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4
Evaluation of Dosimetric Methods and Results
This section presents a brief critique of HTDS
dosimetric methods and evaluates the quality of the dosimetric
component. Our purpose is to assess whether there are any flaws in
the estimation of the individual thyroid doses calculated by the
HTDS, whether uncertainties are properly accounted for, and how
much the NTS and global fallout might have affected the results of
the HTDS.
BACKGROUND
.
One of the disputed aspects of the estimates of
exposures of children downwind from the Hanford site pertains to
the amount of AT released from the site, which in this case
originated in the T and B reactors on the Columbia River; these
reactors became operational in late 1944 or 1945. The fuel rods
were "cooled" for various periods to let the short-lived fission
products decay and then carried to separation plants, where the
plutonium was ultimately reclaimed and the remaining ill{ effluent
was a byproduct of the chemical separation procedures. The
quantities of Lit released to the air from the separation processes
used to obtain plutonium from the uranium fuel rods of the reactors
at the Hanford Atomic Product Operations were estimated, and
meteorologic information and environmental-transfer models were
used to assess the environmental concentrations of ]3~{ in air, milk,
and other foodstuffs. Personal information on the whereabouts and
61
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Review of the HTDS Draft Final Report
dietary habits of the study subjects led to the estimation of the
thyroid doses from islet. When the operation being described is 50
years in the past and the release and deposition of a radioisotope
with only an 8-day half-life are being estimated, reconstructing the
dose to the thyroids of a somewhat mobile population surrounding
the effluent site is extremely difficult, even if it is based on the best
records to be found and if due care is used in the calculations and
interpretation of the scant data that are available.
in the HTDS Draft Final Report, the thyroid doses were
calculated on the basis of information provided by the HEDR
project. It is the subcommittee's understanding that the HEDR
project estimated the daily environmental concentrations of ]3~} (in
indoor and outdoor ground-level air, in milk from a backyard cow
and from commercial sources, in leafy vegetables, and so on) for
the period from December 26, 1944, to December 3l, 1957, and
for geographic locations in the study domain where the subjects
could have been exposed from the Canadian border to central
Oregon and from the eastern border of the Idaho panhandle to the
center of the Cascade mountains. To do that, the HEDR project
performed 100 realizations of its computer codes. In each
realization, the input parameter values were randomly selected
from subjective probability-distribution functions; 100 results (~3~T
concentrations in air, milk, leafy vegetables, and so on) were
provided for each day for each location of interest. Uncertainties
were assigned according to histograms of the results.
Using the calculated HI environmental concentrations
and the answers provided by the subjects as to their residence and
dietary histories, the HTDS estimated their i3~T intakes via
inhalation and via ingestion as a function of time. The products of
the AT intakes and the AT thyroid dose coefficients for inhalation
and for ingestion yielded the thyroid dose estimates. It is unclear to
the subcommittee how the uncertainties in 13~] intakes were
estimated by the HTDS.
One of the difficulties encountered in reviewing the
work done by the HTDS is that the method used to calculate doses
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Evaluation of Dosimetric Methods and Results
63
is not clearly described in the documents that were provided to the
subcommittee. This infonnation is presumably scattered in the
large number of documents that were prepared by the HEDR
project and the HTDS in the course of their work. It is
recommended that the HEDR project develop a single document in
which the methods, assumptions, coefficients, and shape and
magnitude of the coefficient uncertainties are summarized, the
nature of the results provided to the HTDS is clearly descnbed
(with their strengths and wealmesses), and the ways in which the
HTDS made use of the HEDR results to estimate individual
thyroid doses are described in detail.
During the 1950s and early 1960s, two other
environmental sources of ]3~{ contributed to the thyroid doses
received by the populations around Hanford: the nuclear-weapons
tests canted out at the NTS, mainly in 1952, 1953, 1955, and 1957;
and the nuclear-weapons tests conducted outside the United States,
which gave rise to "global" fallout, mainly in 1954, 1956, 1957,
1958, 1961, and 1962. Only the first of those two sources was
taken into consideration by the HTDS. A crude assessment of the
thyroid doses resulting from "global" fallout near Hanford has
been prepared for the purposes of this review (appendix C).
