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What Is Cancer?

  • Updated: 03/07/2014

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Defining Cancer

Cancer is a term used for diseases in which abnormal cells divide without control and are able to invade other tissues. Cancer cells can spread to other parts of the body through the blood and lymph systems.

Cancer is not just one disease but many diseases. There are more than 100 different types of cancer. Most cancers are named for the organ or type of cell in which they start - for example, cancer that begins in the colon is called colon cancer; cancer that begins in melanocytes of the skin is called melanoma.

Cancer types can be grouped into broader categories. The main categories of cancer include:

  • Carcinoma - cancer that begins in the skin or in tissues that line or cover internal organs. There are a number of subtypes of carcinoma, including adenocarcinoma, basal cell carcinoma, squamous cell carcinoma, and transitional cell carcinoma.
  • Sarcoma - cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue.
  • Leukemia - cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of abnormal blood cells to be produced and enter the blood.
  • Lymphoma and myeloma - cancers that begin in the cells of the immune system.
  • Central nervous system cancers - cancers that begin in the tissues of the brain and spinal cord.

(For definitions of other cancer-related terms, see NCI's Dictionary of Cancer Terms.)

To find out more about a specific type of cancer, see the A to Z List of Cancers or the list of Cancers by Body Location/System.

Origins of Cancer

All cancers begin in cells, the body's basic unit of life. To understand cancer, it's helpful to know what happens when normal cells become cancer cells.

The body is made up of many types of cells. These cells grow and divide in a controlled way to produce more cells as they are needed to keep the body healthy. When cells become old or damaged, they die and are replaced with new cells.

However, sometimes this orderly process goes wrong. The genetic material (DNA) of a cell can become damaged or changed, producing mutations that affect normal cell growth and division. When this happens, cells do not die when they should and new cells form when the body does not need them. The extra cells may form a mass of tissue called a tumor.

Image titled Loss of Normal Growth Control. The image shows normal cell division and normal cell suicide or apoptosis of a damaged cell. It also shows cancer cell division, through several mutation stages, ending in uncontrolled growth.

(Image from Understanding Cancer Series: Cancer.)

Not all tumors are cancerous; tumors can be benign or malignant.

  • Benign tumors aren't cancerous. They can often be removed, and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body.
  • Malignant tumors are cancerous. Cells in these tumors can invade nearby tissues and spread to other parts of the body. The spread of cancer from one part of the body to another is called metastasis.

Some cancers do not form tumors. For example, leukemia is a cancer of the bone marrow and blood.

Cancer Statistics

A report from the nation's leading cancer organizations shows that rates of death in the United States from all cancers for men and women continued to fall between 2001 and 2010, the most recent reporting period available. (Read more about the Annual Report.)

Estimated new cases and deaths from cancer in the United States in 2014:

NCI's Cancer Stat Fact Sheets provide frequently requested cancer statistics for a number of cancer types.

Additional Information

Cancers that are diagnosed with the greatest frequency in the United States are listed below. (Read more about Common Cancer Types.)

More cancer topics:

The risk of developing many types of cancer can be reduced by practicing healthy lifestyle habits, such as eating a healthy diet, getting regular exercise, and not smoking. Also, the sooner a cancer is found and treatment begins, the better the chances are that the treatment will be successful.

Contact Us for Help

NCI cancer information specialists can answer your questions about cancer and help you with quitting smoking. They can also help you with using this website and can tell you about NCI's printed and electronic materials. Contact us.

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