A model for the study of the long duration pathogenesis of cancer and its relevance to therapy.

The problem of comparing human cancer, a disease of long evolution, with experimental models in short life span animals is discussed. It is suggested that malignant tumors are more common in long life span animals because of the requirement of long periods of time for tumor progression. Increased malignancy may result from the ability of tumor cells to change, coupled with the long exposure of the neoplastic cells to the selective forces of the organism and the environment. It is proposed that the dynamic nature of tumors and the time required for changes to be expressed, should be taken into account in the study of the changing biological behavior of tumor cells and their changing responses to therapy. The use of transplantable tumors which increase in malignancy during successive transfers, artificially prolongs the existence of the neoplastic cells in isogeneic hosts, permitting thereby tumor progression. Tumor progression, occurs during a period which is accessible to the therapist. This dynamic process should be of primary interest to cancer researchers and therapists.