Biological aspects of radiation carcinogenesis

Ionizing radiation has been observed to increase the incidence of virtually every type of neoplasm, and carcinogenic effects have been noted in all species investigated. Susceptibility varies markedly, however, with species, strain, organ, cell type, sex, age at irradiation, physiological condition, exposure to other physical or chemical agents, and additional variables. Because of the multicausal, multistage nature of the cancer process, and the diversity of mechanisms through which carcinogenesis may be influenced by irradiation, the variety of observed dose-incidence relationships is not unexpected. The data are consistent with the concept that cancer arises from a single transformed cell, through a stepwise succession of undefined changes, and that the process of transformation may be initiated by radiation-induced damage to the DNA of the affected cell. There is also evidence that intercellular interactions may play an important role in the evolution of the neoplastic process, the nature and radiobiology of which remain to be determined