American Journal of Epidemiology Vol. 153, No. 4 : 323-324
Copyright © 2001 by The Johns Hopkins University School of Hygiene and Public Health
ORIGINAL CONTRIBUTIONS |
Sont et al. Respond to "Studies of Workers Exposed to Low Doses of Radiation"
1 Radiation Protection Bureau, Health Canada, Ottawa, Ontario, Canada.
2 Health Protection Branch, Health Canada, Ottawa, Ontario, Canada.
3 Institute of Radiation Medicine, Chinese Academy of Medical Sciences, Tianjin, People's Republic of China.
4 Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada.
5 Health Statistics Division, Statistics Canada, Tunney's Pasture, Ottawa, Ontario, Canada.
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We thank Dr. Gilbert for her thorough commentary (1
) on our paper (2). Its many useful comments and its additional tabulation will help put the paper into perspective.
In the commentary, Dr. Gilbert focuses on the excess relative risk calculations and identifies various forms of bias. We acknowledge the possibility that our excess relative risks may have been overestimated. We do not suspect a large error from the use of probabilistic linkage, as the methodology was similar to what was used in the National Dose Registry mortality study, where the linkage results were supported by follow-up of vital status (3). Confounding by smoking cannot be assessed in our study because of lack of data. The possibility of confounding by socioeconomic status was considered in the mortality study (3) and was not found to be of concern. The most likely potential source of bias may be the underestimation of workers' doses, which was a consequence of recording most single doses below 0.2 mSv as zero and of the lack of doses before 1951. We are investigating the feasibility of addressing this last issue by using methodology developed at Oak Ridge National Laboratory (4).
In comparing excess relative risk results, it should nevertheless be kept in mind, as already pointed out by Dr. Gilbert, that the characteristics and the exposure history of the National Dose Registry cohort are substantially different from those of the atomic bomb survivors. Differences of instantaneous versus prolonged exposure may be especially significant, depending on the mechanism by which ionizing radiation increases cancer risk. Recently, a two-stage model of carcinogenesis has been used (5) to describe the temporal patterns of risk for solid tumors among atomic bomb survivors. Intermediate cells that have sustained mutations in the initiation stage may proliferate from prolonged radiation exposure, thereby increasing cancer risk (6). This prolonged exposure is typical of occupational cohorts as opposed to atomic bomb survivors; therefore, direct observations of occupational cohorts are of importance in assessing cancer risks. When occupational cohorts are compared among themselves, differences in their characteristics are less pronounced. The National Dose Registry cohort stands out from the others in that it contains a large group of medical workers with generally lower doses than would be typical of nuclear power workers who dominate such studies as the one from the National Registry for Radiation Workers (7) and the three-country study (8). It is difficult to assess how much of the difference in excess relative risks for all cancers is due to cohort characteristics and how much is due to biases.
As Dr. Gilbert mentions, the estimation of excess relative risks is hindered by the scarcity of high doses. We agree that risks may be small compared with biases in calculated risks. We believe, however, that the finding of such low risks in occupational studies is in itself information that is of value to the radiation protection community. We have also included excess relative risks for the purpose of identifying cancer types that may be associated with radiation exposure but were missed in the standardized incidence ratio calculations because of the healthy worker effect. The association could have been tested in various ways; however, calculating excess relative risks has the advantage that the results can be more easily compared with those of other studies.
Dr. Gilbert affirms the usefulness of additional data acquired through follow-up. The National Dose Registry of Canada has started a follow-up to the mortality study (3), where the cohort has been considerably enlarged and dose data and mortality data have been included up to 1994. In addition, a follow-up to this incidence study (2) is in its planning stages.
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Reprint requests to Dr. W. N. Sont, Radiation Protection Bureau, 775 Brookfield Road, Ottawa, Ontario K1A 1C1, Canada (e-mail: willem_n_sont{at}hc-sc.gc.ca).
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Gilbert ES. Invited commentary: studies of workers exposed to low doses of radiation. Am J Epidemiol 2001;153:319–22.
[Free Full Text] -
Sont WN, Zielinski JM, Ashmore JP, et al. First analysis of cancer incidence and occupational radiation exposure based on the National Dose Registry of Canada. Am J Epidemiol 2001;153:309–18.
[Abstract/Free Full Text] -
Ashmore JP, Krewski D, Zielinski JM, et al. First analysis of mortality and occupational radiation exposure based on the National Dose Registry of Canada. Am J Epidemiol 1998;148:564–74.
[Abstract/Free Full Text] - Mitchell TJ, Ostrouchov G, Frome EL, et al. A method for estimating occupational radiation dose to individuals, using weekly dosimetry data. Oak Ridge, TN: Oak Ridge National Laboratory, 1993. (ORNL-6778).
- Kai M, Luebeck EG, Moolgavkar SH. Analysis of the incidence of solid tumors among atomic bomb survivors using a two-stage model of carcinogenesis. Radiat Res 1997;148:348–58.[ISI][Medline]
- Luebeck EG, Curtis SB, Hazelton WD, et al. The importance of promotion in lung carcinogenesis for protracted exposures to radon and radon progeny. Radiat Res (in press).
- Muirhead CR, Goodill AA, Haylock RG, et al. Occupational radiation exposure and mortality: second analysis of the National Registry for Radiation Workers. J Radiol Prot 1999;19:3–26.[Medline]
- Cardis E, Gilbert ES, Carpenter L, et al. Effects of low doses and low dose rates of external ionizing radiation: cancer mortality among nuclear industry workers in three countries. Radiat Res 1995;142:117–32.[ISI][Medline]
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Related articles in Am. J. Epidemiol.:
- Invited Commentary: Studies of Workers Exposed to Low Doses of Radiation
- Ethel S. Gilbert
Am. J. Epidemiol. 2001 153: 319-322.[Extract] [FREE Full Text] - First Analysis of Cancer Incidence and Occupational Radiation Exposure Based on the National Dose Registry of Canada
- W. N. Sont, J. M. Zielinski, J. P. Ashmore, H. Jiang, D. Krewski, M. E. Fair, P. R. Band, and E. G. Létourneau
Am. J. Epidemiol. 2001 153: 309-318.[Abstract] [FREE Full Text]
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