ABSTRACT
In practical cancer research the classical concepts of the malignant process are often forgotten. This applies both to the nature of the disease and its many facets. In research there is a tendency to restrict to one model system with a high degree of standardization and reproducibility. Several exogenous causes of human cancer were known during the last century, such as tobacco and cancer of the respiratory tract; aniline dyes and the occurrence of bladder cancer; and light, prolonged local heat exposure and chronic ulcers as causes of skin cancer. Invasive properties seem to be critical for this development. Invasiveness not only means that cells are able to occupy another tissue in a progressive and destructive manner, but also that they are able to migrate and break down normal tissue. Their invasive pattern can both occur as single cells, as groups of cells, or as solid tissue.
