Colon and rectal cancer develop in the digestive system, which is also called the gastrointestinal, or GI, system. The digestive system processes food for energy and rids the body of solid waste matter (fecal matter or stool).

After food is chewed and swallowed, it travels through the esophagus to the stomach. There it is partly broken down and then sent to the small intestine, also known as the small bowel. The word "small" refers to diameter of the small intestine, which is narrower than that of the large bowel. But, the small intestine is actually the longest segment of the digestive system—about 20 feet. The small intestine continues breaking down the food and absorbs most of the nutrients. The small intestine joins the large intestine or large bowel, a muscular tube about five feet long. The first and longest part of the large bowel, called the colon continues to absorb water and mineral nutrients from the food matter and serves as a storage place for waste matter. The waste matter left after this process is feces and goes into the rectum, the final six inches or so of the large bowel. From there it passes out of the body through the anus.

The colon has four sections. The first section is called the ascending colon. It begins where the small bowel attaches and extends upward on the right side of the abdomen. The second section is called the transverse colon since it goes across the body to the left side. There it joins the third section, the descending colon, which continues downward on the left side. The fourth section is known as the sigmoid colon because of its S-shape. The sigmoid colon joins the rectum, which in turn joins the anus, or the opening where waste (fecal) matter passes out of the body.

Each of these sections of the colon and rectum has several layers of tissue. Colorectal cancers start in the innermost layer and can grow through some or all of the other layers. Knowing a little about these layers is important, because the stage (extent of spread) of a colorectal cancer depends to a great degree on which of these layers it affects. For more information, refer to the section of this document on staging.

Colon cancer and rectal cancer have many features in common. Sometimes they are referred to together as colorectal cancer. They will be discussed together in this document except for the section about treatment. At that point they will each be discussed separately.

Cancer can develop in any of the four sections of the colon or in the rectum. Cancer beginning in these different areas may cause different symptoms. Some tests are better at finding cancer on the right side of the colon while others work better at finding cancer on the left side of the colon or in the rectum.

Colorectal cancers are thought to develop slowly over a period of several years. We now know that most colorectal cancers begin as a polyp, also known as adenoma. Over many years, these can slowly change into cancer. A polyp is a growth of tissue into the center of the colon or rectum. Some types of polyps (hyperplastic polyps and inflammatory polyps) are not precancerous. But, having adenomatous polyps increases your risk of developing cancer, especially if there are many polyps or if they are large.

Once a cancer forms in these polyps, instead of growing only into the center of the colon or rectum, it will also grow into the wall of these organs. Cells from the tumor can then break away and spread through the bloodstream or lymph system to other parts of the body. There, they can form "colony" tumors. This process is called metastasis.

Over 95% of colorectal cancers are adenocarcinomas. These are cancers of the glandular cells that line the inside of the colon and rectum. The information given here refers to this type of cancer. Other, less common type of tumors may also develop in the colon and rectum. Carcinoid tumors develop from hormone-producing cells of the intestine. Gastrointestinal stromal tumors develop in the connective tissue and muscle layers in the wall of the colon and rectum. Lymphomas are cancers of immune system cells that typically develop in lymph nodes but may also start in the colon and rectum or other organs.

The treatment and outlook (prognosis) for these rarer types of tumors differ from that of adenocarcinoma and are not covered in this document. A separate document of gastrointestinal (digestive system) carcinoid tumors is available from the American Cancer Society. Information on lymphomas of the digestive system is included in our document on non-Hodgkin's lymphoma.