Overview

Cancer begins when normal cells begin to change and grow uncontrollably, forming a mass called a tumor. A tumor can be benign (noncancerous) or malignant (cancerous, meaning it can spread to other parts of the body).

Colorectal cancer is a disease in which normal cells in the lining of the colon or rectum begin to change, start to grow uncontrollably, and no longer die. These changes usually take years to develop; however, in some cases of hereditary disease, changes can occur within months to years. Both genetic and environmental factors can cause the changes. Initially, the cell growth appears as a benign polyp that can, over time, become a cancerous tumor. If not treated or removed, a polyp can become a potentially life-threatening cancer. Recognizing and removing precancerous polyps before they become cancer can prevent colorectal cancer.

Anatomy of the colon and rectum

The colon and rectum make up the large intestine, which plays an important role in the body's ability to process waste. The colon makes up the first five to six feet of the large intestine, and the rectum makes up the last six inches, ending at the anus.

The colon has four sections. The ascending colon is the portion of the colon that extends from a pouch called the cecum (the start of the large intestine into which the small intestine empties) on the right side of the abdomen. The transverse colon crosses the top of the abdomen. The descending colon takes waste down the left side. Finally, the sigmoid colon at the bottom takes waste a few more inches downward to the rectum.

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.

Colorectal cancer often begins in polyps, noncancerous growths that may develop on the inner wall of the colon and rectum, as people get older. There are several forms of polyps. Adenomatous polyps, or adenomas, are growths that may become cancerous and can be detected with a colonoscopy (see Risk Factors and Prevention). One way to prevent colorectal cancer is to detect and remove polyps before they become cancerous. About 10% of colon polyps are flat and hard to detect through colonoscopy, unless a dye is used to highlight them. These flat polyps have a high risk of becoming cancerous, regardless of their size.

Most colon and rectal cancers are a type of tumor called adenocarcinoma, which is cancer of the cells that line the inside tissue of the colon and rectum. This section covers specifically adenocarcinoma. Other types of cancer that occur far less frequently but can begin in the colon or rectum include carcinoid tumor, gastrointestinal stromal tumor (GIST), and lymphoma.

Find out more about basic cancer terms used in this section.