, leads to increased risk of circulatory diseases.. Emerging evidence of excess risks of circulatory diseases after exposure to doses well below those considered previously as safe warrants epidemiological studies of populations exposed to low levels of ionizing radiation. The purpose of our study was to evaluate the risk of circulatory disease mortality in a Canadian cohort of 337,397 individuals (169,256 men and 168,141 women) occupationally exposed to ionizing radiation and included in the National Dose Registry (NDR) of Canada. The cohort consists of employees at nuclear power stations (nuclear workers), medical, dental and industrial workers. Mean whole body radiation dose was 8.6 mSv for men and 1.2 mSv for women. During the study period (1951-1995) 3,533 deaths from circulatory diseases (3,018 among men and 515 among women) have been identified. The circulatory disease mortality in the cohort was significantly lower than in the Canadian population at large. The cohort showed a significant dose response both in men and in women. Estimates of circulatory disease mortality risk in the NDR cohort are higher than those of most other occupational cohorts and higher than in the Japanese atomic bomb survivors studies. The major limitation of the study is the absence of information on non-radiation lifestyle-related risk factors for circulatory diseases. Lack of adjustment for these factors or for socio-economic status requires caution in interpretation of the study findings. It was estimated that dose errors related to recording doses below the dosimeter detection limit as zero is unlikely to result in an overestimation of risk by more than 15-20%.