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Summary
T
he U.S. Nuclear Regulatory Commission (USNRC) requested that
the National Academy of Sciences (NAS) provide an assessment of
cancer risks in populations near USNRC-licensed nuclear facilities
that utilize or process uranium for the production of electricity (see Sidebar
1.1 in Chapter 1 for the complete statement of task). These facilities pres-
ently include 104 operating nuclear reactors at 65 sites in 31 states and 13
fuel-cycle facilities in operation in 10 states. The operating fuel-cycle facili-
ties include four in situ uranium recovery facilities, one conventional ura-
nium mill, one conversion facility, two uranium enrichment facilities, and
five fuel fabrication facilities (see Sidebar 1.2 in Chapter 1 for a description
of these facilities). There are additional state-licensed conventional uranium
milling facilities and in situ leaching facilities.
This USNRC-requested assessment is being carried out in two consecu-
tive phases. The focus of the Phase 1 scoping study, which is the subject of
this report, is to identify scientifically sound approaches for carrying out an
assessment of cancer risks associated with living near a nuclear facility. The
results of this Phase 1 study will be used to inform the design of the cancer
risk assessment, which will be carried out in Phase 2. This report provides
the committee’s judgments about the strengths and weaknesses of various
study approaches; these approaches differ in their broadness of approach,
anticipated statistical power, ability to assess potential confounding factors,
possible biases, and required effort.
Three findings and three recommendations emerged from this study.
These are presented and discussed below. Additional supporting informa-
tion can be found in the report.
1
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2 ANALYSIS OF CANCER RISKS
FINDING 1: There are several challenges for carrying out epidemio-
logic studies of cancer risks in populations near U.S. Nuclear Regulatory
Commission-licensed nuclear facilities in the United States, including the
following:
• Uneven availability and quality of data on cancer mortality and
incidence at geographic levels smaller than a county. Cancer mor-
tality and incidence are tracked by individual states, and the avail-
ability and quality of data varies from state to state. In general,
cancer mortality data are available electronically from about 1970,
but subject address at time of death is not captured until much later
in some states. (In the absence of subject address at time of death,
mortality data cannot be geocoded at levels of geographic interest
for an epidemiologic study, such as census tracts.) Cancer incidence
data of known quality are generally available from about 1995,
although such data are available for earlier times in some states.
These data include address at time of diagnosis and have been
widely geocoded, although there are residual problems associated
with post office boxes and rural delivery addresses.
• Uneven availability and quality of data on nuclear facility effluent
releases. Effluent release data may not be available and data qual-
ity may be poor for some nuclear facilities. Effluent releases from
many nuclear facilities were much higher in the past and their ra-
dionuclide compositions have changed over time. Uncertainties in
dose estimates may be much higher in years when effluent releases
were highest.
• Inability to reliably capture information on population mobility,
risk factors, and potential confounding factors. There is no cen-
tralized source of information on residential histories or lifestyle
characteristics of individuals who live in the United States. The
U.S. Census provides decadal snapshots of some population char-
acteristics, including population size and distribution with respect
to age, race/ethnicity, gender, educational level, and income. How-
ever, data on population lifestyle risk factors, including exposure
to cigarette smoking and access to healthcare, are limited to state-
level health surveys and are not consistently available from state
to state at the same level of resolution. Moreover, populations near
nuclear facilities receive radiation doses from multiple sources that
are unrelated to facility effluent releases, for example, doses from
natural background radiation and medical radiation. There may be
other risk factors and potential confounding factors, for example,
exposures to toxic chemicals and unidentified lifestyle factors, that
can influence cancer risks.
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3
SUMMARY
• Low expected statistical power. Doses resulting from monitored
and reported radioactive effluent releases from nuclear facilities
are expected to be low. As a consequence, epidemiologic studies
of cancer risk in populations near nuclear facilities may not have
adequate statistical power to detect the presumed small increases
in cancer risks arising from these monitored and reported releases.
The committee paid close attention to these challenges as it assessed the
scientific merit of various epidemiologic study designs.
FINDING 2: An assessment of cancer risks in populations near nuclear
facilities could be carried out using several study designs. Each design has
strengths and limitations for estimating cancer risks.