HEDR CALCULATIONS OF ENVIRONMENTAL CONCENTRATIONS OF
3~} FROM HANFORD AND RESULTING DOSES: ASSESSMENT OF KEY
PARAMETER VALUES
Because of the paucity of ]3~] environmental data,
HEDR used a suite of environmental-transfer models to simulate
the transfer of ]3~{ Dom the point of release to the dietary products
consumed by the subjects and to ground-level air.
The HEDR models have been subjected to numerous
reviews, for example, the Technical Steering Pane} (TSP) and
CDC review of the RATCHET model, the TSP external review of
the environmental-accumulation dosimetry code development, two
workshops held by CDC to address issues related to the dose and
risk assessments, and a recent review prepared by this
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Review of the HTDS Draft Final Report
subcommittee. In addition, the various computer codes have been
tested by staff independent of the developers to ensure correct
implementation of the models. The dose assessment has been
found, on the whole, to be structurally sound for the estimation of
thyroid doses to representative, hypothetical persons, but minor
errors have been found and doubts have been raised about the
validity of the results for particular environmental conditions and
for the estimation of thyroid doses to specific people. In its letter
report dated January 25, 1999, this subcommittee recommended
that
.
The HEDR investigators supplement their
description of the mode} with an account of the
origin of the errors made with regard to the
estimation of the i3~} concentrations in pasture
grass on the basis of measurements, the impact
on the predicted values when the errors are
corrected, and a preliminary assessment of the
effect of reparameterization on estimates of
absorbed dose to the thyroid. Until such a
reassessment has been made it is difficult to
know whether the current estimates of dose to
representative individuals need recalculation.
As of September 1999, the subcommittee had not
received any response to its recommendation.
Hourly releases of AT based on recorded data on the
average reactor power levels and on the burnup of the fuel at
discharge are provided in PNWD-2033 HEDR (volumes ~ and 2)
(Heeb, 1993) for 1944-1947 and in PNWD-2222 HEDR (Heeb,
1994) for 1944-1949. For 1950-1972, only monthly release
estimates are reported in PNWD-2222 HEDR (Heeb, 1994~. The
computer mode! for the source term (STRM) generated ~ 00
realizations of the emissions for each hour for 1944-1949, which
were aggregated into daily releases. The emissions were
aggregated into monthly releases for 1950-1957.
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Evaluation of Dosimetric Methods arid Results
65
The RATCHET model for environmental transport has
a significant source of uncertainty built in that is not accounted for:
the interpolation of the wind fields. This uncertainty is not spatially
uniform. There should at least be a study of the sensitivity of the
deposition estimates with respect to local errors in the wind field.
According to Ramsdell and others (1994) (table D, inverse-square-
distance weighting was used for the interpolation. That method
does not produce any information about the uncertainties
associated with the interpolated values. The paper does not justify
the choice of the method; in particular, it is known to be very
sensitive to the location pattern of the data points. Inverse-square-
distance weighting is known to be "isotropic"; that is, it
incorporates the distance from the data points to the interpolation
point but not the directions. In the case of wind fields, one would
expect the direction from an interpolation point to a data point to
be important. The algorithm is also insensitive to the directions
between the pairs of data points; it incorporates only their relative
distances from the interpolation point.
The environmental-accumulation mode} developed by
the HEDR project (DESCARTES) provides the concentrations of
Al in different plant and animal products for numerous space-time
combinations, whereas the individual-dose mode} (CIDER) is used
to estimate the Fit body burdens and doses in humans of various
ages and both sexes on the basis of ingestion and inhalation
pathways. The equations used in the DESCARTES and CIDER
models and the key input-parameter values and their distributions
can be found in PNWD-2023 HEAR (Snyder and others, 19941.
The DESCARTES code was operated with a daily time step for
1944-1949 and with a monthly time step for 1950-1957 (Farns and
others, 1994; Price, 19941.
The thyroid dose via ingestion is due primarily to the
consumption of milk produced by cows that fed on pasture grass.
