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Executive Summary BACKGROUND It is well recognized that iodine-13 l, or 13~] (a radioactive form of iodine), is an important radionuclide because of the potential for human exposure to it after accidental releases from nuclear reactors and fuel-reprocessing plants. Deposition of |3~} onto pasture grass leads to contamination of cows' milk and ingestion of the radioactivity by humans. Because iodine is concentrated in the thyroid, radiation doses to the thyroid can result. The Draft Final Report of the Hanford Thyroid Disease Study (HTDS) describes a study of the cumulative incidence of thyroid disease and abnormalities among "downwinder" children exposed to ]3~{ from the Hanford Atomic Products Operations. Releases of 13~{ began in December 1944 as a consequence of the chemical removal of plutonium from the fuel rods irradiated at the Hanford nuclear site. The main study objective of the HTDS is described in the Draft Final Report as a "determination of whether thyroid morbidity is increased among persons exposed to releases of radioactive iodine from the Hanford nuclear site." l:n the study, 3,441 subjects who had been born near Hanford in 1940-1946 were contacted in the 1990s and taken to several locations for medical examination for thyroid disease. A detailed dose-reconstruction method developed by the Pacific Northwest National Laboratory was used to assign likely thyroid doses to study participants. Estimating dose also entailed querying parents about study participants' residence and milk-consumption history in 1944- 1957, the period of idol releases. 17
18 Review of the HTDS Draft Final Report The Centers for Disease Control and Prevention (CDC) asked the National Academy of Sciences-National Research Council (NAS-NRC) to give an independent appraisal of the study methodology, results, and interpretation and of the communication of the study results to the public. Specifically, it asked: · Has the analysis been carried out appropriately and completely? · Are the presentation and the discussion of results complete? · Are the conclusions reasonable? · Was the material accurate and appropriate in providing guidance to the public in understanding the study findings? · if these messages about findings need to be amended, how should the revised messages best be communicated to the public? With regard to release of future study reports, how can CDC improve the public communication process? This report constitutes the response of the NRC subcommittee to that request. To respond to the charge, the NRC subcommittee felt that it needed to go beyond the specific questions addressed to it by CDC and develop a broad understanding and critique of the HTDS and the Draft Final Report. As part of those activities, the subcommittee solicited comments from outside experts and members of the public primarily in a public meeting held in Spokane, Washington, in June 1999, where 14 scientists and members of the public made formal presentations to the subcommittee about various aspects of
Executive Summary 19 the Draft Final Report. Other members of the public also spoke during four open-comment sessions at the meeting. in addition, efforts were made to evaluate all information materials prepared for the public and additional CDC communication plans. Information was gathered through interviews with journalists, members of concerned citizen groups in the Hanford region, members of the CDC scientific and media staff in Atlanta, and the HTDS investigators. in this summary, the main points follow the structure of our report and are presented under several headings: epidemiologic and clinical methods and data collection, dosimetry, statistical analyses, statistical power and interpretation of the study, and communication of the study results to the public. We then provide a brief synopsis of our response to the questions raised by CDC. EPIDEMIOLOGIC AND CLINICAL METHODS AND DATA COLLECTION The HTDS eligibility criteria called for including all persons born in the early 1940s in the counties that were predicted to have the highest exposures to Hanford releases (Benson, Franklin, and Adams counties) and randomly selected subjects born in the same period in four counties that were expected to have intermediate exposure (Walla WalIa County) or low exposures (Ferry, Stevens, and Okanogan counties). Attempts were made to determine the vital status of all 5,199 eligible potential participants and, if they were alive, to trace and enroll them in the study. A total of 3,441 people received thyroid examinations; thyroid doses could be estimated for 3, 190 of them. The subcommittee considered the study design to be generally appropriate to address the aims of the study. The investigators also chose the best population to study, namely, those in the most highly exposed areas who were young children at the time ofthe greatest AT releases. The low-exposure group, up wind and more distant from the Hanford site, was also a reasonable choice. However, the one significant weakness of the design was
20 Review of the HTDS Draft Final Report the uncertainty in thyroid dose estimates, with resulting potential for exposure misclassification. The epidemiologic methods were exceptionally good. The sample was based on an almost complete census of eligible subjects, and location and participation rates were high. There was a high level of quality control in the epidemiologic procedures, and interviews and clinical examinations were "blinded" (without knowledge of exposure or disease status). The HTDS collected data on an appropriate set of potential confounding variables (risk factors that might distort or mask findings), including sex; age at first exposure to ill] Tom Hanford; age at examination; history of diagnostic, therapeutic, and occupational radiation exposures smoking history; and ethnicity. The questionnaire to elicit information from parents or surrogates on the participants' milk-consumption patterns was carefully designed and field-tested. However, there is substantial inherent unreliability in recall of dietary habits of 40-50 years