Anatomy of the Colon

The colon is divided into four sections:

• Ascending colon – travels vertically up the right side of the abdomen
• Transverse colon – extends across the abdomen
• Descending colon – extends vertically down the left side of the abdomen
• Sigmoid colon – S-shaped section that extends to the rectum

Normal Anatomy of the Intestines

© 2009 Nucleus Medical Art, Inc.

As we eat, bile acids from the liver and gallbladder and powerful digestive enzymes from the pancreas are secreted into the small intestine, where they break down the nutrient portions of food into forms that can be carried by the bloodstream to the liver. Here they are metabolized. What remains after these acids and enzymes do their work—water, minerals, and a watery form of digested food (called chyme)—is transported through the colon to the rectum, where it is stored as feces and eventually expelled.

Colon cancer may occur anywhere in the large intestine, but most colon cancers occur in the upper rectum and adjoining sigmoid colon.

Precancerous Changes (Polyps)

Colon cancer tends to be a slow-growing form of cancer. Before colon cancer develops, there are often changes that occur in the lining of the colon or rectum. One type of change is the development of polyps—growths that project from the lining of the colon or rectum.

Some types of polyps, such as inflammatory polyps, are benign. However, other types—such as adenomatous polyps and hyperplastic polyps—may be precancerous. The risk of polyps turning into cancer is increased if the polyps are flat, large, numerous in quantity, or have a slender, fingerlike structure, as opposed to a tubular structure.

Polyps are painless. A sign that may indicate the presence of polyps is rectal bleeding. If you notice blood in the stool or rectal bleeding, you should contact your doctor to have a colonoscopy, a test that will check for the presence of polyps. If polyps are found, your doctor will remove the polyps (polypectomy) and have them analyzed at a lab to determine if they are cancerous. (See “ ]]>Screening for Colon Cancer]]> ” for more detailed information on these tests.) Removing polyps before they become malignant is an effective way to reduce your risk of developing colon cancer.

Types of Colon Cancers

Most types (nearly 95%) of colon cancers are adenocarcinomas, cancers of the cells that line the interior of the colon and rectum. Adenocarcinomas are derived from the glandular components of the lining of the bowel. Other types of colon cancer, which are more rare, include:

• 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 – cancers of immune system cells.

Some colon cancers are caused by genetics and run in families. The two most common forms of inherited colon cancer are:

• Hereditary non-polyposis colon cancer (HNPCC) – this is a fast-growing form of colorectal cancer, and accounts for about 5% of all colorectal cancer cases. Typically, people with this form develop cancer in their 40s.
• Familial adenomatous polyposis (FAP) – people with this type of colorectal cancer develop hundreds of polyps at a very young age, sometimes as early as their teens. Initially, polyps are benign but become cancerous over time. This type of colorectal cancer is rare (1% of all colon cancer cases). But if you have these polyps, the likelihood that you will develop colon cancer is almost 100%, and many patients have most of their colon removed as a preventive measure.

Who Is Affected

Colon cancer is the second leading cause of cancer-related death in men and women in the United States, with an estimated 56,600 deaths occurring in 2002. Approximately 148,300 people will be diagnosed with colon or rectal cancer this year; 107,300 will have colon cancer and 41,000 people will develop rectal cancer.

Colorectal cancer is one of the more treatable forms of cancer, and the rate of successful treatment is quite high, especially when the cancer is treated early.

Causes

The exact cause of colon cancer is not known. Since colon cancer is more prevalent in industrialized countries, there has been speculation that colon cancer may be associated with diets that are high in fat and calories and low in fiber. However, studies have yielded conflicting results, and further research is necessary. Genetics are also believed to play a role.

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