What Is Cancer?

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Cancer is a group of diseases characterized by either the runaway growth of cells or the failure of cells to die normally. Often, cancer cells spread to distant parts of the body, where they can form new tumors. Cancer can arise in any organ of the body and strikes one of every two American men and one of every three American women at some point in their lives.

Each year, nearly 1.4 million new cases of cancer are diagnosed in the United States, a figure that does not include the 900,000 cases of skin cancer diagnosed annually. Cancer is the second leading cause of death (after heart disease) in the United States, accounting for 560,000 deaths every year.

There are more than 100 different varieties of cancer, which can be divided into six major categories.

Carcinomas, the most common type of cancer, originate in tissues that cover a surface or line a cavity of the body. Most types of lung cancerbreast cancer, and colon cancer are carcinomas.

Sarcomas begin in tissue that connects, supports or surrounds other tissues and organs.

Lymphomas are cancers of the lymph system, the circulatory system that bathes and cleanses the body's cells.

Leukemias involve blood-forming tissues and blood cells.

As their name indicates, brain tumors are cancers that begin in the brain, and skin cancers, including melanomas, originate in the skin.

Cancers are considered metastatic if they spread via the blood or lymphatic system to other parts of the body to form secondary tumors.

Cancer is caused by a series of mutations, or alterations, in genes that control cells' ability to grow and divide. Some mutations are inherited; others arise from environmental factors such as smoking or exposure to chemicals, radiation, or viruses that damage cells' DNA. The mutations cause cells to divide relentlessly or lose their normal ability to die.

Despite the fact that the cancer mortality rate in the U.S. has risen steadily for the past 50 years, scientific advances appear to have begun to turn the tide. 1997 was the first year in the past half century in which fewer Americans died of cancer than the year before — the start of what cancer researchers hope will be a long-term decline in cancer deaths.

Learn more about how Dana-Farber cares for adults and children with cancer.