The common name for a malignant neoplasm or tumor. Neoplasms are new growths and can be divided into benign and malignant types, although in some instances the distinction is unclear. The most important differentiating feature is that a malignant tumor will invade surrounding structures and metastasize (spread) to distant sites whereas a benign tumor will not. Other distinctions between benign and malignant growth include the following: malignancies but not benign types are composed of highly atypical cells; malignancies tend to show more rapid growth than benign neoplasms, and are composed, in part, of cells showing frequent mitotic activity; and malignant tumors tend to grow progressively without self-limitation. See also Mitosis; Tumor.
Malignant neoplasms that arise from cells of mesenchymal origin (for example, bone muscle, connective tissue) are called sarcomas. Those that develop from epithelial cells and tissues (for example, skin, mucosal membranes, and glandular tissues) are termed carcinomas. Carcinomas usually metastasize initially by way of lymphatic channels, whereas sarcomas spread to distant organs through the bloodstream.
The cause of most types of human cancers is unknown. However, a number of factors are thought to be operative in the development of some malignant neoplasms. Genetic factors are thought to be causally related to some human malignancies such as lung cancer in that the incidence of cancer among persons with a positive family history of cancer may be three times as high as in those who do not have a family history. A number of different neoplasms are known to be genetically related and may be due to damage or changes in chromosome structure.
Radiation in various forms is thought to be responsible for up to 3% of all cancers. In the United States the carcinogens in tobacco account for up to one-third of all cancer deaths in men and 5–10% in women. The increasing incidence and death rate from cancer of the lung in women is alarming, and is directly related to the increasing prevalence of cigarette smoking by women. Cigarette smoking and the heavy consumption of ethyl alcohol appear to act synergistically in the development of oral, esophageal, and gastric cancers. There are several carcinogens to which people are exposed occupationally that result in the development of cancer, although the mechanisms by which they cause neoplasms are sometimes poorly understood.
For example, arsenic is associated with lung, skin, and liver cancer and asbestos causes mesotheliomas (cancer of the pleural, peritoneal, and pericardial cavities). Certain drugs and hormones have been found to cause certain types of neoplasms. Postmenopausal women taking estrogen hormones have a much higher incidence of endometrial cancer (cancer of the lining of the uterine cavity). The role of diet and nutrition in the development of malignant tumors is controversial and still under investigation. Some epidemiologic studies have shown that certain diets, such as those high in saturated fats, are associated with an increased incidence of certain types of neoplasm, such as colon cancer. The role of viruses in the development of human cancers is being studied. See also Mutagens and carcinogens; Radiation biology; Tumor viruses.
It is generally accepted that the neoplastic condition is caused by alterations in genetic mechanisms involved in cellular differentiation. In malignant cells, normal cellular processes are bypassed due to the actions of a select group of genes called oncogenes which regulate cellular activities. A group of these highly conserved genes exist in normal cells and are called proto-oncogenes. These genes appear to be important in regulating cellular growth during embryonic development. It is thought that in carcinogenesis these proto-oncogenes become unmasked or changed during the breakage or translocation of chromosomes. These genes that were previously suppressed in the cell then become functional, and in some instances lead to the excessive production of growth factors which could be important in the neoplastic state. See also Oncogenes.
The physical changes that cancer produces in the body vary considerably, depending on the type of tumor, location, rate of growth, and whether it has metastasized. The American Cancer Society has widely publicized cancer's seven warning signals:
(1) a change in bowel or bladder habits;
(2) a sore that does not heal;
(3) unusual bleeding or discharge;
(4) a thickening or lump in the breast or elsewhere;
(5) indigestion or difficulty in swallowing;
(6) an obvious change in a wart or mole; and
(7) a nagging cough or hoarseness.
In current medical practice, most cancers are staged according to tumor size, metastases to lymph nodes, and distant metastases. This type of staging is useful in determining the most effective therapy and the prognosis.
The progression, or lack thereof, of a given cancer is highly variable and depends on the type of neoplasm and the response to treatment.
Treatment modalities include surgery, chemotherapy, radiation therapy, hormonal manipulation, and immunotherapy. In general, each type of cancer is treated very specifically, and often a combination of the various modalities is used, for example, surgery preceded or followed by radiation therapy. The response to treatment depends on the type of tumor, its size, and whether it has spread.
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