•   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.

EVALUATING THE NASA MODEL

Updating of the Current Model

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;



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