Causes

Rectal cancer begins when healthy cells in the rectum develop changes (mutations) in their DNA. A cell's DNA contains the instructions that tell a cell what to do.

The changes tell the cells to grow uncontrollably and to continue living after healthy cells would die. The accumulating cells can form a tumor. With time, the cancer cells can grow to invade and destroy healthy tissue nearby. And cancerous cells can break away and travel (metastasize) to other parts of the body.

For most rectal cancers, it's not clear what causes the mutations that cause the cancer to form.

Inherited gene mutations that increase the risk of colon and rectal cancer

In some families, gene mutations passed from parents to children increase the risk of colorectal cancer. These mutations are involved in only a small percentage of rectal cancers. Some genes linked to colorectal cancer increase the risk of developing the disease, but they don't make it inevitable.

Two well-defined genetic colorectal cancer syndromes are:

• Lynch syndrome. Lynch syndrome, also known as hereditary nonpolyposis colorectal cancer (HNPCC), increases the risk of colon cancer and other cancers. People with Lynch syndrome tend to develop colon cancer before age 50.
• Familial adenomatous polyposis (FAP). FAP is a rare disorder that causes thousands of polyps in the lining of your colon and rectum. People with untreated FAP have a significantly increased risk of developing colon or rectal cancer before age 40.

Genetic testing can detect these and other, rarer inherited colorectal cancer syndromes. If you're concerned about your family history of colon cancer, talk to your doctor about whether your family history suggests you have a risk of these conditions.