Causes

In general, cancer occurs when healthy cells become altered. Healthy cells grow and divide in an orderly way to keep your body functioning normally. But sometimes this growth gets out of control — cells continue dividing even when new cells aren't needed. In the colon and rectum, this exaggerated growth may cause precancerous cells to form in the lining of your intestine. Over a long period of time — spanning up to several years — some of these areas of abnormal cells may become cancerous. In later stages of the disease, colon cancer may penetrate the colon walls and spread (metastasize) to nearby lymph nodes or other organs. As with most cancers, the exact cause for colon cancer is unknown.

Precancerous growths in the colon
Precancerous cells can occur anywhere in your large intestine, the muscular tube that forms the last part of your gastrointestinal tract. The colon comprises the upper 4 to 5 feet of your large intestine, and the rectum makes up the lower 6 inches.

Precancerous growths most commonly occur as clumps of cells (polyps) that extend from the wall of the colon. Polyps can appear mushroom-shaped. Precancerous growths can also be flat or recessed into the wall of the colon (nonpolypoid lesions). Nonpolypoid lesions are more difficult to detect, but are less common.

Several types of colon polyps exist. Among the most common are:

• Adenomas. These polyps have the potential to become cancerous and are usually removed during screening tests such as flexible sigmoidoscopy or colonoscopy.
• Hyperplastic polyps. These polyps are rarely, if ever, a risk factor for colorectal cancer.
• Inflammatory polyps. These polyps may follow a bout of ulcerative colitis. Some inflammatory polyps may become cancerous, so having ulcerative colitis increases your overall risk of colon cancer.