Radiation-induced neoplastic transformation in vitro: evidence for a protective effect at low doses of low LET radiation.

The study of radiation-induced transformation in vitro has long been an experimental approach to examine mechanisms underlying radiation carcinogenesis. Even though the major concern of exposure to radiation is the risk of cancer induction at low radiation doses, most laboratory mechanistic studies have focused on high dose effects. This, coupled with the fact that epidemiologic data are rarely powerful enough to accurately discriminate this risk at doses <5 cGy, has led in recent years to an increased effort to study low dose effects using the endpoint of neoplastic transformation in vitro. Since transformation frequencies at low doses are typically low (< 10(-4)), such studies are, by necessity, large and labor intensive. However, they have yielded quantitative dose-response data, as well as insights into underlying cellular and molecular mechanisms. An interesting, and potentially important, finding is that low doses of low LET radiation can suppress neoplastic transformation in vitro to levels below that seen spontaneously. Mechanistic studies have revealed that multiple mechanisms are likely to be involved, and these include both the death of a subpopulation of cells prone to spontaneous neoplastic transformation and the induction of DNA repair. The relative contribution of these mechanisms appears to be dose-dependent. The relevance of in vitro studies to carcinogenesis in vivo is discussed.