ago, especially when someone other than the mother was interviewed, as was the case in 26% of the interviews. Compounding that problem is that for 38°/0 of the participants no parent or surrogate was available to be interviewed. Generally, the clinical examinations and laboratory studies were performed with good-quality methods. Subjects were given physical examinations, including thyroid palpation by thyroid specialists, ultrasonography, and appropriate thyroid- hormone and thyroid-antibody blood tests. The subcommittee has mostly minor criticisms of the clinical and laboratory procedures pertaining to inadequate quality-control procedures in assessing and reporting cytopathology results and the possibility that some past thyroid diagnoses were missed, inasmuch as it was impossible to obtain 7°/0 of the death certificates and 37°/0 of the requested historical medical records. The HTDS investigators provided the subcommittee with additional tabulations for examining deaths in the group studied. Although there was a small increase in mortality, mostly
Executive Summary 21 due to perinatal mortality and congenital anomalies, the tabulations indicate that the increase was not due to AT exposure, in that it occurred both before and during the time of the 13~{ releases. However, a more detailed tabulation of perinatal deaths and congenital anomalies should be provided to help readers to interpret these data. Conclusions: The study design was generally appropriate. · An optimal study population was chosen: the most highly exposed young children. The epidemiologic methods were of excellent quality. The clinical and laboratory methods were appropriate and generally had good quality control. Some past thyroid diagnoses might have been missed because medical records and pathology slides were unobtainable. Recommendations: needed. An adequate review of the cytopathology results is · The HTDS investigators should indicate for how many potential past thyroid diagnoses they were unable to obtain any medical confirmation, with a breakdown by reported type of thyroid disease and dose. detail. . The mortality experience should be tabulated in more
22 DOSIMETRY Review of the HTDS Draft Final Report The HTDS relied heavily on the Hanford Environmental Dose Reconstruction (HEDR) method, which in turn relied heavily on the use of environmental-transfer models. Models were necessary to estimate movement of IT in the environment because very few measurements of IT from the 1940s were available. The estimates of the thyroid doses for the 3,190 participants ranged from of O.OOOS mGy to 2,842 mGy, with a mean of T82 mGy. The HEDR models have been subjected to numerous reviews and to independent testing. The NRC subcommittee found the dose assessment, on the whole, to be structurally sound for the estimation of thyroid doses, but minor errors have been found and doubts have been raised about the validity of some assumptions and of results for some environmental conditions. A review of some of the key parameter values that went into the HTDS dosimetric mode} showed that most of them were reasonable, and the resulting dose estimates are generally supported, at least to within a factor of 2 or 3, by the validation studies performed by the HEDR project. Recently, several scientists have asserted that the source term (the amount of ill{ released by Hanford) was underestimated. However, even if those scientists' points are all valid, we estimate that it would increase the total ]3~{ releases during 1944-1957 by only about 30°/O. Systematic dose underestimation has implications for the statistical power of the study. if the doses were underestimated across the board, the study would have greater statistical power than was projected, in which case negative results of the study would be more persuasive. However, if there was variation by subject in the degree of dose underestimation, or in the degree to which subjects vary in sensitivity (because of age differences, and so on), these could result in reduced statistical power. The subcommittee believes that the doses might have been overestimated, in that the HTDS used an estimate of the fraction of ill] eaten by a cow that is transferred to milk that was
Executive Summary 23 about twice as high as estimates in other studies. If the doses were overestimated, the statistical power of the study would be less than was stated in the HTDS Draft Final Report. Revisions of the mode} and recalculation of doses would be required to determine the net effect of those factors. A critical feature of the dosimetry system is that the dose estimates have large uncertainties because they are based on mathematical models, not direct measurements. It is very likely that the uncertainties were underestimated by the HTDS because some sources of uncertainty were not taken into account. A notable deficiency was in accounting for the uncertainties in the sources and amount of milk consumption reported by parents or their surrogates. Those reports were 40-50 years after the fact, so one would expect appreciable unreliability in recall of milk- consumption patterns. The uncertainties would lessen the statistical power of the study and thereby make its results less definitive. During the 1950s and early 1960s, the Hanford population was also exposed to 13~{ from Nevada Test Site CATS) fallout and global fallout from atomic and hydrogen-bomb tests over the Pacific, in the Soviet Union, and elsewhere. The HTDS team performed some analysis of the impact of NTS fallout, but not of global fallout. A rough assessment of the thyroid doses arising Dom global fallout was performed as part of our review. The subcommittee concludes that the variability in thyroid doses from those sources is much less than that in doses from Hanford fallout in the population being studied, so NTS and global fallout are probably not important confounders of the Hanford dose- response associations. Nevertheless, they should be examined carefully to be sure. Conclusions: · The dose-assessment methods and their implementation are difficult to review because the information is scattered among numerous documents.
