Colorectal cancer originates in the inner lining of the colon and/or rectum and usually starts as a polyp, or growth of tissue, which can be noncancerous. These polyps can become cancerous if not removed. If the cancer begins in the rectum, the part of the lower gastrointestinal system where waste is passed from the body, it is referred to as rectal cancer. Likewise, if the colon, the first four to five feet of the large intestine, is the initial site, it is called colon cancer.