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Public Summary
BACKGROUND
In 1986, officials of the US Department of Energy
revealed that the Hanford Atomic Products Operations in Richiand,
Washington, had been releasing radioactive material, in particular
iodine-13 l, into the environment over a period of years. This
information, which confirmed the suspicions of some people in the
Pacific Northwest about what they called the Hanford Reservation
or just Hanford, created quite a stir. Both the US Congress and
citizens of the Northwest became keenly interested in knowing
whether these radiation releases had caused human health effects.
They were particularly concerned about whether Hanford releases
of iodine-131 had led to an increase in thyroid disease among the
population of the area.
In 1988, Congress ordered a study of the human health
effects of exposure to the iodine-131 released from Hanford.
Funded by the Centers for Disease Control and Prevention (CDC),
the study was earned out by the Seattle-based Fred Hutchinson
Cancer Research Center over the last decade. The study examined
estimate of exposures of the thyroid and rates of thyroid disease
because iodine-131 concentrates in the thyroid and that organ
would be the best indicator of radiation damage in the population.
Although dose is the correct technical term, this summary will use exposure to
refer loosely to a person's total radiation dose to the thyroid gland resulting from
either short- or long-term exposure to iodine-131 in the atmosphere and
environment from releases during the period 1944-1957.
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Review of the HTDS Draft Final Report
Scientists have recognized for about 45 years that
iodine-131 intake can lead to substantial radiation exposure of the
thyroid and possibly to the development of thyroid cancer and
other thyroid diseases. The likelihood that a given person will
develop thyroid disease after being exposed depends on the size of
exposure. The amount of radiation received by people living
downwind of the Hanford site depended on specific characteristics
of their individual lives, such as when they were born, where they
lived, what foods they ate, and where they obtained those foods.
The iodine-131 exposure of children occurred mainly through the
milk they drank and to a lesser extent through the leafy vegetables
and fish they ate. Breathing contaminated air also exposed Hanford
area residents and was included in the exposure calculations. The
radiation exposures of the thyroid glands of small children were,
on the average, much higher than those of adults because
children's thyroids are much smaller than those of adults and
children consume a lot of milk.
To conduct the Hanford Thyroid Disease Study
(HTDS), a 9-year $~S million effort, the investigators had to
contact 5,199 eligible people who had been born near Hanford (in
Franklin, Adams, Benton, Walla Walla, Okanogan, Stevens, and
Ferry counties) in the period 1940-1946 because the period of
greatest radiation releases was 1944-1947. Eventually, the HTDS
investigators enrolled 3,441 subjects in the study, gave them
extensive medical examinations to look for evidence of thyroid
disease, and used a questionnaire on risk factors for thyroid
disease. The HTDS investigators estimated individual radiation
exposures for the 3,190 people who, during 1944-1957, had ever
lived in the geographic area for which dose calculations were
made. Estimating radiation exposures of 50 years ago is a daunting
task for scientists because of the many unknowns about people's
lives, habits, and diet. Nevertheless, a detailed method that had
been developed previously by the Pacific Northwest National
Laboratory was used to estimate the exposure received by each
HTDS participant.
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Public Summary
3
Armed with the rates of thyroid disease found among
the 3,441 participants and estimates of radiation exposure received
by 3,190 of them, the HTDS investigators used statistical methods
to determine whether there was a relationship between the rates of
disease found and the estimated radiation exposures. Ordinarily,
one would expect that participants with larger radiation exposures
would have higher rates of disease. The statistical analysis was
complex for a number of reasons, including difficulty in
determining the radiation exposure received by each person.
On January 28, 1999, the HTDS investigators and CDC
released a Draft Final Report (FHCRC, 1999a) of the study to the
public. The report was a draft because, although it had undergone
internal review by CDC, it was still to be reviewed and subjected
to scrutiny and comment by the National Academy of Sciences-
National Research Council (NAS-NRC). The draft was released 2
months earlier than planned, for several reasons, including public
pressure for the report's release without changes made by CDC
and the desire by NAS-NRC to have an open review of the report.
