Tumor growth is the outcome of many variables, including cellular proliferation, the cell cycle, and cell loss from the tumor. Most tumors contain heterogenous populations of tumor cells as well as normal stromal elements, which influence tumor volume doubling times. The number of cells within the tumor actually cycling is also variable, depending upon factors such as intratumoral oxygen content. Cell loss from the tumor is influenced by tumor cell death, immunologic destruction of the tumor, and spread or metastasis of cells away from the tumor. Consequently, every tumor has its own growth rate.
Cell growth is mediated by growth factors and their receptors, as well as by hormones, neuropeptides and nutrients. Mammalian cells secrete a range of growth factors and respond to a variety of these molecules in an autocrine or paracrine manner. In normal tissues, these factors coordinate local tissue organization and proliferation regulation. In malignancy, normal cells acquire an increasingly aggressive phenotype allowing escape from normal regulatory constraints, and deviating from the genetic program characteristics of their normal differentiation state. Cancer cells escape normal growth controls through various methods including autocrine production of growth factors, or responses to aberrant mitogenic signals as a result of oncogene products disrupting normal signal transduction pathways.