Causes

Cancer is caused by damage (mutations) to the DNA within cells. Your DNA contains a set of instructions for your cells, telling them how to grow and divide. Normal cells often develop mutations in their DNA, but they have the ability to repair most of these mutations. Or, if they can't make the repairs, the cells often die. However, certain mutations aren't repaired, causing the cells to grow and become cancerous. Mutations also cause cancer cells to live beyond their normal cell life span. This causes the cancerous cells to accumulate.

In some cancers, accumulating cells form a tumor. But not all cancers form tumors. For example, leukemia is a cancer that involves blood, bone marrow, the lymphatic system and the spleen, but doesn't form a single mass or tumor.

The initial genetic mutation is just the beginning of the process by which cancer develops. Scientists believe you need a number of changes within a cell in order to develop cancer, including:

• An initiator to cause a genetic mutation. Sometimes you're born with this genetic mutation. Other times a genetic mutation is caused by forces within your body, such as hormones, viruses and chronic inflammation. Genetic mutations can also be caused by forces outside of your body, such as ultraviolet (UV) light from the sun or cancer-causing chemicals (carcinogens) in your environment.
• A promoter to cause rapid cell growth. Promoters take advantage of genetic mutations created by initiators. Promoters cause cells to divide more rapidly. This could lead to an accumulation of cells, such as a tumor. Promoters could be inherited, could come from inside your body or could come from outside your body.
• A progressor to cause cancer to become aggressive and spread. Without a progressor a tumor may remain benign and localized. Progressors make cancers more aggressive, more likely to invade and destroy nearby tissue, and more likely to spread to other parts of your body. Like initiators and promoters, progressors could be inherited or they could come from environmental sources.

Your genetic makeup, forces within your body, your lifestyle choices and your environment can all set the stage for cancer or help complete the process once it's started. For instance, if you've inherited a genetic mutation that predisposes you to cancer, you may be more likely than other people to develop cancer when exposed to a certain cancer-causing substance. The genetic mutation begins the cancer process, and the cancer-causing substance could play a role in further cancer development. Likewise, smokers who work with asbestos are more likely to develop lung cancer than are smokers who don't work with asbestos. That's because tobacco smoke and asbestos both play roles in cancer development.