What Is Colorectal Cancer?
Colorectal cancer is a term used to refer to cancer that starts in the colon or rectum. Colon and rectal cancers begin in the digestive system, also called the GI (gastrointestinal) system. This is where food is processed to create energy and rid the body of solid waste matter (stool).
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. The small intestine is really the longest part of the digestive system. It is about 20 feet long.
The small intestine continues breaking down the food and absorbs most of the nutrients. The small intestine joins the large intestine (also called the large bowel or colon), a muscular tube about five feet long. The first part of 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 colon has 4 sections, as shown in the picture above. Cancer can start in any of the 4 sections or in the rectum. The wall of each of these sections (and rectum) has several layers of tissues. 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 which of these layers it affects.
Cancer that starts in the different areas may cause different symptoms. But colon cancer and rectal cancer, together known as colorectal cancer, have many features in common. They will be discussed together in this document except for the section about treatment. There they will each be discussed separately.
In most cases, colorectal cancers develop slowly over a period of several 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. A type of polyp known as an adenoma can become cancerous. Removing the polyp early may prevent it from becoming cancer.
Over 95% of colon and rectal cancers are adenocarcinomas. These are cancers of 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.