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OCR for page 1
Summary
At the request of NASA, the National Research Council’s (NRC’s) Committee for Evaluation of Space
Radiation Cancer Risk Model1 reviewed a number of changes that NASA proposes to make to its model for
estimating the risk of radiation-induced cancer in astronauts. The NASA model in current use was last updated
in 2005, and the proposed model would incorporate recent research directed at improving the quantification and
understanding of the health risks posed by the space radiation environment. NASA’s proposed model is defined
by the 2011 NASA report Space Radiation Cancer Risk Projections and Uncertainties—2010 (Cucinotta et al.,
2011). The committee’s evaluation is based primarily on this source, which is referred to hereafter as the 2011
NASA report, with mention of specific sections or tables cited more formally as Cucinotta et al. (2011).
The overall process for estimating cancer risks due to low linear energy transfer (LET) 2 radiation exposure
has been fully described in reports by a number of organizations. They include, more recently:
• The “BEIR VII Phase 2” report from the NRC’s Committee on Biological Effects of Ionizing Radiation
(BEIR) (NRC, 2006);3
• Studies of Radiation and Cancer from the United Nations Scientific Committee on the Effects of Atomic
Radiation (UNSCEAR, 2006),
• The 2007 Recommendations of the International Commission on Radiological Protection (ICRP), ICRP
Publication 103 (ICRP, 2007); and
• The Environmental Protection Agency’s (EPA’s) report EPA Radiogenic Cancer Risk Models and Projections
for the U.S. Population (EPA, 2011).
The approaches described in the reports from all of these expert groups are quite similar. NASA’s proposed
space radiation cancer risk assessment model calculates, as its main output, age- and gender-specific risk of
exposure-induced death (REID) for use in the estimation of mission and astronaut-specific cancer risk. The model
also calculates the associated uncertainties in REID.
1Biographicalinformation about the members of the committee is presented in Appendix B.
2SeeAppendix C, “Glossary and Acronyms,” for definitions of terms and acronyms.
3The BEIR VII Phase 2 report is the most recent in a series of reports by NRC committees dealing with ionizing radiation; these are widely
known as the BEIR reports.
1
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2 TECHNICAL EVALUATION OF THE NASA MODEL FOR CANCER RISK TO ASTRONAUTS
The general approach for estimating risk and uncertainty in the proposed model is broadly similar to that
used for the current (2005) NASA model and is based on recommendations by the National Council on Radiation
Protection and Measurements (NCRP, 2000, 2006). However, NASA’s proposed model has significant changes with
respect to the following: the integration of new findings and methods into its components by taking into account
newer epidemiological data and analyses, new radiobiological data indicating that quality factors differ for leukemia
and solid cancers, an improved method for specifying quality factors in terms of radiation track structure concepts
as opposed to the previous approach based on linear energy transfer, the development of a new solar particle event
(SPE) model, and the updates to galactic cosmic ray (GCR) and shielding transport models. The newer epidemio -
logical information includes updates to the cancer incidence rates from the life span study (LSS) of the Japanese
atomic bomb survivors (Preston et al., 2007), transferred to the U.S. population and converted to cancer mortality
rates from U.S. population statistics. In addition, the proposed model provides an alternative analysis applicable to
lifetime never-smokers (NSs). Details of the uncertainty analysis in the model have also been updated and revised.
NASA’s proposed model and associated uncertainties are complex in their formulation and as such require
a very clear and precise set of descriptions. The committee found the 2011 NASA report challenging to review
largely because of the lack of clarity in the model descriptions and derivation of the various parameters used. The
committee requested some clarifications from NASA throughout its review and was able to resolve many, but not
all, of the ambiguities in the written description.
PROPOSED MODEL—OVERALL CONCLUSION
In considering NASA’s proposed model as a whole, the committee noted that the general approach to estimating
cancer risks from exposure to low-LET radiation follows that utilized by ICRP, NCRP, EPA, and BEIR VII, and
as such is state of the art. The specific data incorporated into NASA’s proposed model are generally appropriate,
with some exceptions, noted below, relating to new data that have become available since the development of
the model or additional data sets that were already available and not selected for use by NASA. There remains a
need for development of additional data to enhance the current approach and to reduce uncertainty in the model;
specific needs have been identified by the committee. The committee has some concerns about specific model com -
ponents, particularly related to the change to an “incidence-mortality” approach for calculating mortality and to
the risk-transfer approach used by NASA. The question of the effectiveness of the combination of the several
modules into the proposed integrated model was most appropriately answered by the committee’s observing of a
live demonstration by NASA of the application of the model for assessing risk to astronauts under some selected
specific mission conditions. This demonstration showed that the model was indeed an integrated one—something
that was not immediately apparent from the rather complex descriptions provided in the 2011 NASA report. The
committee’s overall evaluation is that NASA’s proposed model represents a definite improvement over the current
one. However, the committee urges that the necessary improvements identified in the specific recommendations
provided below be incorporated before the proposed integrated model is implemented.
