Abstract

The growth rates of solid tumors in experimental animals and in humans were found to be exponential during their observable lifetime. It was concluded that cancer develops over a long clinically undetectable phase, during which the probability of occurrence of metastases is high. Solid tumors became viewed as frequently disseminated at the time of clinical diagnosis, thus requiring a systemic therapeutic approach in addition to the traditional loco-regional approaches offered by surgery or radiation therapy. However, studies in experimental animal models in which tumor growth can be measured over a wide range of doubling times have shown that solid tumor growth rates best fit a Gompertz function, in which the tumor doubling times continuously increase with increasing tumor size. Subsequent studies of the dynamics of tumor growth revealed that tumors are composed of proliferative and non-proliferative compartments, the latter consisting of cells that are able to proliferate again and cells lacking this capacity. In addition to therapeutic implications drawn from the Gompertz and cell-compartment models, experimental tumors characterized by a Gompertzian growth were added to the in vivo systems used to screen for drug activity.