NASA’s current missions to the International Space Station (ISS) and potential future exploration missions involving extended stays by astronauts on the lunar surface, as well as the possibility of near-Earth object (NEO) or Mars missions, present challenges in protecting astronauts from radiation risks. These risks arise from a number of sources, including solar particle events (SPEs), galactic cosmic rays (GCRs), secondary radiation from surface impacts, and even the nuclear isotope power sources transported with the astronauts. The serious early and late radiation health effects potentially posed by these exposures are equally varied, ranging from early signs of radiation sickness to cancer induction. Other possible effects include central nervous system damage, cataracts, cardiovascular damage, heritable effects, impaired wound healing, and infertility. Recent research, much of which has been sponsored by NASA, has focused on understanding and quantifying the radiation health risks posed by space radiation environments. Although many aspects of the space radiation environments are now relatively well characterized, important uncertainties still exist regarding biological effects and thus regarding the level and types of risks faced by astronauts. The career dose limits for radiation exposure to astronauts are based on cancer mortality risks, and so NASA’s current (2005) model and the proposed model reviewed in this report have both been developed for estimating such risks. This report presents an evaluation of NASA’s proposed space radiation cancer risk assessment model, which is described in a 2011 NASA report (Cucinotta et al., 2011). The evaluation in the present report considers the model components, input data (for the radiation types, estimated doses, and epidemiology), and the associated uncertainties.
GENERAL CANCER RISK ESTIMATION APPROACH
The overall process for estimating cancer risks due to low linear energy transfer (LET) 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);1
• Studies of Radiation and Cancer from the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2006),
1The 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.
• 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 processes described in all of these reports are similar. The estimation of human cancer and non-cancer effects at low doses (less than 100 millisievert [mSv]) is based on the epidemiological data from atomic bomb survivors together with selected data for occupational and medical exposures. There is a continued reliance on the assumption that, at these low doses, a given increment in dose produces a directly proportionate increase in the probability of the development of cancer or heritable effects that are attributable to the radiation. This relationship is described as the linear no-threshold (LNT) model. The ICRP, for example “considers that the adoption of the LNT model combined with a judged value of a dose and dose rate effectiveness factor (DDREF) provides a prudent basis for the practical purposes of radiological protection, i.e., the management of risks from low-dose radiation exposure” (ICRP, 2007, p. 51). This is an important position because the LNT hypothesis and some of the other assumptions behind the estimation of risks are based on models and projections and not on direct scientific observation.
The ICRP, UNSCEAR, and EPA have developed cancer risk estimates that include the risks for cancer incidence (as opposed to mortality) now that the cancer incidence data described above have become quite extensively available (UNSCEAR, 2006; Preston et al., 2007). Because incidence data allow for a more accurate diagnosis than do mortality data, the use of incidence data is preferred. For the purposes of radiation protection, the general approach used by the radiation protection community is to calculate sex-specific or sex-averaged detriment-adjusted nominal risk coefficients. The calculation of these nominal risk coefficients for cancer requires the estimation of nominal risks for different organs and tissues, and the adjustment of these for dose and dose rate effectiveness factor (DDREF), lethality, and quality of life to derive a set of site-specific values of relative detriment. The relative detriment values are used to calculate tissue weighting factors to allow for differences in the sensitivity of different tissues to tumor induction (ICRP, 2007). In addition, account needs to be taken of the relative biological effectiveness (RBE) of radiations of different LET values in the derivation of risk estimates. This is of particular importance in the case of exposures to astronauts when high-LET radiations are the major source of exposure. The topic is discussed comprehensively in ICRP Publication 92 (ICRP, 2003). The ICRP, again for example, uses the calculations of detriment-adjusted risk estimates to develop nominal probability coefficients for detriment-adjusted cancer risks of 5.5 × 10−2 Sv−1 for the whole population and 4.1 × 10−2 Sv−1 for adult workers (ICRP, 2007, pp. 53, 177-194). Of importance to the present discussion, these values are intended to be applied to the whole population and not to individuals (ICRP, 2007). Considerably more detail can be found in the reports themselves (NRC, 2006; UNSCEAR, 2006; ICRP, 2007; EPA, 2011). Thus, the intent is that this introductory chapter describe a generalized approach to risk estimation and to the development of nominal risk coefficients as a starting point for the specific discussions of NASA’s proposed model, which is applicable to a specific subgroup of the population.
