2. The pure quadratic model may be used to define the lower limits of risk from low-dose low-LET radiation. 3. For exposure to high-LET radiation, such as neutrons and heavy charged particles, the linear model is considered to be the most appropriate (NAS-BEIR, 1980). 4. For low dose and/or dose-rate exposure where reliable human data do not exist, the evidence from laboratory animal data indicates that a more accurate excess cancer risk may be smaller by a factor of perhaps two or more than that estimated using the linear model (NRC, 1975; UNSCEAR, 1972 and 1977; NCRP, 1975 and 1980). This reduction, however, would probably apply only to the low-LET fraction of the total dose. Estimation of Space Radiation Induced Carcinogenic Risk in Man Additional poorly understood factors affect the quantitative estimation of carcinogenesis at low-dose radiation. These include: the length of the latent period; the RBE for high-LET radiations, particularly for fast neutron and heavy charged-particle radiation relative to gamma and x-radiation; the special health effects of HZE particles; the period during which the radiation risk is expressed; the model used in projecting risk beyond the period of observation; the effect of dose rate or dose fractionation; and the influence of differences in the natural incidence of specific types of cancer. In addition, uncertainties are introduced by the biological risk character!sties of humans, for example, the effect of age at irradiation, the influence of any disease for which radiation was given therapeutically, and the influence of length of observation or follow-up of the study populations. The collective influence of these uncertainties limits the precision with which estimates of human cancer risk can be made for radiation exposure in the space environment. The BEIR—III Report (NAS-BEIR, 1980) is used here as the source for cancer risk estimation. The chief sources of epidemiological data used in that report are the Japanese populations exposed to whole-body irradiation in Hiroshima and Nagasaki, the patients with ankylosing spondylitis and other patients who were exposed to partial body irradiation therapeutically or to diagnostic medical x-rays and the various occupationally-exposed populations, such as uranium miners and radium dial painters. Most epidemiological data do not systematically cover the range of low to moderate radiation doses for which the Japanese atomic bomb sur/ivor data appear to be fairly reliable (NAS— RERF, 1977; UNSCEAR, 1977). Analysis in terms of dose-response, therefore, necessarily rely greatly on the Japanese data. The substantial' neutron component of dose in Hiroshima makes this data of great value in estimating cancer risk for high-LET radiation. The Nagasaki data, for which the neutron component of dose is small, are not reliable for
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