NASA’s proposed model is composed of a number of components or modules that separately address highly
distinct aspects of radiation risk and uncertainty. The committee assessed each of the individual components of
the model as well as the integrated model as a whole. The key results of its evaluations are summarized below.
Possible improvements to components of the model and to the integrated model are provided, together with
recommendations for addressing gaps in the model. In some cases, specific research is identified that could help
NASA address gaps and/or uncertainties in its proposed model for cancer risk projections. The specific research
identified is not necessarily a comprehensive list but is intended to include efforts that would have a significant
impact and at the same time would be feasible to undertake within the short to medium term (less than 5 years).
The recommendations provided in this Summary address those areas for which the committee perceived more
substantial gaps or issues. The model components are discussed in more detail in the main body of the report (see
Chapter 2), which contains advice in addition to the major recommendations and conclusions. It is the integrated
model that will actually be implemented by NASA, and so it is also assessed in detail in Chapter 2 of this report,
particularly with regard to the integration methodology.
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3
SUMMARY
PROPOSED MODEL—ASSESSMENT OF COMPONENTS
Tissue-Specific Particle Spectra
The committee considers that the radiation environment and shielding transport models used in NASA’s
proposed model are a major step forward compared to previous models used. This is especially the case for the
statistical solar particle event model. The current models have been developed by making extensive use of available
data and rigorous mathematical analyses. The uncertainties conservatively allocated to the space physics parameters
(i.e., environment and shielding transport models) are deemed to be adequate at this time, considering that the space
physics uncertainty is only a minor contributor to the overall cancer risk assessment. Although further research in
this area could reduce the uncertainty, the law of diminishing returns may prevail.
Given the above considerations, the committee does not recommend any specific research to improve the pro -
posed model for tissue-specific particle spectra at this time. However, in this report the committee has identified
several specific research areas that could improve the proposed environment models for tissue-specific particle
spectra, including additional statistical analysis of the radial dependence of SPE intensity and solar-cycle depen -
dence of SPE frequency and extreme events. The estimates could be further improved by adding physics-based
studies of particle transport using the current picture of the heliosphere and its magnetic fields. Particle transport
in the interplanetary medium is determined by its electric and magnetic fields. Theoretical and numerical studies
of particle trajectories would certainly result in improved transport models and smaller uncertainties in the envi -
ronmental estimates, but would involve a major effort and a change in modeling approach. NASA would need to
weigh the added value of such an approach to its model outputs.
Cancer Risk Projection Model for Low-LET Exposures
Epidemiology Data
A major change proposed in NASA’s model is to use the “incidence-mortality” approach used by BEIR VII
(NRC, 2006) for the development of a REID. For this approach, risk coefficients from LSS cancer incidence models
are converted into cancer mortality risks. A major reason for the use of the LSS cancer incidence data is that these
are likely to be more accurate with respect to diagnosis than are mortality data, which suffer from misclassification
of causes on death certificates. The approach results in considerable changes in the REID estimates, particularly in
the pattern with age at exposure, and the committee considers this to be an improvement for site-specific cancer
mortality estimation.
Recommendation: Before NASA implements its proposed major change to the “incidence-mortality”
approach, the committee recommends that NASA conduct more research into the specific patterns of the
underlying epidemiological biases that drive these changes. The committee also highlights a specific problem
with the method of estimating the mortality probability from the ratio of cancer mortality to incidence as
developed by the BEIR VII report published by the National Research Council in 2006 and proposed for use
by NASA. In response, the committee recommends that NASA consider alternative methods for improved
estimation of mortality probabilities for each cancer site. For example, as presented in its 2011 report EPA
Radiogenic Cancer Risk Models and Projections for the U.S. Population, the Environmental Protection
Agency has developed an alternative approach for breast cancer mortality estimation, and this could serve
as a suitable approach to be applied by NASA.
Transfer of Cancer Risk Estimates from the Japanese to the U.S. Population
Because underlying cancer incidence rates for some cancer sites differ greatly between the Japanese and the
U.S. populations, risk estimates based on an excess relative risk (ERR) model can give REID values very different
from those based on an excess absolute risk (EAR) model. A number of organizations and committees (ICRP, the
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4 TECHNICAL EVALUATION OF THE NASA MODEL FOR CANCER RISK TO ASTRONAUTS
National Council on Radiation Protection and Measurements [NCRP], BEIR VII) have recommended that a site-
specific weighted average of the ERR and EAR models be used. The proposed NASA approach follows BEIR VII
(NRC, 2006) in calculating a weighted average with uncertain weights and generally follows the recommended
BEIR VII weights.
