A cancer is an abnormal growth of cells (usually derived from a single cell). The cells have lost normal control mechanisms and thus are able to expand continuously, invade adjacent tissues, migrate to distant parts of the body, and promote the growth of new blood vessels from which the cells derive nutrients. Cancerous (malignant) cells can develop from any tissue within the body.

As cancerous cells grow and multiply, they form a mass of cancerous tissue—called a tumor—that invades and destroys normal adjacent tissues. The term tumor refers to an abnormal growth or mass. Tumors can be cancerous or noncancerous. Cancerous cells from the primary (initial) site can spread throughout the body (metastasize).

Types of Cancer

Cancerous tissues (malignancies) can be divided into those of the blood and blood-forming tissues (leukemias and lymphomas) and “solid” tumors, often termed cancer. Cancers can be carcinomas or sarcomas.

Leukemias and lymphomas are cancers of the blood and blood-forming tissues and cells of the immune system. They often harm the body by crowding out normal blood cells in the bone marrow and bloodstream, so that normal functioning cells are gradually replaced by cancerous blood cells. They expand lymph nodes, producing large masses in the armpit, groin, abdomen, or chest.

Carcinomas are cancers of epithelial cells, which are cells that cover the surface of the body, produce hormones, and make up glands. Examples of carcinomas are cancer of the skin, lung, colon, stomach, breast, prostate, and thyroid gland. Typically, carcinomas occur more often in older than in younger people.

Sarcomas are cancers of mesodermal cells, which are the cells that form muscles and connective tissue. Examples of sarcomas are leiomyosarcoma (cancer of smooth muscle that is found in the wall of digestive organs) and osteosarcoma (bone cancer). Typically, sarcomas occur more often in younger than in older people.

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Most Common Cancers in Men and Women* Group Cancer Men Prostate Lung Colon and rectum Bladder Non-Hodgkin lymphoma Women Breast Lung Colon and rectum Uterus Non-Hodgkin lymphoma * In decreasing order of frequency. The order is based on statistics from the American Cancer Society. Skin cancer is probably the most common cancer in both men and women, but only one type of skin cancer—melanoma—is required to be reported, so how common other types are is less clear. Thus, skin cancer figures are incomplete and are therefore generally excluded from statistics.

Talking About Cancer
Aggressiveness: The degree to which (or speed at which) a tumor grows and spreads. Anaplasia: A lack of differentiation. Thus, an anaplastic cancer is highly undifferentiated and usually very aggressive. Benign: Noncancerous. Thus, a benign tumor is unable to spread to adjacent tissues or to metastasize. Carcinogen: An agent that causes cancer. Carcinoma-in-situ: Cancerous cells that are still contained within the tissue where they have started to grow and that have not yet become invasive or spread to other parts of the body. Cure: Complete elimination of the cancer with the result that the specific cancer will not grow back. Differentiation: The extent to which the cancerous cells resemble normal cells—less resemblance means the cancer is less differentiated and more aggressive. Invasion: The capacity of a cancer to infiltrate and destroy surrounding tissue. Malignant: Cancerous. Metastasis: Cancerous cells that have spread to a completely new location. Neoplasm: General term for a tumor, whether cancerous or noncancerous. Recurrence (relapse): Cancerous cells return after treatment, either in the primary location or as metastases (spread). Remission: Absence of all evidence of a cancer after treatment. Survival rate: The percentage of people who survive for a given time period after treatment (for example, the 5-year survival rate is the percentage of people who survive 5 years). Tumor: An abnormal growth or mass.

Last full review/revision August 2008 by Bruce A. Chabner, MD; Elizabeth Chabner Thompson, MD, MPH