24 Review of the HTDS Draft Final Report · Dose estimates appear accurate to the degree one normally expects for environmental-dose reconstruction but minor errors in the parameter values used in the model need to be corrected. · Dose uncertainties were underestimated because errors were not included for all the factors in the dosimetry model. This conclusion is the same as stated in a previous National Research Council (1999) letter report that reviewed the analysis plan for the HTDS. Recommendations: · A single document describing clearly the HEDR dose- assessment methodology, including uncertainties, and its implementation by HTDS should be prepared. · Errors in the dose-estimation mode} should be corrected. account. All dose-related uncertainties should be taken into STATISTICAL ANALYSES The observed number of cases of thyroid cancer, the thyroid-disease end point of greatest interest, was very small: 20, of which only 14 had dose estimates. That makes it difficult to perform a meaningful statistical evaluation. For most other thyroid diseases, the numbers were more substantial, such as 250 cases of benign thyroid nodules. The HTDS used the HEDR thyroid doses as its only, or at least primary, tool to describe patterns of likely exposure. The subcommittee believes that the HEDR dosimetry should not have been the sole method for evaluating the association between AT exposure and thyroid disease, and it suggests that supplemental
Executive Summary 25 analyses be done that could help to confirm (or refute) the dose- response analyses. The dose-response results given are difficult to interpret; all one sees is dose-response regression coefficients with no tabulations to help in interpreting the factors that influence the results. The subcommittee recommends that the following tables, which were absent from the Draft Final Report, be included in the revised report: frequency distribution of individual doses, observed and expected numbers of disease or abnormality outcomes in several dose categories, and average doses according to such important categories as year of birth and milk consumption in childhood. Other potential risk factors were evaluated as possible confounding or effect-modifying variables, but no tables were presented to show the results of those evaluations. The HTDS investigators assigned thyroid doses only for periods when subjects lived as children in the geographic area for which exposures were estimated. They made no attempt to estimate out-of-area doses for persons who were out of the area for part of the exposure period and to perfonn sensitivity analyses to determine the impact of the missing doses. (See chapter 5.) Given that thyroid-disease and thyroid-abnormality rates appeared to differ by geographic area, the subcommittee recommends alternative analyses to address the issue. A set of analyses stratifying on geographic area is needed because the HTDS investigators' tabulations showed that the rates tended to be higher in areas with low fallout, so the geographic variations due to factors other than dose would induce a negative association between ]3~} and thyroid-disease rates. That feature of the data could explain why most of the dose-response estimates were in the negative direction, the question is whether removing its influence would yield a positive association. A concern expressed by members of the public is that a study in which everyone is exposed is not valid and that a completely unexposed "control" group is needed for proper assessment of the risk associated with Hanford 13lI fallout.