Recommendation: Because there are some deviations in NASA’s proposed model from the weights recom-
mended by BEIR VII for the excess relative risk and excess absolute risk models, the committee recommends
that NASA provide additional justification for these alternative weights.
Dose and Dose Rate Effectiveness Factor
A dose and dose rate effectiveness factor (DDREF) value is applied, when appropriate, to reduce the LSS-
based cancer risk coefficients for protracted exposures. A median value of 1.75 was selected by NASA for its
proposed model, based on an assessment made by the National Institutes of Health (NIH) for a previous estimate
and its uncertainty (NIH, 2003). For its proposed model, NASA assumed that the DDREF applies only to low-
LET radiations and consequently that there is no dependence of space radiation risks on dose rate. Differences in
risks between space radiation charged particles and gamma rays at low dose rate are encompassed entirely within
the quality factor, QF, discussed below. A number of publications issued since the NIH report are relevant to this
issue, and although these were discussed in the 2011 NASA report, they were not used by NASA in its choice of
DDREF or in the associated uncertainty analysis. These studies include the Mayak workers study (Shilnikova et
al., 2003), the third analysis of the United Kingdom’s National Registry for Radiation Workers (Muirhead et al.,
2009), and the 15-country nuclear workers study (Cardis et al., 2007), together with the review of these studies
and comparison with the life span study by Jacob et al. (2009).
Conclusion: Although the proposed NASA approach for estimating a DDREF describes a number of limi -
tations in these newer epidemiological studies and in the BEIR VII DDREF methodology, the justification
given for preferring the older approach taken by the National Institutes of Health in 2003 is that it is close to
the average of various recommended values of slightly less than 2. The use of this average value is somewhat
problematic, given that the recommended values used to derive this average are not independent and thus
applying equal weights to these is not justifiable.
Recommendation: The committee agrees with the use of an uncertainty approach for estimating DDREF,
but it recommends that NASA use a central value and distribution that better accounts for the recent epide -
miological and laboratory animal data.
Risk Models for Never-Smokers
The issue of the smoking status of astronauts and the potential implications for risk projections for smoking-
related cancers are important, and it is appropriate that this should be investigated. Most astronauts are non-smokers,
which would likely lower the risk projections for astronauts compared to estimates for the general population (a
mix of never- and ever-smokers).
Recommendation: The proposed NASA approach for estimating lung cancer risks for astronauts who are
never-smokers is limited and does not consider competing risks. Thus, the committee recommends that the
NASA approach be developed further, given the important impact that it has on reducing estimated risk.
The revised approach should use survival probabilities for competing risks that are specific to never-smokers.
Further, the committee recommends that NASA make no changes at this time in the proposed model to
include other smoking-related cancers. The data are not sufficiently robust for use in the modification of
the REID estimate.
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5
SUMMARY
Uncertainties in Low-LET Cancer Risk Model and Overall Uncertainties
in Cancer Risk Projections for High-LET Exposures
The 2011 NASA report addresses risk estimates and their uncertainties associated with exposure to low-LET
radiation. Uncertainties are important because risk protection involves the use of safety factors, and NASA sets
radiation permissible exposure limits (PELs) based on the 95 percent confidence limit that takes into account the
uncertainties in risk projection models (NASA, 2005).
Uncertainty Limits and Methodology
Conclusion: Uncertainty limits on radiation-related risk reflect information about anticipated environmental
radiation dose levels and accumulated knowledge about the relationship between radiation dose and cancer
risk. For the approach used by NASA, more information, if available, might reduce statistical uncertainty
and, assuming that the new information did not increase the central risk estimate, lower the upper 95 percent
uncertainty bound criterion used by NASA to evaluate the acceptability of activity-related mortality risk.
Maximum Likelihood and Empirical Bayes Estimates
In the 2011 NASA report’s description of the proposed model, the discussion of the use of a maximum likeli -
hood estimate (MLE) and/or empirical Bayes (EB) estimate of site-specific ERR per sievert is ambiguous with
respect to the specific approach that was used in specific instances. For example, the site-specific EB estimate of
ERR per sievert for kidney cancer (0.40) would be similar to the MLE (also 0.40 for this particular organ site),
with a lower estimated standard error (0.19) compared to the MLE standard error of 0.32.
