What Is Colorectal Cancer?

Colorectal cancer is a term used to refer to cancer that starts in either the colon or the rectum. Colon cancer and rectal cancer have many features in common. They are discussed together here except for the section about treatment, where they are discussed separately.

The normal digestive system

Colon and rectal cancers begin in the digestive system, also called the GI (gastrointestinal) system (see the picture below). This is where food is processed to create energy and rid the body of solid waste matter (stool). In order to understand colorectal cancer, it helps to know some basics about the normal structure and function of the digestive system.

After food is chewed and swallowed, it travels down to the stomach. There it is partly broken down and sent to the small intestine. The word "small" refers to the width of the small intestine. In fact, the small intestine is the longest part of the digestive system -- about 20 feet.

The small intestine also breaks down the food and absorbs most of the nutrients. The small intestine leads to the large intestine (also called the large bowel or colon), a muscular tube about 5 feet long. The colon absorbs water and nutrients from the food and also serves as a storage place for waste matter. The waste matter moves from the colon into the rectum, the last 6 inches of the digestive system. From there the waste passes out of the body through the opening called the anus.

The wall of the colon and rectum has several layers of tissues. Colorectal cancer starts in the inner layer and can grow through some or all of the other layers. Knowing a little about these layers is helpful because the stage (extent of spread) of a cancer depends to a great degree on how deep the cancer goes into these layers.

Abnormal growths in the colon or rectum

Cancer that starts in these different areas may cause different symptoms. But colon cancer and rectal cancer have many things in common. In most cases, colorectal cancers develop slowly over many years. We now know that most of these cancers begin as a polyp--a growth of tissue that starts in the lining and grows into the center of the colon or rectum. This tissue may or may not be cancer. A type of polyp known as an adenoma can become cancerous. Removing a polyp early may prevent it from becoming cancer.

Over 95% of colon and rectal cancers are adenocarcinomas. These are cancers that start in the cells that line the inside of the colon and rectum. There are some other, more rare, types of tumors of the colon and rectum, but the facts given here refer only to adenocarcinomas.