26 . Review of the HTDS Draft Final Report However, for several reasons, the subcommittee believes that the HTDS investigators were correct in emphasizing dose-response comparisons in the study rather than comparisons with other general populations. The slope of the dose-response curve can provide a valid index of the risk even without an unexposed contra group, as long as a sufficient range of doses are estimated with reasonable accuracy. Comparisons with an external, general population are problematic on several accounts. Persons living in various geographic areas often vary in their baseline risk of thyroid diseases because of differences in dietary iodine intake and other unknown factors. The rates of detected disease in the HTDS are based on thyroid examinations and depend on the methods and criteria of those examinations. That often produces a large screening effect (detection of cases of disease that otherwise would not have been detected until some years later, if at all), so comparisons with rates from other geographic regions without comparable screening are not valid. Conclusions drawn from comparisons with general-population prevalence have more potential for bias than those drawn from dose-response comparisons in the study population. The HTDS investigators analyzed the effect of interview versus default data for milk consumption, although results were not presented. The subcommittee suggests that they also examine associations, using only those with interview information to minimize dose misclassification. Another suggested alternative is to conduct dose-response analyses that stratify on interview versus default milk values. Tabulation of thyroid-disease rates by reported milk- drinking habits is suggested to elucidate further whether 13~{ exposure estimates tend to coincide with disease. The fact that higher thyroid morbidity was found in the less-exposed counties argues that the pattern of thyroid morbidity did not tend to track the likely geographic pattern of exposure to its. However, birth location was not as important a determiner of thyroid dose as were
Executive Summary 27 behavioral habits (as can be seen by the large range of doses in all counties), primarily milk-drinking. Besides the lack of some important tables in the analysis, the subcommittee has concerns about the manner of presentation of the study results. The results were reported in black and white terms: a statistical test did or did not reject the null hypothesis. No confidence intervals on the estimates of the size of effects were given. The subcommittee recommends that confidence levels be provided throughout the report. Furthermore, had the investigators presented confidence intervals for the dose-response mode! on the basis of their statistics, the confidence intervals would almost certainly have been too small because dosimetry errors were ignored. A sophisticated statistical method described in the statistical chapter of the report would have at least partly taken dose uncertainties into account in the confidence intervals, but it was not implemented. Conclusions: · The HTDS report relies too heavily on dose-response analyses without providing sufficient associated evidence from tabulations of factors that could illuminate the results obtained. (See chapter 5.) A number of key tables were absent from the report, for example, tables of frequency distribution of doses; of observed and expected frequencies of each thyroid disease by, say, quartiles of dose; of thyroid disease rates by milk-drinking habits and other risk factors in disease; and of average doses by year of birth, amount of milk consumption in childhood, and the like. · Differences in thyroid-disease rates by geographic area (called "geostrata" in the Draft Final Report) might have been an important confounder of the dose-response association.
28 Review of the HTDS Draft Final Report · The subcommittee believes that the HTDS investigators rightly chose to emphasize comparisons of thyroid-disease rates in the study group rather than comparisons with external general population rates, because there is a potential for serious biases in the latter due to geographic variations and, especially, screening effects. · Too little attention was paid to the range of risk estimates that is compatible with the data. Confidence intervals were seldom given. Recommendations: · A number of key tables should be included in the final report to help readers to interpret the dose-response results. · The HTDS investigators should report on those who were out of the dosimetry area for part of the exposure period and examine the impact of the assumption of zero dose received during such periods. · Analyses designed to control for possible confounding by geographic area should be conducted. . There should be a more thorough set of analyses of thyroid-disease rates by milk-drinking information. . . · Confidence intervals should be given and used in the interpretation of the study results. · The confidence intervals should take into account all the sources of uncertainty in the dose estimates.
