Colorectal cancer is a term used to refer to cancer that starts in either the colon or the rectum (the last five inches of the colon). Often, it starts as a polyp—a slow growth of tissue that begins in the lining and grows into the center of the colon or rectum. A polyp may be benign (not cancerous) or malignant (cancerous). Removing a polyp early may prevent it from becoming cancerous.
Over 95% of colon and rectal cancers are adenocarcinomas that begin in the cells that line the inside of the colon and rectum. The other 5% of tumors are much rarer.