colon cancer, cancer of any part of the colon (often called the large intestine). Colon cancer is the second most common cancer diagnosed in the United States. Epidemiological evidence has shown that a diet high in fat and low in fruits, vegetables, and fiber contributes to the development of the disease. Smoking is also a factor in some types of colon cancer. Statistically, a family history of colon cancer or cancer of the female reproductive organs, a history of colon polyps, or a history of ulcerative colitis puts one at a greater risk of developing colon cancer. Colon cancer is most common in people over age 50.

Several genes that signal a hereditary predisposition to colon cancer have been identified. For example, mutations in either of two genes, MSH2 and MLH1, can predispose a person to hereditary nonpolyposis colorectal cancer (HNPCC). People in HNPCC families can undergo blood tests that can tell them whether they have an affected gene. With the information obtained from such screening, an appropriate course of preventive measures and follow-up tests can be initiated (see genetic screening).

Diagnosis

A sudden change in bowel habits or blood in the feces (often detectable only in a laboratory) may be the first symptoms of colon cancer. In the early stages of the disease there may be no obvious symptoms. Diagnosis is made by physical examination of the rectum and a laboratory examination of blood for carcinoembryonic antigen (CEA), a tumor marker produced by colon cancers. These may be followed by an endoscopic examination of the colon with a sigmoidoscope (to examine the rectum and the adjoining sigmoid colon) or colonoscope (to examine the entire colon). A biopsy of any suspicious tissue, such as a polyp or a flat or depressed lesion, is then examined in a laboratory to determine if cancerous changes are present. If cancer is found, the patient is evaluated to determine the extent of the primary tumor and whether the disease has spread throughout the body.

Treatment

Treatment depends upon the stage of the cancer. The initial treatment is usually local excision of the tumor or excision of a larger part of the colon followed by the joining of the two adjacent ends, a procedure referred to as end-to-end anastomosis. In some cases a colostomy (an opening that allows waste to be expelled through an opening in the abdomen rather than through the anus) is created either temporarily, to allow healing, or permanently, if significant portions of the colon have had to be removed. If the disease is advanced, radiation therapy, chemotherapy, or biological therapies (therapies that stimulate the body's own immune defenses against the disease) may be used in addition to surgery.

colon, in anatomy: see intestine.

colon, in writing: see punctuation.

Malignant tumour of the large intestine (colon) or rectum. Risk factors include age (after age 50), family history of colorectal cancer, chronic inflammatory bowel diseases, benign polyps, physical inactivity, and a diet high in fat. Many of the symptoms are associated with abnormal digestion and elimination. Colorectal cancer is treated by surgery, chemotherapy, or radiation therapy.

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Segment that makes up most of the large intestine. Though the two terms are often used interchangeably, the colon technically excludes the cecum (a pouch at the beginning of the large intestine), rectum, and anal canal. It runs up the right side of the abdomen (ascending colon), across it (transverse colon), and down the left side (descending colon); its last section (sigmoid colon) joins the rectum. It has no digestive function but lubricates waste products, absorbs remaining fluids and salts, and stores waste products until excretion. Problems involving the colon include ulcerative colitis, constipation and diarrhea, gas discomfort, megacolon (enlarged colon), and cancer.

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