Executive Summary STATISTICAL POWER AND INTERPRETATION OF THE STUDY 29 The HTDS investigators were generally successful in achieving the sample size and dose distribution that they had projected as necessary if the study were to have adequate statistical power. The Draft Final Report indicates that for many of or all the thyroid end points the estimated power of the study was good (over 90°/O) for detecting plausible nonzero dose-response relationships. Nevertheless, the HTDS investigators' discussion of statistical power did not present how small the expected excess of thyroid cancers was. Based on their assumptions about the risk coefficient, the dose distribution, the number of persons in the study and the length of the follow-up, about 34 thyroid cancers were expected in the study, of which 19 would have been expected without any 13~{ exposure and ~ 5 were due to radiation exposure. Had these numbers been presented, they might have tempered CDC's evaluation of the scientific value of the study in relation to its cost. Our subcommittee reviewed the factors that influence power, focusing on the impact of uncertainties in the HEDR thyroid doses. We note that neither the power calculations nor analytic techniques used made explicit allowance for uncertainties in the dose estimates. The investigators ignored five sources of uncertainty. First, in making their projections they assumed that the dose- measurement error was all of a type ("Berkson error") that would not reduce the strength of associations. That is different from classical measurement error, which does weaken the strength of associations and therefore requires a larger sample or a wider dose range to attain adequate statistical power. [ndividual-based dose error for example, the uncertainty of a subject's milk-drinking habits represents classical measurement error that needs to be taken into account in estimating the statistical power of a study. Second, multiplicative (as opposed to additive) dose uncertainties (such as errors in the source term or in the coefficient of transfer from cow intake to milk) that apply to everyone will add error to risk estimates and thereby decrease statistical power. Third,
30 Review of the HTDS Draft Final Report correlations among multiplicative dose uncertainties can further reduce statistical power. Fourth, geographic variations in the baseline rates of disease can decrease statistical power if not controlled for. Fifth, some sources of uncertainty in the 13~} environmental pathway apparently were not included in the dosimetry uncertainty estimates. Because those types of uncertainty were not taken into account in the statistical-power calculations, the subcommittee believes that the HTDS projections of statistical power are overestimated, perhaps substantially. The negative results of the study are therefore less definitive than the Draft Final Report and press releases stated. The uncertainties listed above also have an impact on the width of confidence intervals around the estimates of thyroid- disease effects, so the study is less clearly negative than was portrayed. The confidence intervals would be more compatible with (although the best estimate does not support) the larger risks seen in other studies, such as the Utah NTS fallout study and the large pooled study of thyroid-cancer risk associated with external radiation exposure. Conclusions: · The HTDS investigators were successful in achieving the sample size and dose distnbution that they projected as necessary if the study were to have adequate statistical power. · However, the HTDS assumptions regarding statistical power did not include the possibility that dose uncertainty would weaken the associations. They ignored several sources of uncertainty that probably decrease the statistical power of the study. · The subcommittee believes that the HTDS projections of statistical power are overestimated, perhaps substantially.
Executive Summary 31 The negative results of the study are therefore less definitive than the Draft Final Report and press releases stated. · The HTDS results are probably compatible with the risk estimates from the Utah NTS fallout study, because the various uncertainties would yield wide confidence intervals. Recommendations: · The HTDS investigators should describe the sources of uncertainty in as quantitative terms as possible and interpret their results in the light of these uncertainties. · The HTDS investigators should recalculate the statistical power of the study, taking into account the dose uncertainties if this proves feasible. · The compatibility of the HTDS study with other studies of radiation and thyroid disease should be re-examined, taking into account the impact of dose uncertainties. COMMUNICATION OF THE STUDY RESULTS TO THE PUBLIC Compared with the history of a less-than-open public- information policy of the Department of Energy and its predecessor agencies, the early plans by CDC and the HTDS investigators for open communication about the study were enlightened and promising. So was the decision to establish a citizen advisory group for the study and the apparent level of cooperation offered to various other citizen groups in the region over the years of the study. All those early efforts seemed to build trust and credibility for the study. However, when the Draft Final Report was released, a number of communication errors were made that caused a public outcry. The draft report outlined a good communication plan, which included an admirable concern for translating the technical information in the report into an understandable booklet for the
32 Review of the HTDS Draft Final Report public, and included a Web site to share information with the public. But several events forced an early release of the Draft Final Report, which pre-empted the original communication plan. The main message of the report was problematic. The written materials and oral presentations made by HTDS investigators overstated the certainty (the statistical power) of the study and the conclusiveness of the negative findings. Although the public materials factually represented what appeared in the Draft Final Report, the strong statements made publicly were not tempered by expressing the uncertainties. In trying to decide how to present the study, CDC was on the horns of a dilemma: some members of citizen groups had urged agency personnel in advance not to alter the report before its release, and CDC had to respect issues of academic freedom regarding the investigators' views, but after the report's release, they were blamed for not intervening to counter the strong message delivered to the public by the HTDS investigators. On the basis of comments received by the subcommittee from members of the public, it is clear that some people with an interest in the findings of the study were disappointed with the reported (negative) results and upset by how the results were disseminated and described. A number of factors contributed to and complicated the problems surrounding the report's release: an information blackout that included the citizen groups, a complex briefing schedule by telephone in the Hanford area to venous state health agencies and citizen organizations only several hours before the media anal public briefings on the findings, a leak to the New York Times that related the findings to the public before most of the briefings in the Hanford area, and a message that contradicted what most of the public thought would be the outcome of the study. Because serious problems were encountered in the scheduling and conduct of the prerelease briefings, a different briefing strategy should be used in the future, and telephone
Executive Summary 33 briefings should probably be abandoned because they were disliked by all involved. Delivering an unpopular message requires sensitivity to the audience's health concerns and fears. But the media and public briefings, and all written materials emphasized the overall statistical results of the study and did not seriously discuss the outcome for individuals. The implications for individuals and families that have suffered because of thyroid disease could have been explained in the written materials and public briefings. The subcommittee applauds CDC's open- communication policy and strongly recommends that this policy continue with the HTDS and similar studies. it recommends that a new communication plan be devised for the release of the final HTDS report, taking into account the problems that have already been encountered. The final report should outline and explain any significant changes made in the Draft Final Report. The subcommittee suggests that CDC convene a workshop of risk-communication experts, scientists, journalists, and citizens to discuss how to publicly release and discuss controversial unreviewed draft reports more effectively and to discuss other issues that could affect the future release of important CDC reports. Conclusions: · The early enlightened plans by CDC and the HTDS investigators for open communication about the study and for a citizen advisory group for the study should have helped to build trust and credibility. · Early release of the Draft Final Report and public concerns about CDC changes in that draft led to many of the communication problems that resulted from the draft report's release.