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For the purposes of this report, the thyroid dose from this pathway
can be expressed as follows for an acute release of 13lT:
Dm= (ST)(VEG/ST)(Tveg)(Pl)(Fm)(MDF)(MCR)(DCF)'
where
Dm = thyroid dose, Gy,
ST = source term, Bq,
VEG = concentration of i31} in pasture grass at time of
fallout, Bq kg-~,
TVeg = mean time of residence of 13lT on pasture grass, d,
PI = pasture intake by cows, kg d-~,
Em = transfer coefficient of AT to cow's milk, d L-~,
MDF = milk-distribution factor (which tales into account
the fact that milk consumed might not be of local
· ~
ongm,
MCR = milk-consumption rate by subject, L d-l, and
DCF = thyroid dose coefficient, Gy Bq~l.
Those key parameters will be considered in turn. Most of the
parameter values are presented in Snyder and others (19941.
Source term:
The source term for 1944-1947, before the installation
of removal devices for i31l, appears to have been well estimated on
the basis of the available historical data. It is during these early
years that the largest nit releases occurred, but there is concern
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Evaluation of Dos~metric Methods arid Results
67
that the 13lI releases for later dates have been underestimated by
the HEDR project. The following are Hof~nan's analysis of the
HEDR estimates and not necessarily those of the committee.
Several reasons for such an underestimation are listed in a draft
report prepared by Hoffman and others (1999~:
· The HEDR project estimates of the amounts of 13lT
processed and released in 1959 and 1960 are substantially less
than the amounts reported by Warren (1961), which are the
source of HEDR release estimates.
· The HEDR project documents use Warren (1961) as a
source but do not evaluate the credibility of his values; for
example, he projected an unrealistically high scrubber
efficiency.
· The HEDR project misapplied measured release-factor
data from 1959-1960 to the period 1951-1957, when less
emission-control equipment was in place.
The HEDR project incorrectly accounted for operation
of the silver reactors in the B and T plants by inexperienced
personnel during the first IS months after installation in 1951.
· The HEDR project substantially underestimated the
source-term uncertainties for the B. T. and RED OX plants.
The HEDR project inadvertently used the medians
instead of the arithmetic means of the monthly source terms for
the air-concentration and ground-deposition calculations.
· The HEDR project did not propagate the source-term
uncertainties to air concentrations, ground deposition, and
doses.
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The subcommittee did not investigate whether the
reasons for a possible underestimation of the source term, as
proposed in Hoffman and others (1999), are valid.
Hermann and Hermann (1996) evaluated the I'll
releases in 1948-1960; they based their estimates of emission-
contro! efficiency on investigations conducted at fuel-reprocessing
plant WAK in Karisruhe, Germany, and concluded that the HEDR
project had overestimated the efficiency of emission-control
equipment for Gil. They also concluded that the cooling time (time
elapsed between fuel discharge from the reactor and fuel
dissolution in the chemical plant) might not have been properly
taken into account. Hermann and Hermann (1996) estimated an ~3~}
release of about 250,000 Ci in 1948-1960, whereas the HEDR
estimate for the same period is about 54,000 Ci. In comparison, the
AT release in 1944-1947, which seems to have been well
estimated, was about 685,000 Ci. The estimates of the total
releases of AT in 1944-1960 are therefore 740,000 Ci according to
Heeb (1994) and 940,000 Ci according to Hermann and Hermann
(1996~.
Concentration of HI in pasture grass at time of fallout:
The concentration of i3~T in pasture grass at the time of
fallout is characterized by the value of VEG/ST. It is derived from
the daily ground-deposition densities of AT that are provided by
the RATCHET code (Ramsdell and others, 1994~. The deposition
pattern is based on mathematical modeling, with little validation of
the calculated deposition pattern of AT. As shown by Napier and
others (1994), there are discrepancies between the measured and
calculated values of vegetation samples that have been collected
since 1945. It might have been helpful to measure the }29{ pattern
of deposition in the geographic domain considered by the HTDS.
Because ]29], a radioactive isotope of iodine with a very long half-
life, behaves in the same way as }3~{ and because Hanford was a
dominant source of AT as long as the chemical plants were in
operation, measurements of AT concentrations in soil would have
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Evaluation' of Dosimetric Methods and Results
69
produced valuable information regarding the overall geographic
deposition pattern of its, even though the day-to-day temporal
pattern of releases of ]29] iS not highly correlated with those of 13~.
The derivation of the 13~{ concentration in pasture grass
Tom the ground-deposition densities makes use of a relationship
that is valid only for dry deposition processes. It is not clear
whether that relationship was used when precipitation occurred.
