The stability of events in the natural history of neoplasia

Previous studies on the natural history of neoplasia, utilizing mouse skin as a model, have demonstrated that the process of epidermal carcinogenesis may be separated into at least two different phases. The first of these, termed "initiation," is essentially irreversible; the second phase, that of promotion, may be modulated or reversed by a variety of environmental conditions. More recently, similar stages have been demonstrated for other organ systems during carcinogenesis, in particular that of murine liver. At the same time, investigations of a variety of systems including those in plants, amphibians, and, most recently, in mammals have demonstrated that the initiation process of neoplasia may not be as irreversible as previously considered, but in several of these systems, including those in plants and in the mouse teratoma, the neoplastic process appears to be reversible from its initial stages under appropriate conditions. A proposed scheme is presented which takes into account the reversibility of the process of initiation in the natural history of neoplasia.