34 Review of the HTDS Draft Final Report · The information blackout and complex briefing schedule for release of the Draft Final Report worked against effective communication of the report's findings to the public and unnecessarily upset Hanford-area citizen groups that had cooperated with the HTDS over the years. · A key weakness of the communication effort surrounding the release of the Draft Final Report was that the report and the public communications by HTDS investigators overstated the certainty (the statistical power) of the study and the conclusiveness of the negative findings and failed to discuss the uncertainties. CDC officials should have expressed their own interpretations in addition to those of the HTDS investigators about the draft report in the briefings and public documents. Recommendations: · Delivering an unpopular message requires sensitivity to the audience's health concerns and fears. In communications about the HTDS final report, implications for individuals and families that have suffered because of thyroid disease should be carefully explained. if there are plausible alternative interpretations of the results, they should be acknowledged. · The subcommittee supports CDC's open- communication policy and strongly recommends that it continue. it recommends that a new communication plan be devised for the release of the final HTDS report and accompanying public documents, taking into account the problems that have already been encountered. · In the HTDS final report and all public documents, any significant changes made from the Draft Final Report should be clearly outlined and explained, and all remaining uncertaintites should be noted and explained.
Executive Summary 35 . Careful consideration should be given to how to release controversial reports to the public more effectively. The subcommittee suggests that CDC convene a workshop to discuss this and other communication issues of concern. SUMMARY OF RESPONSES TO THE CDC'S QUESTIONS The subcommittee's responses to CDC's six questions are summanzed below. Question 1. Has the analysis been carried out appropriately and completely? Our overall assessment is that the epidemiologic and clinical components of the study were of excellent quality, including the study design, followup success, subject participation rate, interviewing, thyroid examination, and laboratory methods. The design of the dose-assessment mode! has been found, on the whole, to be reasonably sound for the estimation of thyroid doses, but several questionable assumptions have been identified that would have some impact on the estimated individual doses. The estimated-dose uncertainties that the HEDR project produced and the HTDS study used are underestimates of the total dose uncertainty because some significant sources of uncertainty were overlooked. The basic objective in the statistical analysis was to determine whether there was an association between the occurrence of various thyroid diseases and exposure to 13~} released from Hanford. That was appropriately addressed by modeling the relationship between the frequency of a thyroid disease and dose, with consideration of an appropriate set of potential confounding variables. Several other analyses that were not presented could have aided in the interpretation of the apparently negative results, a
36 Review of the HTDS Draft Final Report critical one being an analysis of thyroid dose and disease with apparent geographic variations in disease rates controlled for. The investigators made no attempt to mode} the out-of- area doses for persons who were included in the main analyses. Their approach to that issue could have led to attenuated results in that it potentially estimated the total fallout doses for some people but only partial doses for others. The impact of global fallout on variations in thyroid-disease risk should also be analyzed. The subcommittee believes that the HTDS emphasis on analyses of subjects in the study rather than on comparisons with the general population is appropriate, inasmuch as the latter are potentially subject to serious biases. Question 2. Are the presentation and the discussion of results complete? One serious gap is that the methods used to calculate doses and uncertainties are not clearly or fully described in the dosimetry documents provided to the subcommittee. A number of additional tables are needed. A tabular presentation of the pathways to diagnosis would help readers to assess how the final diagnoses were assigned. A table of the frequency distribution of doses would be informative. Similarly, tables that show observed and expected numbers of disease outcomes according to four or five dose groups would normally be expected. A description of the estimated dose distribution according to such important categories as geostratum, year of birth, and amount of milk consumption in childhood would be helpful. . The discussion of the results was substantially incomplete in that little was said about whether the confidence intervals were wide enough to be compatible with other parallel studies. Most important, there was no adequate discussion of how dosimetric uncertainties might have affected the confidence intervals and the statistical power of the study.