Hoffman and others (1992) showed that the parameter values are
substantially different for dry and wet processes. Because
precipitation amounts in counties remote from Hanford are about
2-3 times higher than for counties near the site (see appendix C),
this could have resulted in differences in a degree of
overestimation or underestimation of the thyroid doses.
Mean time of residence of AT on pasture grass:
The mean time of residence of ]3~] on pasture grass
seems to have been well estimated. Because of the short
radioactive half-life of Ill, the thyroid dose is relatively insensitive
to the choice of value of this parameter.
Pasture intake by cows:
The values chosen for pasture intake by cows are not
presented in Synder and others (1994), because they are extracted
Tom a database described in Beck and others (1992) but later
revised in an unpublished report submitted to the chair of the
HEDR Technical Steering Pane} (oh. Till) in 1994. It would have
been helpfi~} to the subcommittee to have a report available on the
topic.
Transfer coefficient of 13~} from cow's intake to milk:
The central estimate of the transfer coefficient of AT
from cow's intake to milk is taken to be I.2 x lo-2 ~ LO for a dairy
herd and 9.2 x 10~3 ~ is-] for a single cow (Snyder and others,
1994~. Those values seem to be too high by a factor of about 2,
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Review of the HODS Draft Final Report
compared with the central estimates used for NTS fallout 4 x ~ 0-3
L-r in NCT (1997) and 4.2 x 10~3 ~ it-} in Simon and others (1990)
and with values obtained after the Chernobyl accident, which
ranged from 2 x 10~3 to 6 x 10~3 ~ ~-l, according to Hoffman and
others (1988~. However, this is an open issue, in that the measured
values of this transfer coefficient vary over a large range for
reasons that remain largely unexplained.
Milk-distribution factor:
Data on the distribution of milk within the project
domain are not presented in Synder and others (1994), because
they are extracted from a database described in Deonigi and others
(1994~. It would be helpful to know how the uncertainties and the
correlations in the distribution of milk were taken into account by
the HTDS.
Milk-consumption rate:
The HEDR project selected milk-consumption rates for
representative people in 12 age and sex categories (Anderson and
others, 1993; Fares and others, 1994) on the basis of the results of
the 1977-1978 Nationwide Food Consumption Survey and
extrapolation back to 1945-1957. The HTDS used these rates when
reliable information could not be determined from the personal
interviews. However, how they were applied is unclear in the Draft
Final Report. Because a number of assumptions are made (without
quantifying the uncertainty) in Anderson and others (1993) and
extrapolations are made from current food-consumption habits
back to the relevant period, largely on the basis of broad-scale
measures rather than measures particular to the people in the study,
there will be at least an uncertainty associated with the extension
of population or group practices to specific persons. it would be
useful to know whether the milk-consumption rate distributions
given in Anderson and others (1993) were used by the HTDS and
whether milk-consumption rates were assumed to be constant in a
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Evaluation of Dosimetric Methods and Results
71
given age category or were taken to vary with age within an age
category.
Thyroid dose coefficient:
The value of the thyroid dose coefficient depends on
three parameters: the uptake of IT from blood by the thyroid, the
time of retention of 13~{ in the thyroid, and the mass of the thyroid
gland. The values of those three parameters depend to some extent
on the amount of stable iodine in the diet. The thyroid dose
coefficient varies strongly as a function of age, the average value
for infants being about ~ O times greater than that for adults. That is
caused by the variation of the mass of the thyroid gland as a
function of age; the uptake of ill} from blood by the thyroid and
the time of retention of }3~} in the thyroid do not vary substantially
with age.
The central estimates of the thyroid dose coefficients
that are used by HEDR and the HTDS are taken from the
international Commission on Radiological Protection BURP.
~ ,
1990) and assume a sufficient amount of stable iodine in the diet;
the uncertainties are taken from Dunning and Schwarz (1981~.
Those sources of information are generally accepted as the best
available. However, it would be useful to know whether the
thyroid dose coefficient was assumed by the HTDS to be constant
within a given age category.
HEDR DOSIMETRY-MODEL VALIDATION
For an ideal validation of the dosimetry model, one
would like to have measured doses for at least a subset of the
people in the study; then one would compare the doses predicted
for those people with the measured doses. But those data do not
exist. instead, the validation study used other people for whom
some measurements were available.
