Cancer refers to a malignant, spreading tumor and the illness that results from such a tumor. Tumor refers to any swelling, although modern usage has limited the term to swellings that involve neoplastic tissue. Neoplasm (ne'o-plazm) means "new growth" and refers to abnormal tissue growth resulting in unusually rapid cellular proliferation that continues after normal growth of the tissue has stopped or slowed considerably. A neoplasm can be either malignant (with malice or intent to cause harm), able to spread and become worse, or benign (be-nin'; kind), not inclined to spread and not likely to become worse. Although benign tumors are usually less dangerous than malignant tumors, they can cause problems. As the benign tumor enlarges, it can compress surrounding tissues and impair their functions. In some cases (for example, brain tumors), the result can be death.

Malignant tumors can spread by local growth and expansion or be metastasis (me-tas'ta-sis; moving to another place), which results from tumor cells separating from the main neoplasm and being carried by the lymphatic or circulatory system to a new site, where a second neoplasm forms. Oncology (ong-kol'o-je; tumor study) is the study of cancer and its associated problems. A carcinoma is a malignant neoplasm derived from epithelial tissue. A sarcoma is a malignant neoplasm derived from connective tissue.

Malignant neoplasms lack the normal growth control that is exhibited by most other adult tissues, and in many ways they resemble embryonic tissue. Rapid growth is one characteristics of embryonic tissue, but as the tissue begins to reach its adult size and function, it slows or stops growing completely. This cessation of growth is controlled at the individual cell level. Cancer results when a cell or group of cells, for some reason, breaks away from that control. This breaking loose involves the genetic machinery and can be induced by viruses, environmental toxins, and other causes.

The illness associated with cancer usually occurs as the tumor invades and destroys the healthy surrounding tissues, eliminating their functions.

Cancer therapy concentrates primarily on trying to confine and then kill the malignant cells. This goal is accomplished currently by killing the tissue with x-rays or lasers, by removing the tumor surgically, or by treating the patient with drugs that kill rapidly dividing cells. The major problem with current therapy is that some cancers cannot be removed completely by surgery or killed completely by x-rays and laser therapy. These treatments can also kill normal tissue adjacent to the tumor. Drugs used in cancer therapy kill not only cancer tissue but also any other rapidly growing tissue, such as bone marrow, where new blood cells are produced, and the lining of the intestinal tract. Loss of these tissues can result in anemia, caused by the lack of red blood cells, and nausea, caused by the loss of the intestinal lining.

Promising anticancer therapies are being developed in which cells responsible for immune responses can be stimulated to recognize tumor cells and destroy them. A major advantage in such anticancer treatments is that the cells of the immune system can specifically attack the tumor cells and not other, healthy tissues.

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