Recommendation: On the assumption that the empirical Bayes approach has been used in NASA’s proposed
model, the committee recommends that the authors ensure that the off-diagonal covariance information has
been taken into account. If the EB approach has not been used, either this fact should be stated in the text
of the 2011 NASA report (Cucinotta et al., 2011) or the references to the EB approach should be removed
from the text.
Uncertainty in the Value of the Quality Factor
The uncertainty analysis in NASA’s proposed model reveals that the value of the quality factor (QF, as defined
in NASA’s proposed model) is the largest contributor to the uncertainty of REID, introducing about a 3.4-fold
uncertainty in risk. Additional analysis by NASA (Cucinotta et al., 2011) using its proposed model finds that this
component could be reduced to a 2.8-fold uncertainty if two of the track structure parameters were constrained to
a fixed algebraic relationship to one another (such that the Z*2/β2 position of the maximum value of QF is held
fixed). In this context, the committee notes that different values of QF are used for leukemia and solid cancers
based on recent studies using animal tumor models.
Conclusion: According to NASA’s proposed model, the observation that the use of a fixed relationship
between two track structure parameters reduces the uncertainty is a potentially valuable finding that may
provide a method to reduce uncertainty in estimations of the risk of exposure-induced death. However, little
indication is given in the 2011 NASA report as to why such a fixed position might be justified or expected.
The committee suggests that further investigations into the validity and usefulness of this approach would be
worthwhile.
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6 TECHNICAL EVALUATION OF THE NASA MODEL FOR CANCER RISK TO ASTRONAUTS
Radiation Quality and Track Structure Risk Cross Section
The main parameter used to specify radiation quality is Z*2/β2, where Z* is the effective charge number of the
particle and β its speed relative to the speed of light. Z*2/β2 replaces LET used in the conventional quality factor
definition, and also by NASA in its current model. However, three additional empirical parameters ( κ, Σ0/αγ, and
m) are introduced to define the quality factor-risk relationships as a function of Z*2/β2. For NASA’s proposed
model, values for these parameters have been selected by comparison with experimentally observed variations
in relative biological effectiveness (RBE) for different types of radiation for various cellular biological effects
and for selected cancer types. While this approach is broadly appropriate for the proposed model parameters, the
committee was unable to determine from the 2011 NASA report or from inquiries how the particular parameter
values were selected.
Recommendation: The committee recommends that NASA make a detailed comparison of the relative
biological effectiveness versus Z*2/β2 dependence of the experimental data with the proposed form and
parameters of the quality factor, QF, equation in order to improve the transparency of the basis for the selection
of the proposed parameter values for the model and to provide guidance for future research to test, validate,
modify, and/or extend the parameterization. This analysis needs to include the defined selection of different
values for parameters κ and Σ0/αγ for ions of Z ≤ 4 compared to all ions of higher charge.
Conclusion: In the proposed model, different maximum values of quality factor, QF, are assumed for leuke -
mia (maximum 10) and for solid tumors (maximum 40). This is a change from the current NASA risk model.
The committee agrees that it is reasonable to make such a distinction on the basis of the limited animal and
human data available.
Effective Dose
NASA’s proposed model defines a quantity that is analogous to “effective dose” as defined by ICRP, but it
uses different gender-specific sets of normalized tissue weighting factors ( wT) to match the estimated risks to the
various tissues in representative space radiation environments. NASA proposes to use this as a summary quantity
for mission operational purposes and, in NASA’s proposed model, it is simply termed “effective dose.” Effective
dose is, strictly speaking, a quantity defined by ICRP that includes the ICRP-defined specification of numerical
values for weighting factors and sex-averaging. If considerably different tissue weighting factors and radiation
quality specifications are used and “effective dose” is evaluated without sex-averaging, it is problematic for the
resulting quantity still to be termed “effective dose,” and the unit sievert given to its numerical values.
The committee believes that the NASA description of the proposed model would be improved by the use
of terminology and notation that distinguish NASA-defined quantities (especially the quantity termed “effective
dose”) from quantities defined by ICRP.
Other Issues
Non-Cancer Effects (Tissue Reactions)
In its proposed approach to estimating the safe days in deep space, NASA has used a 3 percent REID for
fatal cancer as the limit. In its current model, NASA also considers dose limits for non-cancer effects—lens, skin,
blood-forming organs, heart, and central nervous system. For example, “career limits for the heart are intended to
limit the REID for heart disease to be below approximately 3 to 5 percent, and are expected to be largely age and
sex independent” (NASA, 2005, p. 65). It was further assumed by NASA that the limits established would restrict
mortality values for these non-cancer effects to less than the risk level for cancer mortality. The cancer and non-
cancer risks were not combined into a single REID. More recent data have led ICRP to reconsider the threshold
dose values particularly for the cardiovascular system (and cataracts) (see ICRP, 2011). It is concluded by ICRP
OCR for page 7
7
SUMMARY
(2011) that a threshold absorbed dose of 0.5 Gy should be considered for cardiovascular disease (and cataracts)
for acute and for fractionated/protracted exposures. It is appreciated by ICRP that these values have a degree of
uncertainty associated with them.
