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While the idea of not having to undergo a colonoscopy to screen for colon cancer might be appealing to most patients, there are situations in which virtual imaging does not do an adequate job, says Michael J. Stamos, MD, FACS, a surgeon at Harbor-University of California at Los Angeles Medical Center in Torrance, CA, and chairman of the professional outreach committee of the American Society of Colorectal Surgeons in Arlington Heights, IL. For example, "studies have shown that virtual imaging does not detect polyps less than 6 mm," he says.1
Although proponents of virtual imaging point out that polyps less than 6 mm are generally not malignant, the question that arises is how to determine the interval between exams. "Guidelines for colonoscopy set eight to 10 years as the interval between exams," says Lester Rosen, MD, FACS, professor of clinical surgery and colorectal surgeon at Lehigh Valley Hospital in Allentown, PA. "But we know we can miss small lesions in virtual endoscopy, so I am not as comfortable waiting 10 years to re-examine the patient," he adds.
At this time, there are no guidelines related to intervals between virtual colonoscopies, he says. "An actual colonoscopy also allows the surgeon to see a trail of blood in the colon, while virtual colonoscopy does not," says Rosen. "Dry stool and bowel contour thickness changes also can affect how accurately the image can be read."
Both virtual and actual colonoscopy require patient preparation, but the colon has to be optimally clean for effective virtual imaging, while the surgeon can remove bits of fluid and dry stool with suction during an actual colonoscopy, explains Rosen. "The colon also may be distended more during virtual imaging so the radiologist can get the best pictures, and the extra distension can cause greater discomfort to the patient and increase the risk of injury," he adds.
If the virtual image exam shows any abnormalities that need to be looked at more closely, the patient has to undergo the exam prep again, points out Stamos. "For this reason, and the fact that some things may be missed with the virtual exam, I recommend my high-risk patients undergo the actual colonoscopy only," he says. High risk is defined as patients with a family history of colon cancer, prior history of polyps themselves, symptoms such as bleeding or change in bowel habits, adds Stamos.
The real value to virtual imaging of the colon might be for patients who have a cancer that blocks the surgeon’s view of the colon beyond the cancer, says Stamos. "Virtual imaging of the colon can show me if there are other polyps or potential cancers that I will also need to remove," he says. This helps the surgeon operate more effectively since he or she knows what is beyond the first cancer, he adds.
The noninvasive nature of virtual imaging is a definite advantage for patients, says Stamos. "The current technology is still first generation, and improvements are constantly made, so I do see that virtual imaging might turn into an important screening tool," he says. "If the fear of colonoscopy is removed, more patients may undergo a screening, and we’ll be able to detect and treat colon cancer more effectively."
1. Fenlon HM, Nunes DP, Schroy III PC, et al. A comparison of virtual and conventional colonoscopy for the detection of colorectal polyps. N Engl J Med 1999; 341:1,496-1,503.