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 epidemiological 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 components, 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.