Colorectal Cancer

I

Introduction

Colorectal Cancer, malignancy of the large intestine, the lower portion of the intestinal tract, which consists of the colon and rectum. Colorectal cancer is the third most common cancer among Americans, and the second most frequent cause of cancer-related deaths. From 80 to 90 percent of colorectal cancers can be cured, if diagnosed early before the cancer has spread beyond the intestine. New techniques have greatly improved the ability to detect colorectal cancer in its early stages. These techniques can even detect and remove abnormal growths in the lower intestine before they become cancerous.

The American Cancer Society estimates that more than 100,000 new cases of colon cancer and more than 40,000 cases of rectal cancer are diagnosed annually in the United States. An estimated 55,000 people die from colorectal cancers each year. According to the Canadian Cancer Society, each year about 20,000 new cases of colorectal cancer are diagnosed in Canada, and 8,500 Canadians die from the disease.

II

Risk Factors

The risk of colorectal cancer increases significantly with age. About 90 percent of all colorectal cancers are diagnosed in people over the age of 50. Other factors that increase a person’s risk of developing cancer of the colon or rectum include a family history of colorectal cancer, the presence of polyps (abnormal but usually benign growths) in the large intestine, or a history of chronic inflammatory bowel disease such as ulcerative colitis or Crohn’s disease.

Mutated versions of several genes have been linked to colon cancer. For example, in their normal form the genes MSH2, MLH1, PMS1, and PMS2 correct tiny errors that occur when cells divide and grow. Mutated versions of these genes cannot make such repairs, and eventually an accumulation of many such errors interferes with a cell’s ability to resist the uncontrolled division and growth that characterize cancer.

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Research has linked the consumption of certain foods to colorectal cancer. The more red meat and animal fat that people eat, the greater their risk of developing colorectal cancer. Some studies have indicated that diets high in fiber (the undigestible parts of fruit, grains, and vegetables) may reduce the risk of the disease. However, other studies have found no preventive effect from a high-fiber diet. Obesity, inactivity, smoking, and alcohol use may also increase the risk of colorectal cancer.

Some studies have suggested that certain drugs may lessen the risk of developing colorectal cancer. They include hormone replacement therapy for women after menopause and nonsteroidal anti-inflammatory drugs such as ibuprofen. However, these drugs also carry risks, and the evidence that they offer any protection against colon cancer is inconclusive.

III

Symptoms and Diagnosis of Colorectal Cancer

Colorectal cancer usually develops slowly. There are no apparent symptoms in its early stages. Some individuals with undiagnosed colorectal cancer may detect blood in their bowel movements (feces). They may also experience persistent constipation or diarrhea, other changes in bowel habits, abdominal pain, or unexplained weight loss. Physicians usually recommend a procedure called colonoscopy to check for colorectal cancer in people with such symptoms. Another diagnostic procedure is the barium enema.

In colonoscopy the physician inserts a long, thin, flexible instrument called a colonoscope through the anus into the intestinal tract. This instrument enables a physician to visually examine the interior of the colon and rectum. It has a tiny video camera at one end and is connected by light-conducting fibers to a video screen in the examination room. If the physician sees suspicious tissue, tissue samples can be removed with a special tool attached to the colonoscope. The tissue is then examined under a microscope for signs of cancerous cells. The physician can also use the tool to remove small polyps.

A barium enema is sometimes used for diagnosis instead of colonoscopy. In this procedure liquid barium is inserted through the patient’s rectum into the colon; air is sometimes pumped in after the barium. The physician then takes X rays of the large intestine and examines them for unusual growths in the intestine. The barium makes any growths more visible on the X ray.

IV

Screening for Colorectal Cancer

Screening tests can detect colorectal cancers in their early stages, before symptoms appear. Nearly all colorectal cancers begin with tiny polyps on the wall of the intestine. Although not all polyps turn into cancer, most cases of colorectal cancer could be prevented through the detection and removal of small, precancerous intestinal polyps.

Because colorectal cancers are far more prevalent in older people, the American Cancer Society (ACS) recommends regular diagnostic screening for healthy individuals over the age of 50. Colonoscopy is an extremely effective screening procedure. A procedure similar to colonoscopy, called flexible sigmoidoscopy, is often used to examine the lower colon. Although it is less invasive than colonoscopy, it can miss precancerous growths that develop in the upper colon. The ACS recommends that beginning at age 50 people have either a colonoscopy every 10 years, a flexible sigmoidoscopy every 5 years, a barium enema every 5 years, or a fecal occult blood test every year.

In the fecal occult blood test, a small sample of the patient’s feces is smeared on a card coated with a chemical called guaiac, which reacts with blood. The card is analyzed in a laboratory for occult (hidden) blood. A positive result does not necessarily indicate the presence of cancer, however. Benign conditions such as hemorrhoids can also cause bleeding. If blood is found, a follow-up examination is called for. The physician usually recommends a colonoscopy.

A physician may also perform a digital rectal exam as part of an annual physical. Using a gloved finger, the physician checks the lining of the rectum for any irregularities.

Medical researchers are developing new screening procedures for colorectal cancer. One procedure, known as virtual colonoscopy, uses computed tomography (CT scanning), to create X-ray views of the inside of the colon. Air is pumped into the colon beforehand. Another procedure uses DNA technology to examine a patient’s feces for gene mutations or DNA evidence of cell abnormalities.

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