Technical Evaluation of the NASA Model for Cancer Risk to Astronauts Due to Space Radiation (2012) / Chapter Skim
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2 Review of NASA Model
2 Review of NASA Model
Pages 15-52
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From page 15... ...
It should be noted that the components of the proposed model are not, in every case, described in the 2011 NASA report. Instead they are incorporated by reference to earlier reports, such as the "BEIR VII Phase 2" report from the National Research Council's (NRC's)
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Jokipii (2010) .2 Galactic cosmic rays constitute a major part of the space radiation environment near Earth.
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SOURCE: J.R. Jokipii, The heliosphere and cosmic rays, Chapter 9 in Heliophysics -- Evolving Solar Activity and the Climates of Space and Earth (C.J.
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The simplified GCR model being used in NASA's proposed model is often used by scientists in the field to cat egorize the observations of GCRs. The modulation at Earth as a function of time is represented by one parameter, Φ, called the modulation parameter.
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The impulsive events, associated with instabilities in the solar atmosphere, are rich in heavy ions and show a sharp peak in X rays and gamma rays. These two types of events are illustrated in Figure 2.4.
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SOURCE: J Barth, NASA Goddard Space Flight Center, Figure 2-4 "Modeling Space Radiation Environments," presentation at the IEEE Nuclear and Space Radiation Effects Conference Short Labels editable Course: Applying Computer Simulation Tools to Radiation Effects Problems, July 21, 1997.
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Haggerty, Proton, helium, and electron spectra during the large solar particle events of October-November 2003, Journal of Geophysical Research 110:A09S18, 2005. Copyright 2005 American Geophysical Union.
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indicate that solar heavy ions do not contribute much to the SPE radiation hazard to astronauts. Review of the NASA SPE Model The SPE model used in NASA's proposed space radiation cancer risk assessment model (Cucinotta et al., 2011)
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These interactions are typically modeled using radiation transport codes, which can employ deterministic or Monte Carlo methods. Review of the NASA Shielding Transport Models The transport model used in NASA's proposed model is HZETRN, high charge and energy transport code (and BRYNTRN, a computational model of baryon transport)
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From page 24... ...
CANCER RISK PROJECTION MODELS Overview The cancer risk projection component of NASA's proposed model can be broadly defined as an amalgamation of approaches developed by the 2006 United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2006) and the BEIR VII committee.
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A major proposed change in NASA's proposed model is to use the "incidence-mortality" approach developed by BEIR VII (NRC, 2006) whereby risk coefficients from LSS cancer incidence data are used and then cancer mortality risks are estimated from these incidence risks.
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. In NASA's proposed model, a weight of 0.5 was used for the ERR lung cancer model.
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Dose and Dose Rate Effectiveness Factor A dose and dose rate effectiveness factor is often applied to lifetime cancer risk projections estimated using data from the Japanese atomic bomb survivors because the cancer risk per unit dose from low doses or low-doserate protracted radiation exposures may be lower than the risk per unit dose from acute higher-dose exposures. There are considerable uncertainties, however, about whether such a factor is necessary, and the approach by different committees for estimating DDREF varies.
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UNCERTAINTIES IN LOW LINEAR ENERGY TRANSFER RISK MODEL Overview One of the components in NASA's proposed model (described in Section 4 of the 2011 NASA report: Cucinotta et al., 2011) addresses risk estimates and their uncertainties associated with exposure to low linear energy transfer (LET)
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The proposed model described in the 2011 NASA report continues to depend on the NCRP (1997) Report No.
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However, in the same joint analysis, data from women given radiation therapy for acute postpartum mastitis and for hemangioma demonstrated increased breast cancer risks, but with dose responses inconsistent with the other four studies and with each other, and so the final estimate was based on a combination of the LSS and tuberculosis and enlarged-thymus studies. The 2011 NASA report states at the beginning of Section 6 (Cucinotta et al., 2011)
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The 2011 NASA report is not consist ent in its usage of the terminology Q and QF, and the report's glossary is unhelpful in this respect: it identifies both Q and QF simply as "quality factor," without indicating a distinction between them. For clarity in reviewing NASA's proposed model, the committee uses the term Q exclusively for the ICRP-NCRP-defined quality factor, and it uses QF exclusively for the space radiation quality factor as defined in the 2011 NASA report (Cucinotta et al., 2011)
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Radiation-induced instability is potentially very important in connec tion with current notions of multistep processes involved in carcinogenesis, especially regarding any dependencies on radiation quality that may apply. It would have been useful, for a balanced view on the subject, to include mention in the 2011 NASA report of numerous literature reports in which such instabilities have not been seen (e.g., Kodama et al., 2005; Whitehouse and Tawn, 2001; Dugan and Bedford, 2003)
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As mentioned above, further changes in the radiosensitivity of cells or tissues during prolonged exposure, and differences in the way that these changes may depend on radiation quality, greatly increase the uncertainties involved in use of quality factors that are based on data from single, short exposures. Hence, the 2011 NASA report (Cucinotta et al., 2011)
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. The data presented in the 2011 NASA report support NASA's rationale for replacing the use of LET as a descriptor of track structure and energy deposition events in biomolecules, cells, or tissues for particle radiation, although the use of LET is not questioned by the radiation protection community for the development of risk esti mates for terrestrial radiations.
