Role of low-level ionizing radiation in multi-step carcinogenic process.

In view of our current understanding of experimental in vitro and in vivo studies, as well as of the epidemiological data, carcinogenesis is the result of many endogenous and exogenous factors. No single factor "causes" cancer. A number of extant theories of carcinogenesis and of ionizing radiation's role in the process have been reviewed. An integration of the stem cell theory, the theory of "oncogeny as partially blocked ontogeny," the initiation/promotion/progression model of carcinogenesis, the oncogene/tumor suppressor gene theory, and mutation/epigenetic theories of carcinogenesis was attempted by linking all of them with the process of intercellular communication. This integration was done by examining how extra-, intra- and inter-cellular communication might he affected by the current known facts of the types of radiation-induced biological effects, such as gene and chromosomal mutations, cell killing, including apoptosis and epigenetic alterations of gene expression. Finally, an examination of the possible role of low-level radiation in the multi-step carcinogenetic process, which might have given rise to the excess cancers attributable to radiation exposure in the survivors of the atomic bombs, was attempted.