Studies on the multistage nature of radiation carcinogenesis

The effectiveness of ionizing radiation and ultraviolet radiation (uvr) in the induction of carcinogenesis in mice and humans is discussed. It is clear that with low-dose levels of ionizing or uvr, the number of initiation events exceeds the numbers of tumors that grow to a detectable size. Ionizing radiation, which is a complete carcinogen, appears to be a more effective initiator than an enhancer or promoter. In the case of uvr, with or without photosensitizers such as psoralens, the requirement of a relatively large number of exposures for carcinogenesis suggests that the expression of the initiated cells as frank tumors requires a number of events spread out over the time of the development of the tumor