•   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.

NASA’s Proposed Model

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



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