Preventing colon cancer

Imagine taking a fantastic voyage through the highways and byways of the human body. With a touch of a finger on the controls of your vehicle, you "fly" down strangely scenic routes and dark tunnels. Along the way, you note dangers and relay them back to technicians for future repair. Sound farfetched? Not really.

By using new computer-assisted technology, doctors can visualize a person's colon just as if they were there. Called "virtual colonoscopy," this screening test projects a three-dimensional image of the colon onto a computer screen. The physician "flies" through its length, searching for lumps that might be cancerous. The test is non-invasive and often involves much less discomfort than conventional methods of examining the colon. Sedation is seldom required, and the patient can go home immediately after the procedure.

"Virtual colonoscopy has the potential to revolutionize how we screen for colon cancer," says Brian E. Harvey, M.D., a senior medical officer in the Food and Drug Administration's Center for Devices and Radiological Health. "It's very exciting, and once all the data are in, we may find we can screen the entire population over the age of 50, which can lead to early detection of more colonic polyps and colorectal cancer."

When this technique is perfected, it will be added to the arsenal of tools used for the prevention and early diagnosis of colorectal cancer. Although this cancer remains a very scary disease, such new detection technologies improve the chance of finding the tumor early in its growth when it's most curable. In addition, therapeutic advances offer new hope that, even if the cancer has spread, the diagnosis of colon cancer will not be fatal.

A Killer Disease and Its Risk Factors

Colorectal cancer--cancer of the large intestine and rectum-is second only to lung cancer in the number of cancer deaths it causes. The American Cancer Society estimates that more than 130,000 Americans will be diagnosed with colorectal cancer in 2000, and more than 56,000 will die from the disease this year. On average, one in 20 people will develop the disease in the course of a lifetime. Ninety percent of cases occur in patients over age 50, and the majority of cases--75 percent--occur in people with no known medical risk factors for colorectal cancer. But certain factors can sharply increase risk. They include:

* Family history. Having a first-degree relative--mother or father, for example--with colorectal cancer increases the lifetime risk of developing the disease to as high as eight-fold greater than people without a family history.

* History of bowel disease. Risk increases 30-fold in patients with a history of inflammatory bowel disorders, such as Crohn's disease or ulcerative colitis.

* History of adenomatous polyps. Most colorectal cancers begin as small precancerous growths, called polyps, inside the colon or rectum. Villous adenomatous polyps are the most likely to become cancerous (up to 25 percent). Tubular adenomatous polyps are estimated to become malignant 1 to 5 percent of the time.

* Genetic traits. A genetic syndrome known as Familial Cancer Syndrome or Hereditary Non-Polyposis Colon Cancer markedly increases the risk for developing colorectal cancer at an earlier age than those patients at average risk.

Signs and Symptoms

The colon and rectum make up the large intestine, the end of the long tube of the gastrointestinal tract through which food passes during digestion. (This interconnected gastrointestinal organ system also includes the esophagus, stomach and small intestine.) The colon is the upper five or six feet of the large intestine, and the rectum is the last six to eight inches. Cancer begins to develop when cells in the colon multiply uncontrollably. These cell mutations result in precancerous polyps, small protrusions from the intestine's lining.

There are several types of polyps, and they become increasingly common with age. By age 50, 10 percent of the population has polyps, but by age 65 that number grows to 30 percent. If left untreated, 8 to 12 percent of polyps will become cancerous. If allowed to grow, the tumor can invade nearby organs. Once the disease enters the lymph nodes or bloodstream, it most often spreads to the liver.

As with many cancers, there are usually no symptoms in the early stages. Polyps do sometimes bleed, and there may be some noticeable rectal bleeding. However, most of the time, this blood is invisible to the naked eye and is only detectable microscopically.

Patient symptoms begin to appear once the tumor is large enough to cause obstruction of the bowel. They include:

* anemia

* rectal bleeding with bright red blood

* blood in the stool, characterized by black, "tarry" stools

* a change in bowel habits, such as recurrent diarrhea or worsening constipation

* persistent abdominal pain

* generalized weakness or fatigue

* unexplained weight loss

Early Detection Means Survival

If diagnosed and treated in its early stages, colorectal cancer is highly curable. Patients whose tumors are entirely localized to the bowel have an 80 to 90 percent chance of surviving for 10 years. With tumors that spread to the liver, however, the five-year survival rate is less than 5 percent.

The lack of symptoms in early stages may be one reason colorectal cancer has a high mortality rate. "By the time this disease becomes symptomatic, it's often in the late stage," says Robert Kurtz, M.D., chief of gastroenterology and nutrition at Memorial Sloan-Kettering Cancer Center in New York. "There's no question that the earlier colon cancer is found, the more likely the patient will be cured with surgery."

"In fact," Kurtz says, "prevention is the best solution." Because colorectal cancer begins as a slow-growing precancerous polyp, finding and removing these polyps can prevent cancerous changes from taking place. However, since there is no way to know if a polyp is precancerous without a biopsy, medical professionals generally agree that all polyps should be removed upon discovery.

FDA has cleared, or approved, several screening and diagnostic methods for colorectal cancer. When performed regularly, these tests allow the removal of polyps before they become cancerous, which can reduce the incidence of colon cancer by 40 percent. And, by preventing tumor formation, these tests can cut the death rate from colorectal cancer in half.

Screening for patients with no medical or family risk factors should begin at age 50 and be performed regularly. Available screenings include:

* Fecal occult blood test. Both colon cancer and polyps can cause bleeding, which will be passed into the stool. In this test, a small stool sample transferred to a collection card with a narrow stick is screened for the presence of blood. The sample can be collected at home by patients, who send it to their doctors, or by the doctor during a physical examination. Because other conditions, such as stomach ulcers and hemorrhoids, can cause blood in the stool, this test has a high rate of false positives and may result in unnecessary follow-up screenings. It may also fail to detect some tumors.

* Flexible sigmoidoscopy. A short, flexible fiber optic tube is inserted to inspect the rectum and part of the colon. Although this can be an effective diagnostic tool, it is limited in that it inspects only the lower third of the colon.

* Barium x-ray. In this test, a contrast material is infused through the rectum. This material expands the colon and allows a radiologist to see large polyps or cancers (greater than 10 millimeters) in the entire colon. The bowel must be cleansed by laxatives or enemas before the test is performed. This test involves some discomfort and often fails to detect small polyps.

* Colonoscopy. This is currently the most effective tool for detecting polyps and cancers. Additionally, it allows for removal of small polyps. After bowel preparation with laxatives and/or enemas, the patient is sedated. A long, flexible scope with a video chip is inserted into the entire length of the colon. The chip projects an image of the colon onto a video screen, allowing the physician to view the colon. Small, accessible polyps can be removed and examined for the presence of tumor cells.

The American Cancer Society recommends that patients over 50 have a fecal occult blood test yearly and a sigmoidoscopy every five years. Since Medicare and some insurance companies pay for barium x-ray screening, many physicians also recommend that this test be used in conjunction with the flexible sigmoidoscopy. A colonoscopy should be performed if any abnormalities are seen, or if the patient is experiencing symptoms. Patients with known medical risk factors should be screened more extensively and more often.