ABSTRACT
It has long been known that cancer development includes genetic components. Our most recent understanding is that cancer is a genetically based disease, which is induced in 85-90% of cases by environmental factors (Doll and Peto 1983, Macload and Ford, 1991). Some decades ago viruses were considered to be significant external causes of cancer, but later environmental aspects were implicated (Ember 1993). Discovery of oncogenes and tumor suppressor genes has brought a breakthrough in cancer research, serving as a solid base for the step by step unfolding of the molecular events from the very first alteration to the tumor formation (Figure 4.1). Rapid advances in the field of molecular biology ensured the further development of oncogene and suppressor gene research, unfolding new aspects of carcinogenesis. Comparing molecular changes (affecting the oncogenes
and tumor suppressor genes) with the morphological changes resulted in a focus on molecular and predictive approaches to environmentally-induced carcinogenesis. Since 1941 carcinogenesis (especially chemical carcinogenesis) has been considered as a two-step process of initiation-promotion (Berenblum 1979), but since the early eighties, the five-step model has been widely accepted (Harris 1985). In this model we are able to match specific onco/suppressor changes to these stages, and the model is able to explain the whole process of cancer formation, independently from the immediate causative agent (Figure 4.1). However, this model cannot be
mechanistically applied to all cases, since there is room for individual variation.