The basis for NASA’s current model was NCRP (2000) Report No. 132. The risk estimation model applied in NCRP Report No. 132 was developed several years ago, and the approaches to uncertainty assessment and the underlying epidemiological and biological data incorporated into the model have advanced over the intervening period. NASA has therefore proposed updates to its space radiation cancer risk assessment model. The recent developments important for the NASA update include the following:
• The publication of BEIR VII (NRC, 2006), the UNSCEAR (2006) studies, ICRP (2007) Publication 103, and other reports in the scientific literature have introduced new assessments of human radioepidemiology data and DDREFs;
• New research results from the NASA Space Radiation Laboratory have begun to modify the understanding of radiation quality and dose-rate effects in animal and cellular systems; and
• NASA has a revised evaluation of uncertainty factors (Cucinotta and Durante, 2009)—Chapter 4 found at the NASA site for NASA Human Research Program (HRP) Evidence at http://humanresearchroadmap.nasa.gov/evidence/).
Further, NASA determined that, because its proposed model, described in the 2011 NASA report (Cucinotta et al., 2011), is used to project the cancer risk for current ISS crews and future explorations missions, it requires independent review and validation. Thus, the NRC’s Committee for Evaluation of Space Radiation Cancer Risk Model (see Appendix B for biographical information) was established to review NASA’s proposed space radiation cancer risk assessment model. Based on this recognized need, the statement of task for the committee is broadly as follows (see Appendix A for the full statement of task):
1. The committee will evaluate proposed updates to the NASA cancer projection model taking into consideration the following:
• Current knowledge of low-LET radiation cancer epidemiology,
• Effects of tissue weighting factors, radiation weighting factors, and DDREFs used in projecting risks, and
• Current uncertainties in quality factors, DDREFs, and organ dose assessment.
This will be done taking into consideration possible qualitative differences between low LET and heavy ion biological effects to determine if the use of quality factors are appropriate or inappropriate for GCR risk assessments.
2. The committee will identify gaps in NASA’s current research strategy for reducing the uncertainties in cancer induction risks.
In NASA’s current (2005) model for projecting cancer risk for ISS crews and to support the assessments of risks and uncertainties associated with potential lunar, NEO, and Mars missions, NASA uses the overall approach recommended by the National Council on Radiation Protection and Measurements (NCRP) Report No. 132 (NCRP, 2000). Of note is the fact that the NCRP (2000) approach used cancer mortality data from life span study Report 12 (Pierce et al., 1996). The major cancer epidemiology input data for both the current and proposed NASA models are from the life span study, Report 13, on the effects of atomic bomb radiation, particularly cancer mortality (Preston et al., 2003). For its proposed model, NASA also developed an assessment of the uncertainty in the NCRP model risk coefficients that took into account errors in low-LET human radioepidemiology data, dose and dose rate effectiveness factors, radiation quality factors, and space physics. For astronaut occupational exposures, the 95 percent confidence level is used as a supplementary requirement as part of the permissible exposure limit (PEL) of a no greater than 3 percent increase in the risk of exposure-induced death (REID). REID is defined by ICRP (ICRP, 2007, p. 26) as “the difference in a cause-specific death rate for exposed and unexposed populations of a given sex and a given age at exposure, as an additional cause of death introduced into a population.” However, for the NASA REID calculations, death is considered to be cancer death. The PEL standards are approved by the NASA Chief Health and Medical Officer. A detailed description of the NCRP model, developed in response to a request from NASA, can be found in NCRP (2000) Report No. 132, Radiation Protection Guidance for Activities in Low-Earth Orbit, and so only a brief summary is presented here.
NCRP (2000) Report No. 132 continues the earlier practice of taking into account both age at first exposure to radiation in space, and gender, for estimating risks and setting limits. This is necessary because of age and gender differences in cancer risks. New data for age and gender effects were taken into account compared to the earlier NCRP (1989) Report No. 98. The authors noted that because it was considered that risks to female and male astronauts should be equivalent, the exposure limits were adjusted appropriately, resulting in lower limits for females than males. NCRP (2000, p. 11) Report No. 132 concluded: “The new recommended career dose limits
for males and females of ages 25, 35, 45 and 55 at first exposure, based on a three percent career fatal cancer risk derived from the risks in Pierce et al. (1996) are reproduced in Table 1.3 [see Table 1.1 in the present report]. As noted earlier, the revised risk estimates on which these career dose limits are based have remained essentially the same for a decade now and further revisions, hopefully, should not be necessary.” The values provided in Table 1.1 can be used to estimate the number of safe days in space, provided that information for specific mission conditions is included (e.g., space environment, shielding, age of astronaut). The proposed model has been used by NASA, for specific conditions, to estimate the number of safe days in space; these estimates are provided by NASA: see Table 6.9 in the 2011 NASA report (Cucinotta et al. 2011).