- -- 1- - -1--
The validity of the
comparisons hinges on the cross-applicability of various
assumptions and parameter values between the test population and
the study population. Shipler and others (1996), of the HEDR
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Evaluation of Dosimetric Methods and Results
75
environmental measurements of atmospheric krypton-85 (8sKr).
The number of locations sampled per year increased from four in
1984 to I] in 1987; up to 12 samples per year were taken from
each location. Detailed results were presented for three of the
sampling stations that were in the same grid cell (North of Hanford
Site 300 Area). Air concentrations were calculated from two sets
of meteorologic data: those available in the 1940s and those, more
complete, available in the 1980s. The calculations were in good
agreement with the measurements when the 1980 meteorologic
data were used and were about 3 times higher than the
measurements when only the meteorologic data available in the
1940s were used.
Much less detailed information is presented for the
other sampling stations where 85Kr was measured. It would have
been valuable to present the results in the same fashion as in the
"North of Hanford Site 300 Area" grid cell to identify possible
location-related biases in the calculated values.
Conclusions
On the whole, the results presented are good, inasmuch
as there is agreement between measured and median calculated
values to within a factor of 3. However, the thyroid burdens and
the ~sKr air concentrations could have been analyzed more
thoroughly. In addition, errors in the assessment of the
concentrations in vegetation should be corrected, and many
isolated data were not used in validating the models.
Possible Underestimation or Overestimation of Doses
Hof~nan and others (1999) and He~mann and Hermann
(1996) have reviewed the HEDR dosimetry and claimed that
thyroid doses were substantially underestimated by the HEDR
project. However, the divergences that they identified occurred
during years when 13~t releases were rather small, and their
findings and HEDR results differed by only about a factor of I.3 in
total releases.
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If doses were underestimated, it is important to consider
the implications. To the degree that doses are underestimated, the
imputed risk estimates will be too large, but systematic, across-the-
board dose underestimation does not alter the statistical
significance of dose-response trends. Hence, in the HTDS-in
which, as it turned out, the primary issue became whether there is
any association between ]3~} exposure and thyroid diseases-the
effect of possible dose underestimation might be minimal. If there
was systematic underestimation of doses, the true statistical power
of the study would be greater than one would estimate it to be
given the dose distribution used by the HTDS, in which case
negative results would be more persuasive than they are. However,
if the doses were underestimated more for some study subjects
than for others or if the effect of dose underestimation was greater
for some subjects than for others (for example, because some are
younger than others), variation in the underestimation or in its
effect would act as another source of measurement error and tend
to cancel out the gain in statistical power achieved by having
generally higher doses. Therefore, a simple generalization about
the effects of dose underestimation cannot be given.
In contrast, the HEDR project might have
overestimated the transfer coefficient from cow's intake to milk by
a factor of 2, and some of the validation comparisons also suggest
an overestimation of doses by the HEDR calculations. The effect
of dose overestimation could have implications for the
interpretation of HTDS results because overestimation would mean
that the statistical power of the study was also overestimated; this
would imply that the negative results were not as definitive as the
investigators asserted.
Doses might have been underestimated in some areas
and overestimated in others. Possible reasons are errors in the
deposition pattern of ]3~{ due to inaccuracies in wind flows or
precipitation amounts and errors in the milk-distribution pattern.
it is beyond the mandate of the subcommittee to assess
the direction and magnitude of possible dose misestimation
-
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definitively. As stated in a previous letter report (National
Research Council Letter Report to CDC: HEDR-Issues
Regarding the Hanford Environmental Dose Reconstruction
(HEDR) Atmospheric I-131 Pathway Models dated January 25,
1999), the subcommittee found the dose-reconstruction effort to
be, by and large, a credible effort, although there were unresolved
questions and considerable uncertainty in doses because of the lack
of adequate histoncal measurements and other relevant data.
DOSE UNCERTAINTIES
Considerable efforts were made by the HEDR project to
mode! the uncertainties attached to estimated thyroid doses.
Uncertainties for many parameters used in the environmental
pathways and radiologic dose modules (DESCARTES, CIDER,
and CRD codes) are listed in Snyder and others (19941. The HEDR
investigators assigned probability distributions to the various
parameters on the basis of their assessment of the uncertainties in
the parameter values. They then obtained 100 estimates based on
the computer codes. in each estimate, for each parameter the
values were randomly selected from the probability-distribution
functions, so 100 results (~3~{ concentrations in air, milk, lead
vegetables, and so on) were generated for each day for each square
in their grid that defined a location in their geographic domain.
