Overview of Colorectal Cancer

The colon and rectum are part of the digestive tract. Together, they comprise the large intestine, or large bowel, which is located in the abdomen between the small intestine and the anus. Cancer that originates in the colon or rectum may be called colon cancer, rectal cancer, or colorectal cancer. Colon cancer is the term most commonly used to refer to this type of cancer.

The colon is primarily responsible for absorbing water from food. It is about 6 feet in length and consists of the cecum (connects to the small intestine at the ileocecal valve), the ascending colon (the vertical segment located on the right side of the abdomen), the transverse colon (extends across the abdomen), the descending colon (leads vertically down the left side of the abdomen), and the sigmoid colon (extends to the rectum).

The rectum is the last segment of the large intestine. It is 8 to 10 inches in length and leads to the anus, which is the opening to the outside of the body. Waste material (fecal matter) is stored in the rectum until it is eliminated from the body through the anus.

Most (over 95 percent) colon cancers are adenocarcinomas that develop when a change (i.e., mutation) occurs in cells that line the wall of the colon or rectum. The disease often begins as an intestinal polyp, also called an adenoma, which is an abnormal growth of tissue. Polyps gradually can become precancerous and then cancerous.

Colon Cancer Click here for image enlargement.

Incidence and Prevalence of Colorectal Cancer

Incidence of colorectal cancer is highest in developed countries such as the United States and Japan, and lowest in developing countries in Africa and Asia. According to the American Cancer Society, it is the third most common type of cancer in both men and women in the United States (excluding skin cancers). Incidence is slightly higher in men than women, and is highest in African American men.

The American Cancer Society estimates that about 136,830 cases of colorectal cancer are diagnosed each year in the United States —nearly 97,000 colon cancer cases and 40,000 rectal cancer cases. In the United States, the estimated lifetime risk of developing colorectal cancer is about 1 in 20.

The death rate from colon cancer has declined over the past 15 years due to improved screening methods and advances in treatment. Colorectal cancer is the third leading cause of cancer-related death in the United States.

Publication Review By: Toomas Sorra, M.D., F.A.C.G.

Published: 14 Aug 1999

Last Modified: 22 Jul 2015