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8 Priority Criteria for Dose Assessment Studies RELEASES OF RADIOACTIVE materials have occurred in the past at several places throughout the world. At each location, there is likely to be public demand to determine the effect on health, in the past, present, and future, on the surrounding population in a fair and credible manner. However, given the inevitable limits on resources, a procedure should be considered for setting priorities for studying the effects of releases and for identifying those sites at which past exposures have been particularly significant. The choice of sites for study should be based on an ability to bound the magnitude of the releases and to identify with some confidence, the populations at risk. There should be enough flexibility to permit changes in priority to occur with the disclosure or discovery of additional data. This chapter addresses the process of setting priorities using criteria based on scientific evidence. In practice, however, priorities for specific sites also often are influenced by public concern and political considerations. An early assessment—a scoping analysis—of the relative significance of a particular site for dose assessment should include both radiologic and epidemiologic scoping studies. A scoping analysis requires the establishment of rules that define when a study should lead to a more detailed investigation and when the detail is sufficient for providing an initial indication of potential damage to public health. The criteria need not be absolute, as they are merely intended to help establish a preliminary ranking—and they can vary for different situations or countries. The approach, however, should be generic, so that it can be extended to
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other hazardous situations, such as the release of toxic chemicals. This was recommended in a recent NRC report (NRC 1995b) that called for an iterative approach to risk assessment of chemicals starting with relatively inexpensive screening techniques. The decision-making process also must retain some flexibility to remain responsive to other concerns, such as those of affected communities or states. BASIC CRITERIA The ranking of sites for dose reconstruction and epidemiologic investigation is based on a comparison among the sites of the urgency for such studies and of the potential magnitude of possible health effects. The committee recognizes that ranking can be based on relatively limited data, using simplified models, and should not anticipate the ultimate assessment. The accuracy of the preliminary estimate is expected to be limited, but it will be sufficient to determine which studies should be done. There are several limitations imposed on a comparative study by the wide differences in the facilities involved, which include research laboratories, production facilities, power plants, or other operations. There are appreciable differences in the size of the sites, the magnitude and the duration of the releases, and size of the potentially exposed population. Another significant factor that might need to be considered is whether there are or were any observed or suspected health effects. Evidence of a potential cluster is difficult to interpret, because most investigations of suspected clusters fail to demonstrate that an excess of disease actually exists. Where a real cluster is identified, the cause can seldom be determined (Kheifets 1993). Nevertheless, reported clusters need to be evaluated and could give leads to further research. Quantitative criteria will be used to inform decisions on when to start a study and when to terminate it. And the decision to conduct a study must admit the possibility of a negative outcome obtained by credible and justifiable scientific procedures. There should be a series of decision gates—selected somewhat arbitrarily, but applied consistently—in three areas: the size and structure of the population; the relative size of the estimated dose compared with doses measured at other facilities; and whether anticipated risks exceed regulatory acceptability or are below the doses generally accepted as being of low consequence by authoritative bodies responsible for the protection of human health. Any site for which the scoping study results in low estimates of the feasibility of both radiologic and epidemiologic studies would automatically be ranked low in priority and would be given the least consideration in the first series of studies.
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DECISION CRITERIA The decision or screens for a scoping study must be applied in a consistent and even-handed manner. First, the plausibility of a study must be determined. To establish a credible scenario involving the off-site population, there must be some evidence of significant releases of radioactive materials or corroboration of off-site effects, for example, from environmental measurements. Second, a detailed study is feasible only if there is an adequate database. Bounds must be established for minimum requirements for information from plant records, public records, or environmental data. There must be enough information to permit the definition and estimation of a source term and a specific period of release. Third, the statistical requirements for a meaningful epidemiologic assessment must be met and these requirements should be based on the size of the population, estimated doses, quality and availability of records, and other factors. Fourth, a radiologic assessment should involve an iterative procedure for making dose estimates that increase in detail, once specific minimum dose criteria are exceeded. The preliminary scoping study used should eliminate some sites, at least initially, from inclusion in any ranking. It is suggested that one criterion used to assess severity—such as that stated in the Federal Registry 10CFR20 of 0.001 Sv/year (0.1 rem/year) maximum dose to any individual at a nuclear site boundary—be multiplied times 70 years for an added whole-body lifetime dose of 0.07 Sv (7 rem) and be used as a realistic selection gate for this purpose, especially if the number of competing sites is high. Alternatively, because internal radionuclides could be the primary source of exposure, the severity criterion could be stated in terms of the absorbed organ dose to those organs at greatest risk. FINAL RANKING The final ranking is a relative assessment and is subject to iterative review. Sites that are given a high priority by both the epidemiologic and the radiologic preliminary evaluations would be assigned a high priority for a more detailed dose reconstruction study and epidemiologic feasibility studies. To increase the usefulness and cost-effectiveness of these studies, they should be conducted in parallel and with clear procedures for investigators to follow. Such continuing involvement and interaction ensure that both studies are designed with a common goal and this approach is more likely to ensure that dose reconstruction studies account for such issues as avoidance of bias and evaluating all sources of potential
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uncertainty. Similarly it ensures that an epidemiologic study will focus on the population most likely to have been affected and will collect all the necessary information for detailed dose reconstruction. Sites that receive a high rating in the dose assessment but a low one on epidemiologic grounds (or vice versa) would receive a lower priority ranking and would need to be evaluated case by case. Sites that pass the decision criteria, but still rate low on both scales would receive the lowest priority and could be dropped from further detailed studies. For example, for a large population that is concerned but has received relatively low doses, an epidemiologic study designed to detect health effects might not be feasible. In this case, extensive dose reconstruction might not be necessary or justifiable. Instead, the concerns of this population might best be addressed by establishing and maintaining surveillance registries for continuous public health monitoring so that any health effects from any future exposures would be addressed quickly. In the event of high exposures to a small population, comprehensive epidemiologic studies will usually be warranted, but before they begin, additional dose reconstruction could be necessary to provide the best possible estimates of the doses involved and the nature of the exposure (ambient or through ingestion or inhalation of radionuclides). Finally, it is crucial to maintain a distinction between scientific criteria and other considerations in risk assessments. These other considerations include public concern and the fact that the public might have difficulty in understanding the concepts of statistical significance, uncertainty, and dose-response relationships. SUMMARY AND RECOMMENDATIONS Given the inevitable limits on available resources and to avoid capricious judgments in the selection of sites to be studied, a procedure must be devised for setting priorities for dose reconstruction studies and for identifying those sites where past exposures may have been particularly significant. A series of objective criteria should be applied, including the ability to bound the magnitude of the historic releases with some confidence, and the criteria should be sufficiently flexible to permit changes in priority to occur with the disclosure or discovery of additional data. The committee has, therefore, set forth a tentative process of setting priorities using criteria based on scientific evidence. In practice, however, it is recognized that priorities for specific sites also are influenced by other considerations. The committee makes the following three recommendations:
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Rules and criteria should be defined for a scoping analysis to determine the desirability of a dose assessment and epidemiologic study. These should include demonstration of the feasibility and plausibility of the study, evidence of an adequate database, demonstration of an adequate range of doses, and appropriate numbers of subjects at the higher end of the dose range to meet the statistical needs of an epidemiologic study. Quantitative criteria should be used to arrive at a credible and cost-effective ranking of sites for study. An iterative procedure should be used for making dose estimates that increase in detail after specific minimum dose criteria are exceeded.
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