The primary finding of the HTDS draft report was that there was
no evidence linking radiation exposure from Hanford to the rate of
thyroid disease found in the study population. The lack of evidence
of an effect, in scientific terms, is often caller! a "negative" finding.
While presenting their findings to the media and regional citizen
groups, the HTDS investigators overstated the certainty of their
results.
Many Northwest citizens were upset not only about the
findings of the study, but also about how the results of the study
were conveyed by the investigators. Shortly after the draft's
release, at CDC's request, NAS-NRC began an independent and
comprehensive appraisal of the study methods, results, and
interpretation and of how the study's findings were communicated
to the public. This report is a fulfillment of that request.
The NRC subcommittee studied the HTDS Draft Final
Report and discussed its contents in a series of meetings and e-mail
communications over about 9 months, in February-October 1999.
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Review of the HTDS Draft Final Report
The subcommittee arrived at consensus views on six specific
questions asked by CDC: how well the HTDS investigators
analyzed their data, how well the results were presented, whether
their conclusions were reasonable, whether the material provided
to the public was accurate and useful in helping the public to
understand the study findings, how the presentation might need to
be changed for the final report, and how CDC might improve
communication with the public in the future. The subcommittee
also developed a number of issues of its own to evaluate.
Detailed comments concerning the HTDS Draft Final
Report are included in various chapters of the main report. The
executive summary following this section highlights the views of
the NRC subcommittee. Answers to the questions mentioned
above are summanzed in the executive summary and answered
fully at the end of the subcommittee's report. This public summary
is intended to review the main points of the executive summary in
nontechnical language.
For its report, the NRC subcommittee concentrated on
five main subjects for evaluation: design of the HTDS, estimated
radiation exposures, data analysis, statistical power, and
communication issues. Its major findings and recommendations
appear below in boldface type.
DESIGN OF THE HTDS
The NRC subcommittee considered the HTDS
design to be appropriate to address its goals. The methods to
determine who the participants should be and where they were
living were exceptionally good, and the HTDS collected the
appropriate data on participants to enable the proper type of
analysis. Although the subcommittee founts the stiffly methods
to be of high quality, there are considerable uncertainties in
some of the information.
The investigators chose the most relevant population to
study: those in the most highly exposed areas who were young
children at the time of the greatest iodine-13 1 releases. It was also
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Public Summary
-
s
reasonable to study, as the subcommittee did, a low-exposure
group upwind of and more distant from the Hanford site. The
investigators were able to examine a high percentage of eligible
persons, and this was a strength of the study. The information
collected included such items as sex, history of other radiation
exposures (such as from medical procedures), smoking history, and
ethnicity.
Knowing the childhood milk-drinking habits of the
participants in the study was particularly important because iodine-
131 is most readily transferred to children through cow's milk as a
result of the fallout that settles on pasture grass. The investigators
attempted to question a parent or other close relative about each
participant's residence history, where milk was obtained, and the
amount of milk that was consumed during the period of the iodine-
131 releases (1944-1957~. If relatives were not available, then
participants were given a questionnaire at the time of the medical
examinations to get their history of residences and sources of milk.
For 38°/0 of the subjects, no parent or close relative was available
to provide detailed information about childhood milk-drinking.
The NRC subcommittee found that the clinical
examinations and laboratory studies were performed with
good-quality, scientifically valid methods.
Ultrasound and palpation methods were used in the
examinations. In palpation, a physician feels a person's thyroid
gland in the neck with his or her fingers to determine its size and
detect Jumps. The subcommitee's only criticisms of the medical
procedures were related to some quality-control procedures in the
pathology review and to the fact that some requested medical
records could not be obtained. But those criticisms were not
important enough to invalidate the findings of the study.
Estimated Radiation Exposures
The NRC subcommittee's review found that the
precision of the exposure estimates ranged from one-third or
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Review of the HERS Draft Final Report
one-half the best estimate to 2 or 3 times greater than the best
estimate. That range is reasonable for historical-dose
reconstructions. Evaluations of the mode} by other scientists
have cast doubt on some of the factors involved in the model.
The subcommittee also has concerns about some factors that
might lead to greater overestimation or underestimation of the
radiation exposures than was acknowledged by the HTDS
investigators.