Conclusion: The revised value for the threshold dose value proposed by ICRP suggests that NASA may need
to consider how it might account for cardiovascular disease in its calculations of dose limits. However, it is
noted that to date there exists very little of the information on relative biological effectiveness for non-cancer
effects that is needed for estimates of risks posed by exposure to space radiation.
Delayed Effects
Delayed effects pertinent to the assessment of risk principally relate to observations whereby ongoing
radiation-induced genomic instability is expressed, even at long times after radiation exposure. Such effects could
have important implications for radiation protection in view of current notions of the multistep mutational pro -
cesses involved in carcinogenesis. An early induced change in subsequent and ongoing mutation rates in irradiated
somatic cells could accelerate this process.
Conclusion: There are conflicting reports on the generality of the phenomenon of radiation-induced delayed
genomic instability and some question about variation in the susceptibilities of cells from different individuals
with regard to this effect. Thus, the committee concludes that it is appropriate that genomic instability not be
incorporated into NASA’s proposed model, in agreement with the proposed NASA approach. However, the
committee considers that further investigation of the phenomenon is certainly warranted.
Non-Targeted Effects
Non-targeted effects (NTEs) largely refer to the so-called bystander effects, by which responses can be pro -
duced in an unirradiated cell as a result of the transfer of a signal from an irradiated cell. For high atomic number
and energy (HZE) radiations, doses that may be received by astronauts are very non-uniform in the sense that
some cells will be traversed by the primary particle itself, whereas other cells will not be traversed; thus, an NTE
is also a phenomenon that is of considerable interest.
Conclusion: Although the 2011 NASA report (Cucinotta et al., 2011) contains an extended discussion on non-
targeted effects and their potential impact on risk estimates, NASA appropriately chose not to include these
NTEs in its proposed model at this time. Little is known in qualitative or quantitative terms of the contribution
of these NTEs directly related to radiation-induced carcinogenesis, but the committee believes that studies to
elucidate any such relevance should be encouraged.
Qualitative Differences
It is recognized that there are qualitative differences in the nature of the initial energy depositions and hence
in initial chemical, biochemical, and biological damages from different types of ionizing radiation. Differences are
particularly great between low-LET gamma rays and the wide variety of high-LET heavy ions in space radiation.
This may lead to observed differences in responses of cells, tissues, and organisms such as differences in spectra
of mutations and chromosome aberrations, altered gene-expression patterns, and different spectra and latencies
for carcinogenesis. There is some experimental evidence for qualitative differences at each of the above levels of
biological effect. As a result, it may not be entirely appropriate to apply universal values for quality factors as quan -
titative scaling factors, based on empirical data such as RBE that assume similar underlying biological processes.
The committee notes that this is an area in which experiments quantifying types, frequencies, and latencies of
various cancers—for example, lung, colon, and breast cancer, with further study of liver cancer and leukemia—are
sorely needed for radiations of varying LET, especially for high-LET particles at low particle fluences such as
OCR for page 8
8 TECHNICAL EVALUATION OF THE NASA MODEL FOR CANCER RISK TO ASTRONAUTS
occur in space. Furthermore, the committee suggests that the tumor studies should be coupled with appropriate
mechanistic investigations to provide an understanding of the underlying carcinogenic processes.
Probabilistic Risk Assessment
The committee notes that the risk projections discussed in NASA’s proposed space radiation cancer risk
assessment model and uncertainties are not presented or intended as being based on a probabilistic risk assess -
ment (PRA) approach. NASA’s proposed model is a health-effects model intended to provide estimates of cancer
risk and uncertainties for defined space radiation exposure scenarios. More generally, however, the cancer risk to
astronauts is dependent on much more than a defined scenario model of health effects, with engineered barriers,
in the space radiation environment. Experience with full-scope PRAs of complex systems indicates the importance
of accounting for the “what can go wrong during actual operations” scenarios, as such scenarios generally drive
the overall risk. Thus, the committee suggests that comprehensive, mission-specific PRAs also be considered so
as to enable accountability for the “what can go wrong” scenarios in the overall risk projections.
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