• Risk-projection models estimate cancer risks by combining popula-
tion radiation dose and/or dose surrogate (e.g., distance and direc-
tion from a nuclear facility) estimates with risk coefficients derived
from epidemiologic studies of other exposed populations, for ex-
ample, Japanese atomic bombing survivors. Risk-projection mod-
els can be used to estimate population-based cancer risks for any
facility type, population size, and time period. However, because
risk estimates are based on extrapolations from other epidemio-
logic studies and not on actual cancer incidence and/or mortality
rates in populations near nuclear facilities, risk-projection models
cannot be used to assess whether any predicted excess cancer risks
correspond to observed patterns of cancer incidence or mortality.
• Ecologic studies estimate cancer risks by comparing observed can-
cer incidence and/or mortality rates in populations, considered as a
group rather than as individuals, as a function of average radiation
doses and/or dose surrogates for those populations. This design al-
lows for the study of multiple cancer types during past and recent
times, which helps to improve statistical power and provides a
comprehensive picture of cancer risks. However, ecologic studies
involve a large number of comparisons among population age
groups, nuclear facilities, years of operation, and cancer types.
This can lead to false associations resulting from chance alone.
Moreover, ecologic studies can account only for population char-
acteristics and potential confounding factors using group averages
that are available from the decennial census and from survey infor-
mation that can be linked to the census data (such as the American
Community Survey). Individual characteristics can diverge sharply
from group averages.
• Cohort studies estimate cancer risks by following individuals for a
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4 ANALYSIS OF CANCER RISKS
specified period of time to determine the rate or risk of cancer as a
function of doses and/or dose surrogates. In a prospective cohort
study, subjects are followed from the present to a future time; in
a retrospective cohort study, subjects are followed from a past
time to a more recent time, usually via records. Prospective cohort
studies can in principle provide the least-biased estimates of asso-
ciations of multiple cancer types and radiation doses and/or dose
surrogates compared to studies that rely on retrospective collection
of information, such as case-control studies (described below) or
retrospective cohorts. However, prospective cohort studies need to
follow subjects for long time periods and could therefore require
decades to complete. Retrospective cohort studies are more efficient
than prospective studies because the follow-up period has already
occurred. However, such studies rely on linkages such as those
between birth certificates and state cancer registries; logistical and
administrative barriers to such linkages could limit the feasibility of
this study design in some states. Moreover, in- and out-migration
issues need to be considered.
• Case-control studies estimate cancer risks by comparing radiation
dose and or dose surrogates between individuals selected because
they have (cases) or do not have (controls) cancer. The individuals
under study and cancer outcomes of interest must be predefined
and for practical reasons would be limited to one or a few cancer
types (for example, pediatric cancers). A challenge in case-control
studies is to select suitable controls in a way that does not bias the
study results.
In the absence of information on residential history, most studies by
necessity make assumptions about relevant exposures based on information
about location of residence at one time point in the lifetime of the study
cases, such as place of residence at time of birth or place of residence at time
of diagnosis or death, with the equivalent time for controls. This single time
point of place of residence may not be the most relevant regarding exposure
from the nuclear facilities.
Studies that are based on individuals, such as cohort and case-control
studies, can potentially provide stronger evidence for or against an associa-
tion between radiation exposure and cancer compared to an ecologic study
that is based on groups of individuals (i.e., populations). However, such
studies are likely to involve fewer cancer cases than an ecologic study due
to the effort involved in subject selection and individual data collection.
The effort involved in conducting a cohort or a case-control study could
be reduced by partnering with existing multistate cancer studies that have
already linked cancer and birth registration data.
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5
SUMMARY
Case-control studies can involve contacting subjects to collect residen-
tial history and lifestyle information through interviews and questionnaires.
Such studies would need to be limited to recently diagnosed cancer cases
(i.e., diagnoses made during the past 5 years) and would likely be subject to
additional selection and information biases. There are added difficulties in
obtaining appropriate approvals from the cancer registries before subjects
could be contacted. However, such studies can also be carried out without
subject contacts by using information from birth and other administrative
records.
FINDING 3: Effluent release, direct exposure, and meteorology data, if
available, can be used to obtain rough estimates of annual variations in
dose as a function of distance and direction from nuclear facilities.
Effluent release and direct exposure data collected by facility licensees
are likely to be sufficiently accurate to develop a population-level dose
reconstruction that provides rough estimates in annual variations in dose
as a function of distance and direction from nuclear facilities. However,
such data would not be sufficient to support detailed reconstructions of
doses to specific individuals living near nuclear facilities. However, it will
be necessary to develop a methodology for estimating releases of carbon-14
prior to 2010 to support dose estimation (carbon-14 may be a significant
contributor to dose from nuclear plant releases, especially in recent years).