Pacific Northwest National Laboratory developed the
computer mode! used to estimate the radiation exposures in the
Hanford Environmental Dose Reconstruction (HEDR) project.
This mode! had to take into account many factors: how much
iodine-131 was vented from the Hanford site, the wind directions
and other weather-related measures, how fast the iodine-131
settled to Earth, how much stayed on vegetation, how much
vegetation was consumed by cows (which depended on the
season), the fraction of the iodine-13 ~ eaten or drunk by cows that
was transferred to their milk, the length of time between when the
fanner milked the cow and when the milk was consumed by a
child, where the milk consumed by a child came from (for
instance, a local versus a distant dairy), how much milk was
consumed by the child at various ages, the fraction of the iodine-
131 consumed (or breathed) that was deposited in the thyroid
gland, and how long it stayed there. The mode! had to be able to
estimate thyroid exposures of persons of different sexes, ages,
places of residence, and dietary habits. The subcommittee found
that the general method used in the mode] was suitable for the
HTDS, assuming that the proper information about each
participant could be obtained and used.
The NRC subcommittee found that the resulting
exposure estimates for the HTDS participants were probably fairly
accurate, mostly within a factor of 2 or 3. This statement is based
on the results of valiclation exercises using the HEDR models
(Napier and others, 1994~. Recently, however, several scientists
have claimed that the amount of iodine-131 released from the
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Public Summary
7
Hanford site was higher than calculated by HEDR developers and
that the HEDR model therefore underestimated the thyroid
exposures by roughly 30°/0. And, the NRC subcommittee thinks
that the model overestimates the iodine-131 that was transferred
from pasture grass to cows' milk, this would mean that the mode]
overestimated exposures. A careful reassessment of these elements
of the mode! by the mode! developers is needed.
Errors like those can affect a study's findings about a
relationship between disease rates and estimated radiation
exposures. The ability of the HTDS to find the true relationship is
called its "statistical power" and has been a focus of attention by
the NRC subcommittee.
The NRC subcommittee found that the HEDR and
HTDS investigators probably assessed individual exposures as
being more precise than they actually were because some
sources of uncertainty were underestimated or not dealt with.
The subcommittee noted that exposures that took place
40-50 years ago could not be precisely estimated and that such a
situation could substantially reduce the ability of the study to
detect a radiation effect. (Uncertainty and the power of the study
are discussed further in this summary.)
The HTDS did examine the impact of fallout exposures
Tom nuclear weapons tests conducted at the Nevada Test Site but
overlooked the other sources of fallout exposure (such as nuclear
tests in the Pacific anal the Soviet Union). The NRC made a crude
assessment of the exposures from global fallout and found that, on
the average, the thyroid doses from global fallout were somewhat
smaller than those from NTS fallout. In addition, the global fallout
exposures occurred during the teenage years and early 20s among
the study population. The NRC concluded that global fallout is not
likely to have a large impact on the results of the epidemiologic
study.
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Review of the HTDS Draft Final Report
Analysis of HTDS Data
The subcommittee fount! some limitations in the
HTDS data analysis, including exclusive use of the HEDR
estimates of thyroid exposures from the Hanford releases,
possible inaccuracies in exposure estimates for people who had
lived only part of the time in the Hanford area, the need to
analyze thyroid-disease rates by geographic area, and the
absence of some key tables.
it is difficult to analyze the results of a study of the
occurrence of disease if the number of cases is small. Although
more than 3,000 people were evaluated for thyroid disease in the
HTDS study, only 20 had thyroid cancer; and only 14 of those
lived in the region covered by the HEDR mode! during 1944-1957
and could therefore have exposure estimated. The numbers were
greater for most other thyroid diseases; for instance, benign thyroid
nodules (noncancerous lumps) were found in 250 people. The
radiation effect in causing this disease could be estimated with
more certainty because of the larger number of cases.
The NRC subcommittee was critical of the HTDS
investigators' exclusive use of the HEDR estimates of thyroid
exposure for the data analysis and suggests supplemental analyses
that could help to confirm or weaken the conclusions of the study.