Uncertainties were assigned according to the frequency
distributions ofthe 100 results.
A key point here is the possible distinction between
variability (added to the data) and uncertainty inherent in the data.
If one has an estimate for a parameter value and adds a random
component, such as meteorologic data (Ramsdell and others, 1994,
p. 571), this does not characterize the uncertainty in the original
estimate (although the authors claim that it does). Adding the
random component does not result in a random component with
the same distribution as the added part, but rather is the
convolution of the distribution of the original and the distribution
of the random component. Regrettably, the distribution of the
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Review of the HTDS Draft Final Report
original estimate, the distribution of the error of estimation, is
urlkno~vn.
in other places in the publications mentioned above, the
authors indicate that they have sampled from a subjective
distribution, as opposed to using the estimate plus a random
component. Aside from the question of whether a proper subjective
distribution has been chosen, this approach is more appropriate.
However, it is difficult to quantify the uncertainty associated with
choosing or fitting the subjective probability distribution, and the
authors have not incorporated this uncertainty into the calculations.
There might be adequate explanations for these
discrepancies that do not appear in the publications or in the Draft
Final Report. The omission and the lack of clarity of what the
HEDR project dice constitute inadequacies in communication and
perhaps in scientific methods. Without adequate explanations in
the Draft Final Report, the committee cannot confirm that the
propagation of error in the dose reconstruction (the source term in
particular) was the best choice of methods and is correct. This
situation adds to our overall concern about the integrity of the
uncertainties of the dose estimates and how they could affect the
power calculations. By "integrity of the uncertainties" the
subcommittee means the inclusion of all of the uncertainties and
then understanding and using them in an appropriate way in the
epidemiologic analyses.
It is not clear to the committee what was done by the
HEDR project about the parameters that are not listed in Snyder
and others (1994) or about the mode} uncertainties. For example,
. According to Hoffnan and others (1999), the HEDR
project did not propagate the source-term uncertainties to air
concentrations, ground deposition, and doses for the 1950-1972
period. Evidence of uncertainty propagation could not be found
in the HEDR reports made available to the subcommittee.
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· No information could be found on the uncertainties
related to the cows' feeding practices or to the commercial
distribution of milk.
. No information could be found on the correlations
between parameter values.
· No information could be found on the uncertainties
related to the use of models to calculate thyroid doses from
exposures.
The uncertainty estimates attached to the HTDS thyroid
doses appear to be too low. The geometric standard deviations
(GSDs) of some of those doses are estimated to be about 1.4. The
GSD of the thyroid dose-conversion factor alone is taken to be 2.0
(Snyder and others, 1994), so, the GSDs of thyroid dose estimates
should be at least 2. The authors of the report should explain why
GSD values of less than 2 are found for many HTDS thyroid-dose
estimates. It is to be noted that the GSDs of the thyroid doses
calculated by the HEDR project are 2 or greater (Harris and others,
1996~. An increase in the GSD from I.4 to 2.0 quadruples the
width of the 95°/0 confidence interval.
The HTDS did not take into account the uncertainties
associated with the recall 5 decades after the period of
exposure-of the origin of milk, the milk-consumption rate, or
changes in residence. That is a serious flaw of the methodology,
which is discussed further in chapter 6.
In summary, although the subcommittee believes that
the methods used by the HEDR project to estimate thyroid doses
and their uncertainties are on the whole structurally sound, there
remain quite a few errors or omissions that should be corrected,
and supplementary work is desirable. in a National Research
Council letter report dated July 27, 199S, the committee stated that
the resulting dose estimates are highly uncertain, and the use of
self-reported information in the construction of dose estimates
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presents an opportunity for bias, such as overreporting or
underreporting of activities that might substantially affect exposure
estimates. The investigators are urged to evaluate further the use of
some of the interview data to corroborate the HEDR exposure
estimates and the default options. Modeling has inherent
limitations that need to be acknowledged candidly. Given the
insufficient data available on Hanford, there will always be
considerable imprecision in the individual doses, and it is unlikely
that further refinements in the mode} will increase the precision of
these doses substantially. It warrants noting, too, that although the
uncertainties in some of the components of the mode! have been
accounted for, this is not true for all of them, and the total
uncertainty is likely to be greater than has been publicly stated.
