What Is Cancer? - The American Cancer Society

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What Is Cancer?
Revised: 03/11/2008
The American Cancer Society

Cancer is the general name for a group of more than 100 diseases in which cells in a part of the body begin to grow out of control. Although there are many kinds of cancer, they all start because abnormal cells grow out of control. Untreated cancers can cause serious illness and even death.

How a normal cell becomes cancer

Normal body cells grow, divide, and die in an orderly fashion. During the early years of a person's life, normal cells divide more quickly until the person becomes an adult. After that, cells in most parts of the body divide only to replace worn-out or dying cells and to repair injuries.

Cancer cells develop because of damage to DNA. This substance is in every cell and directs all of the cell's activities. Most of the time when DNA becomes damaged, either the cell dies or is able to repair the DNA. In cancer cells, the damaged DNA is not repaired. People can inherit damaged DNA, which accounts for inherited cancers. Many times though, a personís DNA gets damaged by things in the environment, like,chemicals, viruses, tobacco smoke or too much sunlight.

How cancers differ

Cancers can begin in many different parts of the body. But, different types of cancer can act very differently. For example, lung cancer and breast cancer are very different diseases. They grow at different rates and respond to different treatments. That's why people with cancer need treatment that is aimed at their particular kind of cancer.

How cancer spreads (metastasis)

Because cancer cells keep growing and dividing, they are different from normal cells. Instead of dying, they outlive normal cells and continue to grow and make new abnormal cells.

Cancer usually forms as a tumor (a lump or mass.) Some cancers, like leukemia, do not form tumors. Instead, these cancer cells involve the blood and blood-forming organs, and circulate through other tissues where they grow.

Cancer cells often travel through the bloodstream or through the lymph system to other parts of the body where they begin to grow and replace normal tissue. This spreading process is called metastasis.

Even when cancer has spread to a different part of the body it is still named for the place in the body where it started. For example, breast cancer that has spread to the liver is metastatic breast cancer, not liver cancer. Prostate cancer that has spread to the bone is called metastatic prostate cancer, not bone cancer.

Remember that not all tumors are cancerous. Benign (non-cancerous) tumors do not spread to other parts of the body (metastasize) and are very rarely life-threatening.

How common is cancer?

Half of all men and one-third of all women in the US will develop cancer during their lifetimes. Today, millions of people are living with cancer or have had cancer. The risk of developing most types of cancer can be reduced by changes in a person's lifestyle, for example, by quitting smoking, limiting time in the sun, being physically active, and eating a better diet. The sooner a cancer is found and treated, the better the chances are for living for many years.

No matter who you are, we can help. Contact us anytime, day or night, for cancer-related information and support. Call us at 1-800-ACS-2345 or visit www.cancer.org.

Copyright 2009 © American Cancer Society, Inc.