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The committee endorses the decision that an NTE component not be included in the NASA model at this time. However, the committee points out that, although it is appreciated that the inclusion of considerations of NTEs in the 2011 NASA report was done to indicate potential future enhancements of NASA's proposed model, such an extensive discussion in the 2011 NASA report seems distracting for an element that is then not incorporated into the final version of the proposed model.
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INTEGRATION AND COMPLETENESS OF THE MODEL Background General As noted above, NASA's proposed space radiation cancer risk assessment model follows essentially the same overall methodology as that of the current NASA model, developed in 2005, which is based on recommendations by the NCRP (2000) in Report No.
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The diagram is reproduced from the 2011 NASA report describing the proposed model. NOTE: Acronyms are defined in Appendix C of the present report.
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Summing cancer risks for each tissue is proposed in NASA's proposed model as a more accurate approach to the estimating of SPE cancer risks. However, to define a summary metric of space radiation exposures under realistic space environments, which NASA calls effective dose, it is proposed to choose genderspecific tissue weights to represent that environment.
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. The tissue-specific cancer mortality risk equation in its track structure-based form for heavy-ion exposures in NASA's proposed model is described in the 2011 NASA report as �γM ��E � �� Σ0 ��T ��� ���� � ���� ��� � ���� ���T ��� ��� �ZM ��T � �E � �� � ����� �γ (2.1)
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A major proposed change in NASA's proposed model is to use the "incidence-mortality" approach developed by BEIR VII (NRC, 2006) whereby risk coefficients from LSS cancer incidence models are used, and then cancer mortality risks are estimated from these incidence risks.
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Key Element: Radiation Quality and Track Structure Risk Cross Section Model Completeness As in the current NASA model, quality factors in NASA's proposed model are applied to convert the low-dose low-LET cancer risk coefficients to risk coefficients for the variety of charged particles from space radiation. The underlying assumption is that the age- and tissue-specific risks can be simply scaled from low-LET radiation to HZE particles.
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From page 42... ...
However, as is noted in the 2011 NASA report, the uncertainties related to the quality factor are still the largest contributor to the overall uncertainty of REID. Model Improvements and Recommendations For NASA's proposed model, values for the three parameters that define QF have been selected by comparison with experimentally observed variations in biological effectiveness with radiation type for some cellular biological effects and by considering the few available data on the induction of cancer by high-LET radiation.
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The committee considers NASA's proposed model to be an appropriate tool for calculating this summary metric. It should be noted that the partly substantial changes in weighting factors that NASA uses for evaluating its effective dose in comparison to the ICRP effective dose do not lead to major differences under many space radiation environments, as illustrated in Figures 6.4 and 6.5 in the 2011 NASA report (Cucinotta et al., 2011)
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, are obtained as input to the cancer risk calculation, together with all of the components described above. The committee concludes that NASA's proposed model uses standard and well-studied methods for the specifica tion of the space radiation environment and computation of the tissue-specific particle spectra.
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, are logi cal and appropriate as applied to statistical errors, bias in cancer incidence data, dosimetry errors, transfer model weights, and DDREF for low-LET radiation, and for radiation quality factors and risk cross sections for space radiation, as well as for the physics uncertainties of space radiation. It is not clear, however, that the use of empirical Bayes estimates for the calculation of cancer risks for mul tiple organ sites, as discussed in Sections 3, 3.1, and 4.2 and in Table 4.1 of the 2011 NASA report, offers any advantage for estimating cancer mortality risk for all organ sites combined, as discussed in the following section.
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From page 46... ...
However, it is noted that to date there is very little information on RBE for non-cancer effects that is needed for risk estimates for space radiation exposures. By continuing to monitor developments in the area of potential cardiovascular effects at low doses of radia tion, NASA would have the opportunity to determine if there is a need to modify the proposed estimates of REID.
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From page 47... ...
Probabilistic Risk Assessment The performance of a probabilistic risk assessment of spaceflight cancer risk implies the consideration of a comprehensive set of radiation exposure scenarios involving an array of radiation hazards over a long period of time and an assessment of the vulnerability of a complex engineered system under a variety of threats. Although it was not the intent of NASA's proposed model to be comprehensive in terms of the risk scenarios, as the focus was limited to the health effects component of a total system risk model, it is important to recognize the limitations of the model in reference to best practices in total-system PRA.
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1997. "Modeling Space Radiation Environments," presentation at the IEEE Nuclear and Space Radiation Effects Conference Short Course: Applying Computer Simulation Tools to Radiation Effects Problems, July 21, 1997, IEEE NPSS Radiation Effects Committee.
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2010. Radiation and smoking effects on lung cancer incidence among atomic bomb survivors.
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2005. Clonally expanded T-cell populations in atomic bomb survivors do not show excess levels of chromosome instability.
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2004. Effect of recent changes in atomic bomb survivor dosimetry on cancer mortality risk estimates.
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1994. Cancer incidence in atomic bomb survivors.
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