The basis for the career dose limits in NASA’s current (2005) model and in NASA’s proposed model requires some explanation because it is rather different from that used for estimating occupational dose limits for occupational exposures on Earth, although the outcome is, perhaps coincidentally, quite similar. On the one hand, using the NCRP recommendations in place in 2000 (NCRP, 1993), to limit a worker’s occupational exposure to no more than 50 mSv y−1 in any one year, with a cumulative limit (age × 10 mSv) after age 18, results in an estimated average maximum lifetime risk of fatal cancer of approximately 3 percent (NCRP, 2000). On the other hand, for NASA’s career dose limit estimate, the average lifetime risk of accidental death in occupations such as construction and agriculture is taken to be about 1.5 to 3 percent, and for significantly more dangerous occupations (e.g., test pilot) it is assessed at 10 percent or more. There are clearly some problems with using comparative risks as the basis for dose limits, particularly because there has been a very significant drop in death rates for many occupations in the United States in recent years (starting during the 1990s) (see the Bureau of Labor statistics website, http:// www.bls.gov/iif/oshcfoi1.htm). NCRP (2000) Report No. 132 concluded that this was still a reasonable approach to take at the time. That report further concluded that “the choice of a three percent career excess risk of cancer mortality [made in NCRP (1989) Report No. 98] remains reasonable and justified” (NRCP, 2000, p. 13). It was also recognized that a number of uncertainties are associated with the estimates of cancer risk, and that to obtain the range on these estimates, it is necessary to establish the quantitative value of these uncertainties—an approach taken in NCRP (2000) Report No. 132. These same model uncertainties carry over to NASA’s proposed model, reviewed in the present report.
The committee’s review of NASA’s proposed model considers each component of the revised approach and the model as a whole in the context of how it addresses its task of developing dose limits for astronauts conducting space explorations and thereby providing adequate protection against radiation-induced cancer. The model is composed of the following components: Space Radiation Environmental and Transport Models, Cancer Risk Projections for Low-LET Radiation, Uncertainties in Low-LET Risk Model Factors, Cancer Risks and Radiation Quality, and Revised NASA Risk Projections for Cancer Risks and Uncertainties. In addition, as requested by NASA in the statement of task presented above in this chapter, the committee has considered what it views as gaps in the NASA approach and has recommended research that could address these gaps.
TABLE 1.1 Ten-Year Career Limits Based on Three Percent Excess Lifetime Risk of Fatal Cancer
|Age at Exposure (y)||E (Sv)|
NOTE: Limits are expressed in effective dose (E). A 3 percent excess lifetime risk of cancer mortality has additional components associated with it: namely, the risk of heritable effects (0.6 percent) and of nonfatal cancer (also 0.6 percent) for a total detriment of 4.2 percent. These nominal risks are given in ICRP (1991) and NCRP (1993).
SOURCE: Table 1.3 in National Council on Radiation Protection and Measurements (NCRP), Radiation Protection Guidance for Activities in Low-Earth Orbit, NCRP Report No. 132, NCRP, Bethesda, Md., December 31, 2001, reprinted with permission of the National Council on Radiation Protection and Measurements, http://NCRPpublications.org. Based on data from D.A. Pierce, Y. Shimizu, D.L. Preston, M. Vaeth, and K. Mabuchi, Studies of the mortality of atomic bomb survivors, Report 12, Part I. Cancer: 1950-1990, Radiation Research 146(1):1-27, 1996.
