The rectum is the lower six inches of the large intestine. The colon absorbs water and nutrients from food, and the rectum serves as the holding place for the stool.
The colon is divided into four sections:
Descending or left colon
Most colorectal cancers arise in the sigmoid (the portion just above the rectum) or the right colon. They usually start in the innermost layer and can grow through some or all of the several layers of tissue that make up the colon and rectum. The extent to which a cancer penetrates the various tissue layers determines the stage of the disease.
A small percentage of colorectal cancers — usually hereditary forms of the disease — can cause large numbers of polyps to appear, but these types of colorectal cancers are rare.
Colorectal cancers are usually contained within the colon, but when they become advanced the cancer can metastasize, or spread, to other organs. When colorectal cancer spreads, it tends to move to the peritoneum, liver and lungs.