It is difficult to separate epidemiological considerations of rectal cancer from those of colon cancer because epidemiological studies often consider colon and rectal cancer (i.e., colorectal cancer) together.

The rectum is a part of the digestive system, also called the gastrointestinal (GI) tract. The colon is the first 4 to 6 feet of the large intestine, also called the large bowel. The rectum is the last part of the large intestine, which ends in the anus.

Rectal cancer develops slowly and usually starts as polyps, which are overgrowths of tissue in the lining of the colon. Rectal cancer may develop within a polyp, but not all polyps contain cancer.