CANCER Cancer is the second leading cause of death in the general population in the United States. It is estimated that about 25 percent of the U.S. population will eventually develop cancer (not including non- melonoma skin cancer) and that about 16.5 percent of the population will eventually die of cancer. Cancer is expected to be the most important delayed somatic effect in workers exposed to radiation in the space environment. Radiation causes an increase in the cancer risk after whole-body exposure to low LET radiation at dose equivalents greater than 50 to 100 rem. At lower levels, it is difficult or impossible to demonstrate an increased risk even from epidemiological surveys of large populations exposed to low- level radiation. Risk estimates of possible cancer-induction at low- dose levels must necessarily depend more on the dose-response models and on the projection models used in the risk coefficient calculations than on the epidemiological data themselves. The risk of cancer from low-dose, low-LET whole body radiation is a problem of detection of statistically increased incidence of the disease over the normal expectation in the population in the absence of any additional radiation exposure. Radiation-induced cancer may, as at least an initial step, involve random changes in DNA. Hence, late effects occur only on a chance basis in the relatively rare individual (i.e., almost all survivors of hign-dose exposure will live out their normal life span). For example, in the 1974 ABCC study in Hiroshima and Nagasaki of the 70,000 deaths from all causes among 285,000 Japanese atomic Domb survivors exposed at all dose levels, it is estimated that approximately 400 to 500 persons, or 0.7 percent, nad died from radiation-induced cancer (Finch, 1979). Cancers induced by radiation are indistinguishable from their normally-occurring counterparts; cause and effect cannot be established with certainty on an individual basis. This, together with the low overall induced cancer incidence rate even at high doses, make it difficult or impossible to detect directly or define the carcinogenic effects of low dose equivalents of radiation (in the range of a few rem or less). Although the cancer risk may be increased at the time of irradiation, a cancer induced does not begin to appear until after a minimal latent period has passed. The duration (or plateau) of the apparent radiation-induced increase in incidence of solid cancers (i.e., cancers other than leukemia), which tend to have long latent periods, are not
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