The subcommittee also found the analyses of the radiation effect
(called "dose-response relationship" in the study) difficult to
interpret for a variety of reasons. The subcommittee believes that a
more complete analysis should be carried out to estimate exposures
of people who were out of the study's geographic area for some of
the time when the iodine-!3 ~ releases took place.
The subcommittee recommends that the HTDS
investigators conduct more analyses to address the fact that the
thyroid disease rates in the HTDS appeared to differ in unexpected
ways between one geographic area and another. The geographic
area in which each person was born should be taken into account to
explain the unusual finding that thyroid disease rates tended to be
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Public Summary
9
higher in areas that were expected to have the smallest amount of
iodine-131 deposited on the ground.
The subcommittee believes that the HTDS
investigators were correct in emphasizing analyses of the
radiation effect rather than comparisons with another
population. It does not believe that comparing the HTDS study
group with some unexposed general population wouIc! be
useful.
Members of the public have repeatedly questioned why
no unexposed control group was involved in the HTDS so that
disease rates could be compared. There are several major reasons
why the pane] does not think that that would be a valid
comparison. First, for reasons unrelated to radiation, persons living
in various geographic areas can vary in their likelihood of
developing thyroid cancer. Second, the rates of disease found in
the HTDS are based on thyroid examinations. Intensive medical
examinations usually find more thyroid disease than would
otherwise be known about from routine medical practice. Because
no other population in the Northwest has been examined this way,
a valid comparison with other populations cannot be made. Any
conclusions drawn from comparisons with another population that
is defined as a "control group" would have more potential for error
than the conclusions drawn from the analyses that the HTDS
investigators conducted. Third, the analysis of a radiation effect is
a valid guide to the risk to the Hanford population even without the
use of an unexposed control group, as long as there is a sufficient
range of exposure levels and they are estimated with reasonable
accuracy.
The subcommittee is concerned that the results of
the study were reported- and interpreted-in black and white
terms of whether a statistical test was passed or failed. It
recommends that confidence limits be provided throughout the
report to allow readers to judge how large a radiation effect
might be consistent with the data. It feels that the HTDS
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Review of the HTDS Draft Final Report
investigators probably overstated the strength of their fiddling
that there was no radiation effect.
Usually, scientists provide both a best estimate and a
range of estimates called "confidence limits" or "confidence
intervals" to use to interpret statistical results. However, the
HTDS investigators provided only their best estimate, not the
confidence limits, for the size of possible radiation effects in the
report or in their public statements. That made their findings seem
more solid than they actually were.
Furthermore, the HTDS investigators should have
calculated confidence limits that account for both the imprecision
in the exposure estimates and the conventional statistical
imprecision. By not presenting confidence limits, especially ones
that consider imprecision in exposure estimates, the HTDS
investigators overstated the strength of their main findings in the
draft report.
Statistical Power and the HTDS Interpretation
The subcommittee believes that the assumptions
used by the HTDS investigators to estimate the needed sample
size and to calculate statistical power were incorrect; their
assumptions did not acknowledge that exposures could be
estimated only very imprecisely. The subcommittee found that
HTDS ignored five sources of imprecision, which decreased the
ability of the study to detect a small radiation effect. That
means that the negative results that the study obtained are less
definitive than the report and press releases stated.
Because the HTDS results found no increase in thyroid
disease with an increase in radiation exposure Tom iodine-13 l, a
cntical issue is how to interpret those findings correctly. To
evaluate the HTDS interpretation, the subcommittee asked a series
of questions. For example, were the data good enough? Do the
underlying patterns of exposure and disease agree or disagree with
the negative findings? Was the statistical power of the study high
enough to make the negative findings convincing? (The higher the
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Public Summary
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statistical power of the study, the more confidence people can put
in the study's findings.)
The subcommittee reviewed the factors that influence
statistical power, focusing on the impact of lack of precision in the
thyroid exposures calculated by the HEDR project. It found that
the statistical-power calculations made inadequate allowance for
imprecision in the dose estimates. Given that situation, the
subcommittee believes that the HTDS did not have as much
statistical power to detect radiation effects as the investigators
claimed. That means that the results of no effect ("negative"
findings) reported by the HTDS are less definitive than the report
and related public documents stated. Hence, this subcommittee
recommends that. if possible. the HTDS investigators redo the
~ 1~ 7 _
statistical-power calculations to take into account all the sources of
imprecision and that they reinterpret the study results in
accordance with the limitations of statistical power.
