Radiation Dose Reconstruction for Epidemiologic Uses (1995) / Chapter Skim
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7 Epidemiologic Considerations
7 Epidemiologic Considerations
Pages 62-81
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From page 62... ...
Although, in principle, much could be learned from experience, for example, with the accident at Chernobyl in the Ukraine, it must be emphasized that deriving new scientific knowledge from studies of exposed populations around nuclear facilities is difficult. Radiation doses received by the latter individuals probably cannot be quantified precisely and the number of radiation-induced illnesses generally is smaller than the number one would expect to find from all causes in that population.
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From page 63... ...
As a rule, dosimetric and epidemiologic scoping (screening) studies should be undertaken in parallel, so that, at the conclusion of the dosimetric scoping study, an informed estimate of the expected magnitude of risk and the statistical power of a potential epidemiologic study can be derived.
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From page 64... ...
The risks are expressed as excess fatal cancers per 104 person-gray for various tissue sites. The differences in risk depend on the different projection models used and differences in the interpretation and adjustment of the basic data derived from studies of the Japanese atomic bomb survivors and other highly exposed populations.
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From page 65... ...
A range of risk factors can therefore be chosen for computing excess cancer risks from radiation exposures; however, most dose estimates encountered in dose reconstruction studies are usually for specific tissues (thyroid or bone, for example) and at low dose rates for cumulative total doses that would generally be considered as low.
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From page 66... ...
The problem for the statistician is to determine whether, given the variability in cancer occurrence, an expected excess of 7 or 8 cancer deaths could be detected with a particular level of confidence. Would the study have the statistical power to detect such an excess?
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From page 67... ...
Epidemiologic studies done in conjunction with dose reconstructions near nuclear sites or in the aftermath of accidents are subject to several limitations. The exposed population could be poorly defined, inhomogeneous, and transient.
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From page 68... ...
As noted in the Introduction, if a preliminary dose assessment results in estimates of exposures that are of potential concern, delaying the epidemiologic investigation until a detailed dose reconstruction is complete could not only diminish the usefulness of the reconstruction but could jeopardize a warranted epidemiologic study. For example, the more time has elapsed from the period of interest the more difficult it usually becomes to assemble the complete population needed and to reconstruct the information required for exposure assessment.
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From page 69... ...
Registries, hospitals, and other potential case sources in the area of interest will fail to identify cases who have left the area, and a large proportion of the reported cases will probably be persons who were not in the area at the time of the exposure. However, conducting a casecontrol study within a retrospective cohort study often is useful for obtaining more detailed information on nonradiation exposures, lifestyle factors, and the like.
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From page 70... ...
Although it is not a formal epidemiologic study, interest in possible health effects resulting from a particular exposure source is sometimes generated through identification of a cluster of cases of a specific disease in a particular location, time period, or both. Most clusters are chance occurrences, but it can be difficult to evaluate whether a particular cluster can reasonably be attributed to chance and even more difficult to communicate to the public the role of chance or the magnitude of the purported risks (Slovic 1987~.
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From page 71... ...
The last category requires information on age, sex, length of follow-up, and possibly other factors, and it is determined using available disease rates. At the time the power calculations are made, there is likely to be uncertainty both about the doses and about the size and characteristics of the exposed population.
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From page 72... ...
It is assumed that each member of a population with an age and sex distribution typical of the United States as a whole was exposed to a specific radiation dose and followed over a full lifetime, and the risk model recommended by BEIR V (NRC 1990) was used to project lifetime risk for radiation-induced fatal cancers (about 7.5 per 10 millisieverts (mSv)
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From page 73... ...
generally agree that gamma or beta irradiation delivered at low doses and low dose rates probably causes from 2 to 10 times fewer cancer cases per millisievert than do higher, acute doses. Hence, for the non-leukemic cancers included in Tables 7-2 and 7-3, the effects could be overestimated, because they are extrapolated from high-dose studies.
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From page 74... ...
Considering all these factors together, the sample sizes in Tables 7-2 and 7-3 probably err on the side of underestimating the required sample sizes. Outcomes The health outcomes to be studied should be chosen in keeping with information available about which health effects are expected derived primarily from other studies of exposed populations.
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From page 75... ...
Before undertaking an epidemiologic study, it is essential to determine the feasibility of identifying the study population and of ascertaining whether the health effects of interest have occurred. These difficulties could preclude a study regardless of the study's potential statistical power.
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From page 76... ...
Because even small differences in smoking habits can have a greater influence on lung cancer risks than does the exposure of interest, it is almost never possible to be certain that one has fully adjusted for smoking even when reasonably detailed smoking histories are available. For this reason, if lung cancer is the health effect of primary interest, it might be necessary to apply stricter criteria in determining whether to conduct a study, and such studies should probably not be conducted if smoking data cannot be obtained.
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From page 77... ...
If the expected magrutude of the excess risk resulting from radiation does not exceed the baseline risk by more than a few percent, the results of an epidemiologic study cannot be interpreted unambiguously and could therefore have little value regardless of the size of the population. Statistical Analysis Statistical analyses should be designed to make optimal use of available information on doses.
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From page 78... ...
SUMMARY AND RECOMMENDATIONS Radiation dose reconstruction provides detailed quantitative information about individual exposures for epidemiologic study of populations near nuclear facilities. The quality and quantity of the dose information are central to any good epidemiologic study.
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From page 79... ...
Fourth, the particular health end points to be targeted in an epidemiologic study will be defined primarily by the organ doses that members of the public received (which may vary appreciably when there are radionuclide exposures) and by the radiosensitivity of various organs and tissues.
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From page 80... ...
If an epidemiologic study is conducted, the retrospective cohort or case-control designs are usually the methods of choice. Ecologic (correlational)
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From page 81... ...
31. A full-fledged dose reconstruction and epidemiologic study should be proposed only if the scoping studies show that adverse health effects are likely to be statistically detectable, given the probable dose distribution and size of the exposed population.
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