APPROACH TO THE COMMITTEE’S EVALUATION
The committee held three closely spaced meetings during the course of the study and communicated continuously between meetings by way of telephone conferences and e-mail exchanges. In briefings to the committee, representatives of NASA’s operations support and research staff provided details of the space operations for which NASA’s proposed space radiation cancer risk assessment model, under review, was developed, together with an extended description of the model itself. From research scientists currently funded by NASA, the committee heard summaries of research pertinent to cancer risks in the space environment. This research was based on cellular and laboratory animal studies. The committee used the 2011 NASA report Space Radiation Cancer Risk Projections and Uncertainties—2010 (Cucinotta et al., 2011) as its primary source in understanding the content of NASA’s proposed model. That report was originally provided to the committee by NASA in draft form and then, at the request of the committee, in a published form that could be referenced following the second committee meeting. In order to gain a better understanding of the content of the 2011 NASA report and the details of NASA’s proposed model, the committee posed numerous clarification questions to NASA during and between meetings. The questions were intended to help the committee understand both the content of NASA’s proposed model and the supporting scientific data behind the model. Although the expedited schedule of the study precluded the committee from indefinitely continuing an iterative process of questioning, answers were provided by NASA that in most cases offered sufficient clarification for purposes of the committee’s review. However, the 2011 NASA report (Cucinotta et al. 2011) will continue to serve in the future as the principal source of explanation of and justification for the proposed model. Therefore, in addition to a discussion of possible improvements to NASA’s proposed model itself, the committee has included numerous suggestions in this report for additions and improvements to the 2011 NASA report that describes the proposed model.
As noted throughout this report, the committee also relied heavily on an extensive list of pertinent reference resources in the form of journal publications and past reports produced by groups such as the National Council on Radiation Protection and Measurements and the National Research Council. Of these, the committee paid particular attention to those reports that provided the basis for approaches utilized in NASA’s proposed model, and which were incorporated by reference into the 2011 NASA report describing the proposed model.
Cucinotta, F.S., and Durante, M. 2009. Risk of radiation carcinogenesis. Chapter 4 in NASA Human Health and Performance Risks of Space Exploration Missions (J.C. McPhee and J.B. Charles, eds.). NASA Johnson Space Center, Houston, Tex.
Cucinotta, F.A., Kim, M.-H.Y., and Chappell, L.J. 2011. Space Radiation Cancer Risk Projections and Uncertainties—2010. NASA/TP-2011-216155. NASA Johnson Space Center, Houston, Tex. July.
EPA (Environmental Protection Agency). 2011. Radiogenic Cancer Risk Models and Projections for the U.S. Population. EPA, Washington, D.C.
ICRP (International Commission on Radiological Protection). 1991. 1990 Recommendations of the ICRP. ICRP Publication 60. Pergamon Press, Elsevier Science, Oxford, U.K.
ICRP. 2003. Relative Biological Effectiveness, Quality Factor, and Radiation Weighting Factor. ICRP Publication 92. Pergamon Press, Elsevier Science, Oxford, U.K.
ICRP. 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Pergamon Press, Elsevier Science, Oxford, U.K.
NASA (National Aeronautics and Space Administration). 2005. NASA Space Flight Human System Standard. Volume 1: Crew Health. NASA Technical Standard NASA-STD-3001 (Approved 03-05-2007). NASA, Washington, D.C.
NCRP (National Council on Radiation Protection and Measurements). 1989. Guidance on Radiation Received in Space Activities. Report No. 98. Bethesda, Md.
NCRP. 1993. Risk Estimates for Radiation Protection. Report No. 115. Bethesda, Md.
NCRP. 2000. Radiation Protection Guidance for Activities in Low-Earth Orbit. Report No. 132. Bethesda, Md.
NRC (National Research Council). 2006. Health Risks from Exposure to Low Levels of Ionizing Radiation, BEIR VII Phase 2. The National Academies Press, Washington, D.C.
Pierce, D.A., Shimizu, Y., Preston, D.L., Vaeth, M., and Mabuchi, K. 1996. Studies of the mortality of atomic bomb survivors, Report 12, Part I. Cancer: 1950-1990. Radiation Research 146(1):1-27.
Preston, D.L., Shimizu, Y., Pierce, D.A., Suyama, A., and Mabuchi, K. 2003. Studies of the mortality of atomic bomb survivors. Report 13: Solid cancer and noncancer disease mortality: 1950-1997. Radiation Research 160(4):381-407.
Preston, D.L., Ron, E., Tokuoka, S., Funamoto, S., Nishi N., Soda, M., Mabuchi, K., and Kodama, K. 2007. Solid cancer incidence in atomic bomb survivors: 1958-1998. Radiation Research, 168:1-64.
UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation). 2006. Studies of Radiation and Cancer. Report to the General Assembly, with Scientific Annexes A and B. United Nations, New York.