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Stanford Report, March 8, 2000

Virtual colonoscopy is unobtrusive tool for detecting colon cancer  


The latest in virtual reality has moved out of the arcade and into exam rooms at Stanford Medical Center. But the radiologists using the technology are playing a deadly serious game -- attempting to accurately diagnose precancerous masses in the colon and rectum using a new, noninvasive technique to scan the interior of the digestive tract.

If the new method passes muster, it may allow thousands of people to avoid a more invasive test that can be somewhat uncomfortable and embarrassing. Aversion to such procedures causes many people to put off recommended screening tests for colorectal cancer, even though such tests stand a very real chance of saving their lives.

Colorectal cancer is second only to lung cancer as the leading cause of cancer-related deaths in this country. About 150,000 people will be diagnosed with colorectal cancer this year, and more than 50,000 people will die of the disease. March is National Colorectal Cancer Awareness Month, and physicians and researchers alike are hoping that coupling increased public awareness with the development of more palatable tests will motivate people to undergo regular screening for the disease, which is highly curable if detected in its early stages.

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Colorectal cancers almost always begin as abnormal growths, or polyps, springing from the lining of the large intestine or rectum. If the polyps are detected and removed before they become malignant (the chance of malignancy increases significantly as the polyp grows) the patient can escape this deadly cancer.

"Many of these cases are totally preventable if adequate screening is done," said R. Brooke Jeffrey, Jr., MD, a professor of radiology and member of the gastrointestinal multidisciplinary tumor board at Stanford's Clinical Cancer Center. Jeffrey is a member of the Stanford team working to develop a virtual method reliable enough to replace the current gold standard of colorectal screening, the colonoscopy. Other members of the team include Chistopher Beaulieu, MD, PhD, assistant professor of radiology and the team's principle investigator, and Sandy Napel, PhD, associate professor of radiology and co-director of Stanford's 3-D Medical Imaging Laboratory. David Paik, a graduate student in the medical information sciences program is also involved in the study.

During traditional colonoscopy, a flexible fiber-optic tube is inserted into the rectum and threaded up through the colon, giving the physician an unobstructed view of the interior of this part of the digestive tract. Any unusual growths can be immediately removed and biopsied.

Although its effectiveness for preventing and detecting colorectal cancer is unquestioned, many patients shy away from the test, which in addition to being uncomfortable requires sedation and a day off from work.

In contrast, the virtual colonoscopy test being developed at Stanford and other academic centers uses a machine that spirals around their body to generate multiple computerized tomography (or CT) scans -- a series of about 400 images of the patient's abdomen and pelvis. The entire scan takes less than a minute, after which patients can return to their home or work. About 70 patients have undergone the procedure at Stanford since the study began four years ago.

The images, each an incremental 'slice' of the patient's torso, are assembled by computer into a three-dimensional representation of the patient's colon. Sophisticated software takes physicians on a virtual tour of the interior of the large intestine, allowing them to zoom back and forth at will, occasionally pausing to rotate the image for a better view. The overall effect is somewhat like watching a bobsled race from the hood of the sled. Small bumps in the track may indicate the presence of polyps that should be removed.

Stanford's "virtual colonoscopy," may someday replace the more invasive procedure as a screening method for people at low risk for colon cancer. But for now, the volunteers for the NIH-funded study have to content themselves with the knowledge that they are contributing to the eventual fade-out of the traditional method; a direct comparison between the two, requires volunteers to submit to both the virtual and the traditional test. That way the researchers can tell if the new method is as effective as the old.

So far the results have been promising, said Napel.

"We appear to have very high sensitivity, meaning we don't miss many polyps," he said, "but we occasionally have problems with false positives."

Because the virtual procedure is completely noninvasive, a positive result would require a traditional colonoscopy to confirm and remove a suspected polyp. Too many false positives can make the test less cost-effective, since it doesn't result in a significant decrease in the number of traditional tests that must be done.

Eventually the researchers plan to develop software that can help pinpoint potentially troubling areas and reduce the amount of time the radiologist must spend on each patient. Such computer-assisted diagnosis may help reduce the number of false positives and make the whole process more efficient.

"It's literally like looking for a needle in a haystack," said Jeffrey of the hunt for small polyps in the nearly five feet of the colon. "So in order for this to be cost-effective, the process has to be highly automated."

Unfortunately for patients, preparation for the new procedure is far from virtual. Just like for a traditional colonoscopy, the bowels must be completely emptied the day before with a combination of fasting and purging. Drinking a special concoction the night before helps move things along. Immediately prior to the CT scan, the bowels are inflated with air that is pumped in through a thin tube inserted in the rectum, allowing better visualization of the walls of the colon.

Even with these drawbacks, however, many people will probably consider the virtual colonoscopy more appealing than the real thing. But Napel estimates it may be four or five years before the new procedure has gained enough credibility to replace the old standby. And people with a personal or family history of polyps will probably still have to regularly undergo traditional colonoscopy.

In the meantime, said Napel and Jeffrey, it's important to do what is necessary to prevent colorectal cancer. The American Cancer Society recommends that every American get a traditional colonoscopy at age 50, and every 10 years thereafter. Other screening tests, including collecting stool samples to check for the presence of blood or having a digital rectal exam by your physician are also important and should be conducted regularly. If colorectal cancer is discovered before it has spread, the five-year survival rate is greater than 90 percent.

If you are concerned that you may be suffering from symptoms of colorectal cancer, such as bloody stool, a change in bowel habits, abdominal discomfort, unexplained weight loss, vomiting or tiredness, talk to your doctor. SR