Investigating time patterns of variation in radiation cancer associations.

AIMS: In occupational settings, carcinogenic exposures are often repeated or protracted over time. The time pattern of exposure accrual may influence subsequent temporal patterns of cancer risk. The authors present several simple models that may be used to evaluate the influence of time since exposure or age at exposure on cancer incidence or mortality in an occupational cohort. METHODS: A cohort of 40,415 nuclear industry workers was identified via the Canadian National Dose Registry. Vital status and cause of death were ascertained through 1994. Associations between ionising radiation and mortality due to lung cancer, leukaemia, and cancers other than lung and leukaemia were quantified using conditional logistic regression models with risk sets constructed by incidence density sampling. A step function, a bilinear function, and a sigmoid function were used to evaluate temporal variation in exposure effects. RESULTS: Step and sigmoid functions were used to explore latency and morbidity periods. For analyses of lung cancer, leukaemia, and other cancers the best fitting models were obtained when exposure assignment was lagged by 13, 0, and 5 years, respectively. A bilinear function was used to evaluate whether exposure effects diminished with time since exposure. In analyses of lung cancer and leukaemia, there was evidence that radiation effects attenuated with protracted time since exposure. In analyses of age at exposure, there was evidence of variation in radiation mortality associations for analyses of lung cancer and leukaemia; discounting radiation doses accrued at younger ages (for example, 15-35 years) led to significant improvements in model fit. CONCLUSIONS: This paper illustrates empirical approaches to evaluating temporal variation in the effect of a protracted exposure on disease risk.

First analysis of cancer incidence and occupational radiation exposure based on the National Dose Registry of Canada.

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.

First analysis of mortality and occupational radiation exposure based on the National Dose Registry of Canada.

A cohort mortality study of occupational radiation exposure was conducted using the records of the National Dose Registry of Canada. The cohort consisted of 206,620 individuals monitored for radiation exposure between 1951 and 1983 with mortality follow-up through December 31, 1987. A total of 5,426 deaths were identified by computerized record linkage with the Canadian Mortality Data Base. The standardized mortality ratio for all causes of death was 0.61 for both sexes combined. However, trends of increasing mortality with cumulative exposure to whole body radiation were noted for all causes of death in both males and females. In males, cancer mortality appeared to increase with cumulative exposure to radiation, without any clear relation to specific cancers. Unexplained trends of increasing mortality due to cardiovascular diseases (males and females) and accidents (males only) were also noted. The excess relative risk for both sexes, estimated to be 3.0% per 10 mSv (90% confidence interval 1.1-4.8) for all cancers combined, is within the range of risk estimates previously reported in the literature.

Paternal radiation exposure and leukaemia in offspring: the Ontario case-control study.

OBJECTIVES: To test the hypothesis that there is an association between childhood leukaemia and the occupational exposure of fathers to ionising radiation before a child's conception. DESIGN: Case-control study with eight matched controls per case. SETTING: Regions of Ontario, Canada, with an operating nuclear facility. SUBJECTS: Cases were children (age 0-14) who died from or were diagnosed as having leukaemia from 1950 to 1988 and were born to mothers living in the vicinity of an operating nuclear facility. Controls were identified from birth certificates, matched by date of birth and residence at birth. There were 112 cases and 890 controls. MAIN OUTCOME MEASURES: Paternal radiation exposure was determined by a record linkage to the Canadian National Dose Registry. RESULTS: Six fathers of cases and 53 fathers of controls had had a total whole body dose > 0.0 mSv before the child's conception, resulting in an odds ratio of 0.87 (95% confidence interval 0.32 to 2.34). There was no evidence of an increased leukaemia risk in relation to any exposure period (lifetime or six months or three months before conception) or exposure type (total whole body dose, external whole body dose, or tritium dose), except for radon exposure to uranium miners, which had a large odds ratio that was not significantly different from the null value. CONCLUSIONS: The findings of this study in Ontario did not support the hypothesis that childhood leukaemia is associated with the occupational exposure of fathers to ionising radiation before the child's conception.

Doses and population irradiation factors for Canadian radiation technologists (1978 to 1988).

Individual and collective radiation doses received by Canadian radiation technologists (RTs) working in diagnostic radiology, nuclear medicine and radiotherapy are summarized for the period 1978 to 1988. The data were obtained directly from the National Dose Registry, Department of National Health and Welfare. Over the 11-year study period the mean annual dose equivalent fluctuated around 0.2, 1.8 and 1.1 mSv for RTs working in diagnostic radiology, nuclear medicine and radiotherapy respectively. Over the same period the occupational collective dose equivalent decreased in diagnostic radiology (by 44%) and radiotherapy (by 35%) and increased in nuclear medicine (by 45%). Approximately 10,000 RTs are monitored each year, with an estimated total occupational collective dose equivalent of about 3.6 person-sieverts. Analysis of dose distribution data showed that only 1.3% of all monitored RTs received an annual whole-body dose equivalent greater than the current legal limit for members of the public (5 mSv). Approximately half of the RTs working in nuclear medicine and radiotherapy received an annual dose equivalent in excess of 0.5 mSv; only 7.3% of their diagnostic radiology counterparts exceeded this level. Demographic data showed a high preponderance of young women in all three RT classifications, and an analysis of the radiation risks to this occupational group revealed increases of up to 12% above the risk associated with a "standard" adult working population exposed to the same collective dose equivalent.

Occupational doses to medical radiation technologists in Manitoba (1978-1988).

In Canada, occupational exposure to medical technologists accounts for about 8 per cent of all occupational exposure. In this paper, occupational doses to Manitoban radiation technologists (RTs) in diagnostic radiology, nuclear medicine and radiotherapy are presented for the period 1978-1988. Particular attention is paid to the distribution of dose among this population. The importance of age and sex demographics on radiation detriment is also estimated.

Radiation exposure risks to nuclear well loggers.

This report is based on statistical data from the Canadian National Dose Registry (As82) and information obtained from visits to 1 supplier and 9 oil-well service companies in the Province of Alberta. The companies are representative of most in this industry and provide services at the well head from logging, perforating and fracturing to cementing and tracer work. The information obtained indicates that typical exposures can account for an average dose of 1 to 2 mSv/y. The observations of well-logging procedures revealed a number of potentially hazardous situations which could lead to unnecessary exposure and based upon these, several recommendations are included.

Ender's nailing of hip fractures: the operation and the radiation hazards.

Sixty-two intertrochanteric and 3 subtrochanteric fractures of the femur were treated by Ender's nailing. The method was safe and effective in stable fractures but in unstable fractures malunion was common. Exposure of the operating team to radiation during the procedure was well within acceptable limits.

Projected whole-body career doses for radiation workers in Canada.

Projected whole-body career doses have been calculated from Canadian exposure data contained in the National Dose Registry, using the straight extrapolation of accumulated annual doses to a 40-y period, as described in the 1977 UNSCEAR report (UNSCEAR77). The calculations are broken down by type of employment. The results are compatible with earlier publications. Double regressions provided trends of projected career doses with increasing length of employment and with progressing date of enrollment into the registry. The career doses fit a log-normal or a hybrid log-normal distribution for occupations with low and high exposures, respectively.