The subcommittee believes that the findings of the
HTDS cannot be reliably distinguished from the findings of the
study of thyroid disease among children in Nevada and Utah who
had been exposed to fallout resulting from atmospheric nuclear
weapons tests conducted at the Nevada Test Site in the 1950s. A
marginally positive radiation effect was found in that study. It is
likely that, given the confidence limits for both studies, there
would be an overlap, even though one appears positive and one
negative. That is because the findings of both studies are very
. .
Imprecise.
Communication of HTDS Results to the Public
The subcommittee believes that the original
communication plan developed for the HTDS, particularly the
parts that emphasized open public communication, was well
developed and should have been moderately successful if
implemented as planned. However, several factors led to an
early release of a draft report, rather than a final report. When
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Review of the HTDS Draft Final Report
the Draft Final Report was released, a number of
communication errors were made that caused public outcry.
Compared with the history of less than open public
information from the US 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 were 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 should have helped to build trust and
credibility for the study.
Some of the public outcry on release of the draft report
might have been avoided if the onginal communication plans
outlined in the HTDS draft had been followed. The draft report
outlined a good communication plan for its release, which included
an admirable concern for translating the technical information in
the report into an understandable booklet for the public and other
efforts, including a Web site, to share information with the public.
But the plan also called for delivery to the public of final
information about the study, not a draft that had not been subject to
review by outside scientists. Instead, several events forced the
early release of the Draft Final Report and pre-empted the original
communication plan.
Not only the early release of the report was a problem,
but so was the main message in the report (namely, a strong
statement that iodine-131 releases had caused no thyroid disease).
In trying to decide how to present this message, CDC was on the
horns of a dilemma. CDC personnel had been urged by some
members of citizen groups not to alter the report before its release;
they wanted the report to be released just as the HTDS
investigators had written it. CDC also had to respect issues of
academic freedom regarding the principal investigators' views. But
after the draft's release, the CDC people were blamed for not
intervening to counter the strong message delivered to the public
by the HTDS investigators.
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Public Summary
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A key weakness of the communication effort
surrounding the release of the draft report was that the public
materials written and the oral statements made by HTDS
investigators overstated the certainty of the study (the
statistical power) and the conclusiveness of the negative
findings, but did not report any of the uncertainties.
The public materials factually represented what
appeared in the draft report. But, given the state of a draft
document that had not been reviewed externally and a number of
uncertainties in the data, the strong statements that the
investigators made publicly were unwarranted. On the basis of
comments received by the NRC subcommittee from members of
the public, it is clear that many persons with an interest in the
findings of the study were not only disappointed with the reported
negative results, but also upset by how the results were
disseminated and described.
A number of factors contributed to the problems
surrounding the craft report's release, including (~) a perceived
need for an information blackout that included the citizen groups
that had been privy to most other parts of the study; (2) a complex
schedule of briefings of groups in person in Washington, DC, and
by telephone in the Hanford area to various state health agencies
and citizen organizations only several hours before the media and
public briefings on the findings; (3) a leak to the New York Times
that related the findings to the public before most of the briefings
in the Hanford area; and (4) a message that contradicted what most
of the public thought would be the outcome of the study.
The subcommittee believes that in the media and
public briefings the HTDS investigators paid insufficient
attention to the audience's health concerns and fears and that
HTDS investigators and CDC officials should have offered
more balanced, and possibly alternative, interpretations of the
findings and discussed their implications for individuals.
During the media briefing and public meeting held to
announce the findings of the HTDS, the investigators emphasized
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Review of the HTDS Draft Final Report
the overall statistical results of the study and did not senously
discuss the outcome for individuals. That approach angered many
members of the community who had thyroid-related health
problems. More care should have been taken to explain the
differences between statistical relationships and individual
outcomes. The subcommittee recommends that when the final
report is released, implications for individuals and families that
have suffered because of thyroid disease be explained and
highlighted in the written matenals and the public bnefings. in
addition, legitimate differences in viewpoints regarding study
findings between the HTDS and CDC personnel should be
explained and discussed.