Moreover, facility-specific evaluations will be required to determine the
quality and availability of effluent release and meteorology data as well
as meteorology data for batch releases. Obtaining and digitizing effluent
release and meteorology data for use in an epidemiologic study will be a
large and costly effort.
Environmental monitoring data have limited usefulness for estimating
absorbed doses from effluent releases around nuclear plants and fuel-cycle
facilities. Almost all environmental measurements reported by facilities are
either below the minimum detection limits or are not sensitive enough to
allow for the development of useful dose estimates.
Computer models have been developed to estimate absorbed doses
resulting from airborne and waterborne radioactive effluent releases. These
models combine information on effluent release timing and magnitude,
transport of the released effluents through the environment, and the expo-
sure of individuals to radiation from these releases to estimate absorbed
doses. Such models could be used to obtain rough estimates of doses to
support an epidemiologic study. An existing model could be adapted for
this purpose or a new model could be developed. Regardless of the ap-
proach used, it is essential that the model reflect modern practices for dose
reconstruction, including approaches for estimating uncertainties.
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6 ANALYSIS OF CANCER RISKS
Absorbed doses near nuclear facilities are anticipated to be low, in
most cases well below variations in levels of natural background radiation
in the vicinity of individual facilities. Absorbed doses are also anticipated
to be below levels of radiation received by some members of the public
from medical procedures and air travel. Consequently, dose estimates used
in an epidemiologic study would ideally account for these other sources of
radiation exposures and possibly for other risk factors such as exposure to
hazardous (and potentially carcinogenic) materials released from nearby
industrial facilities.
RECOMMENDATION 1: Should the U.S. Nuclear Regulatory Commis-
sion decide to proceed with an epidemiologic study of cancer risks in
populations near nuclear facilities, the committee recommends that this
investigation be carried out by conducting the following two studies, sub-
ject to the feasibility assessment described in Recommendation 2: (1) an
ecologic study of multiple cancer types of populations living near nuclear
facilities and (2) a record-linkage-based case-control study of cancers in
children born near nuclear facilities.
Brief descriptions of these recommended studies are provided below. A
list of strengths and weaknesses of the recommended studies and additional
details on the study designs can be found in Chapter 4.
The ecologic study should assess cancer incidence and mortality in
populations within approximately 50 kilometers (30 miles) of nuclear facili-
ties for the operational histories of those facilities to the extent allowed by
available data. A study zone of this size would incorporate both the most
potentially exposed as well as essentially unexposed regions to be used for
comparison purposes. The study should examine all relatively common
cancer types by age interval and gender, including cancers that are not con-
sidered to have a radiogenic origin (presumed nonradiogenic cancers such
as prostate cancer can serve as useful negative controls) and also take into
account temporal changes in estimated radiation dose. A subanalysis should
specifically be carried out for highly radiogenic cancers such as leukemia
in children. The study should examine associations between (i) cancer and
distance and direction from the nuclear facility and (ii) cancer and estimated
radiation dose, both at the census-tract level. The committee recommends
that absorbed doses to individual organs be estimated using the methodol-
ogy outlined in Chapter 3.
The record-linkage-based case-control study should assess the associa-
tion of childhood cancers (diagnosed at younger than 15 years of age) in
relation to maternal residential proximity at the time of birth of the child,
among those whose address at time of delivery was within a 50-kilometer
radius of a nuclear facility. The study period for individual facilities should
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7
SUMMARY
be based on the quality and availability of cancer registration in each state.
Controls born within the same 50-kilometer radius as the cases should be
selected from birth records to match cases on birth year at a minimum.
Absorbed doses and/or dose surrogates should be based on address of the
mother’s place of residence at time of delivery, as determined from birth
records.
These recommended studies are complementary: The ecologic study
would provide a broad investigation of both cancer incidence and mortality
over the operational histories of nuclear facilities to the extent allowed by
available data. The analysis will be based on place of residence at time of
cancer diagnosis or at time of death from cancer. The committee’s recom-
mended approach for carrying out this study would improve on the 1990
National Cancer Institute survey1 (these improvements are described in
Chapter 4). The record-linkage-based case-control study of childhood can-
cers would attempt to provide a more focused assessment of the association
of these cancers in relation to early life exposure to radiation during more
recent operating periods of nuclear facilities. An analysis based on maternal
residence at time of delivery of the child may be considered more appropri-
ate for capturing relevant exposures.