Executive Summary Question 3. Are the conclusions reasonable? 37 In concluding its review of the HTDS Draft Final Report, the subcommittee considered the notion raised by the public that the HTDS was inconclusive in its findings. The subcommittee believes that the certainty of the interpretations from a complex study, such as the HTDS, is always a matter of degree. Its members believe that the high certainty with which the HTDS investigators presented the negative findings of the draft report was overoptimistic. Still, the main finding of the final HTDS report might indeed be that no radiation effect could be observed: the lack of evidence of a dose-response relationship for any type of morbidity suggests that overall risks were unaffected by Hanford releases. Given the substantial degree of imprecision in the exposure estimates and the effect of other statistical issues, the absence of any observable radiation effect does not rule out the possibility that a small effect exists, although it does mean that large effects of the 13~} exposure can be excluded as incompatible with the data. Until estimates are given with appropriate confidence limits, we will not know how much risk to the thyroid is compatible with the data. The evidence does not rule out (although it provides no particular support for) the possibility of a weak association that could affect, for example, those already susceptible to thyroid disease because of predisposing genetic factors. This carefully designed study, with sound followup and medical methods, has examined a substantial fraction of the most highly exposed population and failed to find any obvious evidence of a radiation effect; that is, there was no evidence of abnormally high rates of thyroid disease in the Hanford "do~vnwinders" examined who had the largest estimated exposures. Thus, at face value, the study was negative and found no increased risk. The pattern of individual exposure estimates is in accord with such basic factors as the prevailing wind direction and distance from the Hanford site. Finding negative results of both geographic and
38 Review of the HTDS Draft Final Report exposure comparisons implies that AT had no strong impact on thyroid disease. If a similar exposure occurred elsewhere, one could not predict the results with confidence. The small numbers of thyroid-cancer cases and the lack of precision in estimating individual exposures mean that one can have little confidence in the risk estimates found in the HTDS. At the various public- comment meetings, people who lived in down-wind areas stated that they and their families experienced more thyroid disease than would have been expected in the population at large. That could be due to genetic factors in families or even to chance, but the possibility that their disease was the result of unusual fallout or ingestion patterns or of unusual susceptibility to a thyroid radiation effect cannot be excluded. Question 4. Was the material accurate and appropriate in providing guidance to the public in understanding the study f n clings? For the most part, the written and oral messages about the Draft Final Report were accurate, but they were occasionally misleading in that they included statements that were too strongly worded, given the uncertainties that applied to the study. Keeping the study process and activities as "transparent" that is, open visible and understandable as possible for the public is a valuable approach that should not be abandoned because of the problems encountered with the release of the Draft Final Report. Question 5. If these messages need, to be am encled, how should the revised messages best be communicated to the public? Given all the communication problems that resulted Dom the release of the Draft Final Report, another detailed communication plan needs to be drawn up for release of the final report, including planning for unanticipated situations. Messages must take into account the various audiences being addressed and
Executive Summary 39 show concern and sensitivity for the thyroid-health issues that people perceive affect them. The full picture of the study results should be given, including all the uncertainties and other problems. A plan to brief the active citizen groups should be developed so that they have enough information to be able to respond to media inquiries about the report. In addition, an embargoed release of the report to journalists should be used so that they have a few days to read through the report and develop informed questions before the briefing. Question 6. With regard to release of future study reports, how can CDC improve the public communication process? The briefing structure should be simplified to try to eliminate leaks, and citizens who have participated in the advisory process all along should be given higher priority in the briefing structure. It could prove helpful to CDC to conduct a workshop on improving the public-communication process that includes experts in risk communication, journalists, outside scientists, and members of citizen groups.