This subcommittee stresses again that a review of the
dosimetry work conducted by the HEDR project and the HTDS is
difficult in the absence of a single dosimetry document in which
the HEDR work is summarized, the nature of the results provided
to the HTDS is clearly described (with their strengths and
weaknesses), and how the HTDS made use of the HEDR results to
estimate individual thyroid doses is described in detail. In the
absence of such a document, however, it seems that the
uncertainties in the thyroid doses were underestimated by the
HEDR project and consequently by the HTDS.
~ FROM THE NEVADA TEST SITE AND GLOBAL FALLOUT
During the 1950s and early 1960s, two other
environmental sources of i3~{ contributed to the thyroid doses
received by the populations around Hanford: the nuclear-weapons
tests carried out at the NTS, mainly in 1952, 1953, 1955, and 1957;
and the nuclear-weapons tests conducted outside the lower 48
states of the United States, which gave rise to so-called global
fallout, mainly in ~ 9 5 4, ~ 9 5 6, ~ 9 5 7, ~ 95 8, ~ 96 I, and ~ 962. if
global or NTS fallout resulted in significant thyroid doses in the
counties used in the HTDS, and if the variation in doses among
counties used in the HTDS was large for the sources of additional
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exposure, then it would be important to take the global and NTS
fallout into account in the HTDS. Only the first source was taken
into consideration by the HTDS.
For the purposes of this review, a crude assessment of
the thyroid doses resulting from global fallout near Hanford has
been prepared in order to obtain preliminary evidence as to
whether global fallout is likely to be an important confounding
variable in the analyses pertaining to Hanford fallout and thyroid
disease. Details of the assessment are provided in appendix C. The
following paragraphs present a brief summary of the methods used
and the results.
The global tests were carried out primanly in the
Pacific at Bikini and Eniwetok and in the Soviet Union at Novaya
Zemlya. The total fission yield from the tests was over ~ 50
megatons, compared with a total of about ~ megaton for the tests
cattier out at the NTS. However, fission yield is only a crude
indicator of the fallout deposition in the continental US because
about 80°/O of the debris was injected into the stratosphere, where
i3~} decayed and therefore did not contribute to human exposure.
Of the 20% of the debris injected into the troposphere, a
considerable fraction was probably deposited locally or regionally;
the remainder was deposited primarily in the latitudinal band of the
location of the test site and was mainly associated with rain. It
would be extremely difficult with atmospheric-dispersion and-
deposition models to predict with reasonable accuracy the fallout
deposition at a given site in the lower 48 states on the basis of the
yield of a particular test. The dose-assessment methods used were
based on the sparse fallout data and on the small number of IT
concentrations in milk in the Hanford area for some of the testing
period that are available.
The basic methodology used by Beck (see appendix C)
to estimate the 13~} doses near Hanford was as follows:
. For 1961 and 1962, the doses for Franklin, Adams, and
Benton counties were estimated directly from the measured ]3~{
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in milk at farms near Hanford after the milk data were
corrected for Hanford plant contributions. All three counties
are part of the same milk shed and receive similar amounts of
rain, so the milk-concentration estimates were assumed to
apply to all three counties, and a single set of dose estimates
was made for these three counties. (The doses in Yakima and
Kittitas Counties, which are in the milk-producing area, would
also be similar to those in Franklin, Adams, and Benton
counties because of the similar rainfall pattern.)
. The strontium-89 (89Sr) depositions in 1957, 195S,
1961, and 1962 were then estimated from the 89Sr depositions
at sites in the western United States and the measured monthly
rainfall in counties in the Hanford area. The ratios of the
deposition in WalIa WalIa and Stevens counties to deposition
in Franklin, Adams, and Benton counties were used to estimate
the milk concentrations in those counties in 1961 and 1962
from those measured in milk near Hanford.
The calculated deposition in 1958 relative to 1961 and
1962 was then used to estimate the relative concentrations in
milk in 1957 and 1958 in three counties.