The subcommittee recommends that a new
communication plan be developed for the release of the final
report, taking into account the serious problems encountered
with the release of the draft report. In the final report and all
public documents related to it, any important changes made
from the draft report and all remaining uncertainties should
be clearly outlined and explained. The subcommittee applauds
CDC's open-communication policy and strongly recommends
that this policy continue with the HTDS and similar studies.
The complicated briefing strategy used for releasing
the Draft Final Report did not work well, and the subcommittee
suggests that a more simple and efficient briefing plan be devised
for releasing the final report. in particular, it recommends that
telephone briefings be abandoned because all involved with release
of the draft report disliked them. Citizen groups that have
participated in a study over the years should not be kept out of the
information flow concerning the study report's release until the
very last minute, as they were with the briefings on the draft report.
The subcommittee also suggests that a small group of
risk-communication experts, scientists, journalists, and citizens be
convened to consider the more effective public release and
discussion of controversial draft reports that have not been peer
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Public Summary
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reviewed, as well as other issues that could affect the future release
of important CDC reports.
Concluding Remarks
in concluding its review of the HTDS Draft Final
Report, the NRC pane] considered the notion raised by the public
that the HTDS is inconclusive in its findings. The subcommittee
believes that the issue cannot be answered as simply as "agree" or
"disagree", because the certainty of the interpretations Tom a
complex study like the HTDS is always a matter of degree. The
subcommittee members believe that the high certainty with which
the HTDS investigators presented the negative findings of the draft
report amounted to an overstatement. But the main finding of the
HTDS final report could prove to be that no radiation effect can be
observed. Given the imprecision in the exposure estimates and the
effect of other statistical issues, the absence of any observable
radiation effect is not proof that there is none. it does mean that the
iodine-131 exposure did not have large effects. However, until
estimates are given with appropriate confidence limits, we do not
know how much risk to the thyroid is compatible with the data.
it seems doubtful that a better study could have been
conducted in the downwind area, short of having some way to
improve the exposure estimates greatly an unlikely prospect
because so little information is available on the exposures of 45
years ago. This carefully designed study, with sound followup and
sound medical methods, has examined a large fraction of the most
heavily 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
"downwinders" examined who had the largest estimated
exposures. Thus, at face value, the study was negative, and no
increased risk was found. The pattern of individual exposures is in
accord with such basic factors as the prevailing wind direction and
distance from the Hanford site, and this accord generally supports
the exposure modeling. Finding negative results of both
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Review of the HTDS Draft Final Report
geographic and exposure comparisons implies that the iodine-131
exposures had no strong impact on thyroid disease.
However, if a similar exposure occurred elsewhere, one
could not predict with confidence whether a positive or negative
result would be seen. 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 size of the risk estimates
found in the HTDS.
At the time of the initial release of the Draft Final
Report, it was indicated by the HTDS investigators that residents
of downwind areas should fee] relief that being close to the
Hanford nuclear site did not result in increased risk of any thyroid
disease. Such statements are too broad, but they might be
reasonable in specific instances. For example, a healthy 55-year-
old who lived near Hanford and drank a large amount of milk as a
child can take comfort in learning that there is no evidence that he
or she will have a greater risk of thyroid disease than other people
in the general HTDS study area.
At various public-comment meetings, people who lived
in downwind areas stated that their families experienced more
thyroid disease than would have been expected in the population at
large. Their disease could have been the result of unusual fallout or
eating patterns or unusual susceptibility to radiation effects. But
one should bear in mind that some cases of thyroid disease occur
for reasons not understood by medical science. For example,
thyroid disease tends to run in families, and family clusters could
be related to genetic factors in the families or to chance. The lack
of evidence of a dose-response relationship for any type of thyroid
disease in the HTDS suggests, but does not prove, that the overall
risk was not affected by Hanford fallout. The evidence does not
rule out (although it does not support) the possibility that a weak
association could affect, for instance, people who are already
susceptible to thyroid disease because they are predisposed to it by
genetic factors.
Representative terms from entire chapter:
htds investigators