What Is Colorectal Cancer?

This content has been reviewed and approved by

Howard Burris, MD
Director, Drug Development
Sarah Cannon Cancer Center
Tennessee Oncology

Colorectal cancer is cancer that starts in either the colon or the rectum. Cancer that begins in the colon is called colon cancer and cancer that begins in the rectum is called rectal cancer.

The colon and rectum are part of the large intestine, which is the lower part of your digestive system. The colon is a muscular tube that takes up the first 4 or 5 feet of the large intestine. The rectum is the last 6 to 10 inches of the large intestine.

Colorectal cancer usually starts as a growth of noncancerous tissue known as a polyp. Polyps are formed when cells lining the colon grow, divide, and reproduce in an unhealthy, disorderly way, producing a growth.

According to the American Cancer Society, about 148,810 new cases of colon and rectum cancer (77,250 men and 71,560 women) will be diagnosed in 2008.

Colon and Rectum

The colon and rectum are part of the large intestine—a winding organ typically 5 to 6 feet long. The large intestine is the lower part of your digestive system. The digestive system processes food to collect energy and gets rid of solid waste from the body. The digestive system is also known as the gastrointestinal tract or GI tract.

After you chew and swallow food, it goes through the esophagus to the stomach. The food is partly broken down in the stomach and then goes to the small intestine, or small bowel, which is narrower than the large intestine. The small intestine continues to break down the food and absorbs most of its nutrients. The small intestine joins the colon in the lower right abdomen.

The colon is also known as the large intestine or large bowel. It is a muscular tube that takes up the first 4 or 5 feet of the large intestine. The colon absorbs water and minerals from broken down food and stores waste.

The waste that is left after this process is stool, which goes into the rectum. The rectum is the last 6 to 10 inches of the large intestine. The waste passes out of the body from the rectum through the anus.

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The colon has four sections:

1. Ascending colon - The ascending colon begins where the small bowel attaches to the colon and stretches upward on the right side of the abdomen, next to the appendix.
2. Transverse colon - The transverse colon goes across the body to the left side.
3. Descending colon - The descending colon continues downward on the left side.
4. Sigmoid colon - The sigmoid colon, which has an S-shape, joins the rectum. The rectum joins the anus, which is the opening where waste matter (stool) passes out of the body.

Colorectal cancer can begin in either the colon or the rectum. Cancer that begins in the colon is called colon cancer and cancer that begins in the rectum is called rectal cancer.

How Colorectal Cancer Starts

Colorectal cancer usually grows very slowly over several years.

Colorectal cancer usually starts as a growth of noncancerous tissue known as a polyp. Polyps are formed when cells lining the colon grow, divide, and reproduce in an unhealthy, disorderly way, producing a growth.

There are several kinds of polyps:

• Adenomatous polyps (also known as adenomas) - These polyps can become cancerous.
• Hyperplastic polyps - Most hyperplastic polyps do not become cancerous. But some doctors believe that certain hyperplastic polyps can turn into cancer. It is also possible that people with hyperplastic polyps have a greater chance of developing adenomatous polyps and cancer.
• Inflammatory polyps - Most inflammatory polyps do not become cancerous. Inflammatory polyps are swollen growths on the bowel wall. They develop in people who have certain diseases, such as ulcerative colitis or Crohn’s disease.

Once cancer begins in a polyp, it can grow into the wall of the colon or rectum. Once they are in the wall, cancer cells can grow into blood vessels or lymph vessels (tiny thin channels that carry waste and fluid away). The lymph vessels drain into nearby lymph nodes, which are bean-shaped structures that help fight infections.

After the cancer cells spread into blood or lymph vessels, they can spread (metastasize) to other parts of the body; colorectal cancer cells often spread to the liver. When cancer cells metastasize from the place where they started to another part of the body, the new tumor has the same kind of cells and the same name as the original tumor. For example, if colorectal cancer spreads to the liver, the cancer cells in the liver are colorectal cancer cells.

Types of Colorectal Cancer

Most colorectal cancers are adenocarcinomas. These cancers start in the glandular cells (which release fluids) that line the inside layer of the colon and rectum wall.

Other less common types of tumors can also develop in the colon and rectum. These include:

• Carcinoid tumors - These tumors develop in specialized cells in the intestine that produce hormones (substances that affect an organ’s function).
• Gastrointestinal stromal tumors (GISTs) - These tumors develop in specialized cells in the wall of the colon. These cells are called the "interstitial cells of Cajal." Some GISTs are benign, or noncancerous, but others are malignant, or cancerous. These cancers can occur anywhere in the gastrointestinal tract, but they are rarely found in the colon.
• Lymphomas - Lymphomas are cancers of immune system cells. The body’s immune system helps protect the body against infection. Lymphomas usually start in lymph nodes, which are tiny organs shaped like beans. They can be found throughout the body and help the body fight infection. Lymphomas sometimes start in the colon and rectum.