. The 89Sr deposition in 1956 and 1954 relative to 1958
was estimated from the gummed-fiIm data, and the
concentrations in milk were assumed to vary with the estimated
deposition.
· Finally, the thyroid doses in each county were
estimated by using the conversion factors for milk
concentration to dose that were used for that county in the
National Cancer Institute study (NCT, 1997) for the estimation
of )3~{ thyroid doses resulting from tests earned out at the NTS.
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Evaluation of Dosimetric Methods and Results
83
The thyroid doses from global fallout calculated as
described above for the Hanford area are lower but of the same
order of magnitude as those from NTS fallout, as shown in table
4.~. Considering the uncertainties in both the NTS and global
fallout dose estimates, the differences between the two sets of dose
estimates are probably not statistically significant. Thyroid doses
for infants, children, and teenagers are also provided, by year of
fallout and by county, in appendix C.
Table 4.~ Comparison of average estimated 13~] doses (mGy) from
Hanford, the Nevada Test Site (NTS), and global sources for study
counties
County HTDS Global fallouta NTS fadeouts
Benton 172 3.8 12
Franklin 248 3.8 ~ ~
Adams 169 3.8 12
Walla WalIa 86 8.7 30
Fee Stevens 39 ~ I.8 14-19
Okanogan ~ ~not available
a Estimated doses for teenagers (see appendix C).
b Estimated doses for children born January I, ~ 945.
in theory, global fallout is a potential confounding variable
in the analyses of thyroid-disease risk posed by Hanford fallout.
But in reality, the degree to which global fallout could confound
the analytic results appears to be limited. The range of variation in
global fallout between the Hanford high-dose and low-dose
counties is small compared with the variation in Hanford dose (see
table 4.~. in addition, the global-fallout exposures occurred during
the teenage years and early 20s for the study population. Data from
the best studies of external radiation exposure to the thyroid
strongly indicate that the thyroid is much more sensitive to
radiation-induced cancer in childhood, especially before the age of
5 years, than in adolescence or adulthood (Ron and others, 1995~.
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Hence, the effect of the Hanford exposures on thyroid-disease risk
would be expected to be much greater per unit dose than exposures
from the NTS or global fallout. Nevertheless, since no actual
analysis of the impact of global fallout on the results has been
done, the committee recommends that the HTDS investigators do
so.
OTHER SOURCES OF RADIATION EXPOSURE
Ideally, estimates of the diagnostic and therapeutic
medical radiation exposures received by each person in the HTDS
should be factored into a dose-response assessment. The
investigators did include questions about history of substantial
medical radiation exposures in the questionnaire administered to
subjects. in a supplementary set of analyses, they evaluated
whether those exposures were confounders or effect modifiers of
the dose-response results for id versus thyroid diseases. However,
they did not state how the radiation exposures were coded for
analysis or how reliable the responses were, and this could be
problematic: a crude, dichotomous coding or unreliable reports
might adjust only partially for potential confounding. Nevertheless,
it is not very likely that medical radiation exposure would be a
confounding variable, in that the frequency and intensity of such
exposures would have to be correlated with the magnitude of the
Hanford IT doses for confounding to occur.
The thyroid doses from the medical radiation exposures
asked about would probably vary appreciably among medical
procedures and radiological practices. For example, in the 1940s
- a Or r ~-a
___-~--~- r~
and 1950s, the radiation exposure from taking a full-mouth
diagnostic x-ray picture was often several hundred milligrays to
the cheek. The machines often had poor collimation (Budowsky
and others, 1956; Nolan, 1953) (that is, there was appreciable
scatter of the radiation), and the adequacy of neck shielding was
probably variable. Hence, some persons could have received
thyroid doses from diagnostic dental procedures that exceeded
those from Hanforc! fallout.
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85
Questionnaire information on historical exposures is
usually insufficient to impute thyroid doses to individuals, so one
would prefer to have documented medical radiation doses to use in
the analyses. However, it would be impossible to retrieve records
to document many radiation exposures that occurred 2-5 decades
ago, so it would not be feasible to estimate lifetime medical
radiation exposures. In lieu of that, the investigators did list (in
table VIl]:-46 of the Draft Final Report) the reported medical
diagnostic and therapeutic radiation exposures for each thyroid-
cancer case, and this is informative.
Representative terms from entire chapter:
hedr project