The committee has recommended these two studies based primarily on
scientific merit, feasibility, and utility for addressing public concerns about
cancer risks. However, the decision about whether to carry out one or both
of these studies is the responsibility of the USNRC. In making this decision,
the Commission will consider a number of factors, some of which are out-
side the charge for this Phase 1 study such as cost and priority of addressing
public concerns about cancer risks near Commission-licensed nuclear facili-
ties versus other agency priorities. As noted in this summary and discussed
in detail in Chapter 4, the statistical power of epidemiologic studies of
cancer risks in populations near nuclear facilities is likely to be low based
on currently reported effluent releases from those facilities. Moreover, the
magnitude of the variation of other risk factors that may not be measurable
such as smoking or exposure to medical radiation may surpass the expected
effect from the releases of the nuclear facilities and therefore overwhelm the
actual effect attributed to the releases. Nevertheless, there may be sound
policy reasons for proceeding with these studies: They can help to address
public concerns about cancer risks and also demonstrate the USNRC’s com-
mitment to working constructively with its stakeholders.
1 Jablon,S., Z. Hrubec, J.D. Boice, Jr., and B.J. Stone (1990). Cancer in populations living
near nuclear facilities, Volumes 1-3, NIH Publication No. 90-874; Jablon, S., Z. Hrubec, et al.
(1991). Cancer in populations living near nuclear facilities. A survey of mortality nationwide
and incidence in two states. JAMA 265(11):1403-1408.
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8 ANALYSIS OF CANCER RISKS
RECOMMENDATION 2: A pilot study should be carried out to assess the
feasibility of the committee-recommended dose assessment and epidemio-
logic studies and to estimate the required time and resources.
Additional work beyond the scope of this Phase 1 study will be re-
quired to assess the feasibility of these recommended studies and to estimate
the time and resources needed to carry them out. The recommended pilot
study is designed to develop this information. The pilot study should focus
on the four activities described below. Additional details can be found in
Chapters 3 and 4.
• Obtain effluent release and meteorology data for six nuclear plants
and one fuel-cycle facility (the committee suggests Dresden, Mill-
stone, Oyster Creek, Haddam Neck, Big Rock Point, San Onofre,
and Nuclear Fuel Services; see Chapter 2) and digitize these data
into a form that is usable for dose estimation. The pilot should also
develop a methodology for estimating releases of carbon-14 from
the six nuclear plants for all years of operations for which effluent
release data are available.
• Develop a computer model (i.e., by modifying an existing model
or developing a new model) to obtain estimates of absorbed doses
to individual organs resulting from airborne and waterborne ef-
fluent releases, and use this model to obtain dose estimates as a
function of distance (0 to 50 kilometers from the plant) and direc-
tion for each of these seven facilities. Methodologies should also
be developed to account for natural background radiation and, to
the extent feasible, other sources of radiation in the dose estimates,
especially medical radiation. An analysis should be carried out to
estimate dose uncertainties.
• Retrieve cancer incidence and mortality data at the census-tract
level within 50 kilometers of these seven facilities to assess feasibil-
ity of the recommended ecologic study.
• Confer with investigators who are conducting linkages of cancer
and birth registration data to identify eligible cases of pediatric can-
cers and matched controls to assess feasibility of the recommended
record-linkage-based case-control study. Where such linkages are
not already in place, link birth registration and cancer incidence
data to identify eligible cases of pediatric cancers and matched
controls.
RECOMMENDATION 3: The epidemiologic studies should include pro-
cesses for involving and communicating with stakeholders. A plan for
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SUMMARY
stakeholder engagement should be developed prior to the initiation of data
gathering and analysis for these studies.
Stakeholder engagement is an essential element of any risk assessment
process that addresses important public interests (see Chapter 5). Several
approaches were used in this Phase 1 study to engage with stakeholders.
The Phase 2 study can build on these Phase 1 efforts to achieve effective
collaboration with local people and officials and increase social trust and
confidence. To this end, the Phase 2 study should develop and execute an
engagement plan that includes processes to:
• Identify key stakeholders and stakeholder groups with whom en-
gagement is essential.
• Assess stakeholder concerns, perceptions, and knowledge.
• Communicate the questions that the Phase 2 study can address and
its strengths and limitations, and communicate the results from
the Phase 2 study in forms that are useful to different stakeholder
groups.
• Make the information used in the Phase 2 study publicly accessible
to the extent possible.
It is important that the plan be developed prior to the initiation of data
gathering and analysis to ensure early engagement with stakeholders in the
Phase 2 study.
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