Cancer is a disturbance of growth characterized by an excessive proliferation of tissues without apparent relation to the physiologic demands of the organ involved. Proper treatment planning and estimation of prognosis are dependent upon differentiating benign from malignant tumors. The features that distinguish a malignant from a benign tumor are:
Invasion is the hallmark of the malignancy of a tumor. Malignant tumors invade and destroy normal tissues. In contrast, benign tumors grow by expansion, are usually encapsulated, and do not invade surrounding tissues.
Malignant tumors metastasize via blood and lymphatic vessels to lymph nodes and distant sites, whereas benign tumors remain localized. Metastasis is the lethal event in the clinical course of most neoplastic diseases.
Cellular differentiation - malignant tumors tend to be anaplastic or less well differentiated than normal cells of the tissue in which they arise, whereas benign tumors usually resemble normal tissues more closely.
Malignant tumors generally grow more rapidly than benign tumors. Malignant tumors continue to grow even in the face of starvation (cachexia) of the host.
It is generally acknowledged that the development of cancer is a complex, multistep process involving stages of initiation, promotion and progression. This process is an active phenomenon induced by chemical, physical, biologic or genetic factors. This overall pattern of tumor development is known to occur in the liver, skin and urinary bladder, and is probable is several other tissues or organs including the colon, mammary gland and pancreas.
The stage of initiation that occurs first in the natural history of neoplastic development reflects a permanent and irreversible structural change in the genome of the initiated cell.
During promotion, there is expansion of the initiated cell population. The principal characteristic of promotion that distinguishes it from the stages of initiation and progression is its operational reversibility; there is reversible increase in replication of progeny of the initiated cell population, and reversible alterations of gene expression.
The stage of progression may be characterized primarily by its karyotypic instability and evolution. The resultant alterations in the structure of the genome of the neoplastic cell during this stage are related to the increased growth rate, invasiveness, metastatic potential, and biochemical changes in the malignant cell.