A
tumor is a mass of new
tissue growth (neoplasm) that is unresponsive to normal controls or the organizing influence of adjacent tissues; it has no useful physiological function. This definition applies to both
malignant and
benign tumors. Malignant, or cancerous, tumors, however, are additionally defined by their invasion of local tissue and their ability to spread, or metastasize, in other parts of the body (see cancer).
A benign tumor generally grows slowly and does not spread to any other part of the body. As it grows it can press on the surrounding tissues and organs, causing distortion, obstruction of ducts and vessels, pain if it presses on nerve pathways, and destruction of surrounding tissue. However, a benign tumor can kill its host only if it interferes in some way with a critical function, as might a benign brain tumor, for example. The cells of benign
tumors closely resemble the cells of the tissue of origin and are usually encapsulated within a membrane. Types of benign tumors include warts and moles; lipomas (fat-containing tumors); and fibroids of the uterus.
A malignant tumor, by definition, always kills (unless treated) because of its invasive and metastatic characteristics. Solid tumors, which develop in the breast, colon, lung, and other organs, contain an inner core with high-pressure zones that compress and collapse blood vessels, often preventing the penetration of blood-borne
cancer treatments. The tumor grows locally by encroachment into surrounding tissues. It spreads to distant sites when malignant cells break off and move through the blood and lymphatic systems and begin to grow as new colonies. Malignant tumors are diagnosed by examination of their vascularity, shape, form of cell division, and differentiation. More than a hundred different types of malignant tumors have been identified in humans. In general, those derived from epithelial tissue are carcinomas, and those from connective tissue are sarcomas.
The factors controlling tumor growth are poorly understood, although genetics seems to play a role. Tumors in laboratory animals may be transplanted to a second host using only a single tumor cell, suggesting that only one normal cell need become cancerous for tumor growth to begin. The exact causes of tumor formation are unknown. Tumors have been experimentally induced in animals by chemical, physical, and viral agents, and by radiation and chronic irritation. For example, the ultraviolet radiation from the Sun is known to cause skin cancer. People who have been exposed to radiation at their jobs, such as atomic-industry workers, X-ray technicians in the early years of the technology, and the painters of watch faces with luminous paint, have had higher than normal incidence of cancer. Some minerals or chemicals have been linked to particular cancersÑfor example, asbestos to lung cancer. Some viruses are also thought to cause cancerÑfor example, the human papilloma virus is associated with cervical cancer.
Cancer researchers no longer believe that a single drug, or "magic bullet," will be able to cure cancer. Experts now believe that a combination of drugs will be the best method to kill tumors. One route being researched utilizes antibodies that bind to specific receptors on the tumor cell, thus inhibiting tumor growth by blocking certain tumor-growth factors from entering the cell. Tumor necrosis factor (TNF), an immune-system protein, has been found effective in cutting off the blood supply to tumors, although too much TNF causes severe side effects.
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