“Virtual Colonoscopy” effective in finding most polyps but may miss small ones

A type of noninvasive “virtual colonoscopy” can diagnose medium to large polyps as well as traditional colorectal probes can, but it may miss some smaller yet potentially cancerous growths, according to a new review of studies.

The review — limited to comparing the traditional procedure to use of computer tomography as a method — also finds that despite seeming advantage for the person being screened, there is no evidence that availability of the CT scan results in more screenings being done.

CT scanning uses computers and software to display images of the colon as if the viewer were seeing the inside of it through an endoscope, the instrument used in traditional colonoscopy. It does not require sedation, although it does require the same bowel-cleansing preparation as the older method.

The review is published by ECRI, a nonprofit health services research agency that produces systematic evidence reviews on medical devices, drugs, biotechnologies and procedures.

Because anything detected by a CT scan would necessitate a follow-up colonoscopy to confirm the finding and to remove the polyps, there is controversy over its value both to the individual and to the health care system.

Although the review included 23 studies of CT colonography, only one study, of 1,123 patients, directly compared colonoscopy and CT colonography in patients without any previous history of colorectal problems.

In that study, CT colonography detected 94 percent of polyps 10 millimeters and larger in diameter, while colonoscopy detected 88 percent of these polyps. CT colonography detected 89 percent of polyps six millimeters in diameter and larger, compared with 92 percent for colonoscopy.

“CT colonography can detect most large colorectal polyps and masses, but is less sensitive for detecting smaller polyps,” the review concludes.

Other findings:

• There is no direct evidence as to the effect of any method of screening on the prevention of colon cancer incidence or death.

• Virtual colonoscopy is safer, although traditional colonoscopy has a very low rate of complications.

• CT imaging reduces to almost nothing the chance that a colon exam cannot be completed, and it does find incidental conditions such as kidney stones and gall stones that would not be picked up by the older method.

• Because the invasive colonoscopy is the standard by which all other screening methods are measured but is not perfect, there is a statistical chance that the findings underestimate the accuracy of the virtual colonoscopy.

Most colon polyps are not cancerous or become malignant so slowly that an individual will die of other causes before colorectal cancer develops, according to the report. But those larger than 10 millimeters are more likely to become malignant. Between six millimeters and 10 millimeters, there was no statistically significant difference between the methods in the studies reviewed.

“In some studies, patients who have undergone both conventional colonoscopy and CT colonography have expressed a preference for CT over the more invasive examination, but this does not necessarily mean they will accept one and not the other,” according to the report.

Patients might find advantages to each type of test.

“CT is faster and noninvasive, but polyps can be diagnosed and removed during a single colonoscopy procedure, and patients may prefer being sedated for colonoscopy to being awake for CT,” says Charles Turkelson, Ph.D., chief research analyst at ECRI.

“Since colonoscopy can achieve both the diagnosis and removal of polyps during one procedure, some patients may prefer it to colonography because they will only have to undergo the required bowel cleansing preparation once rather than twice, for the diagnostic CT colonography and then again for any necessary polyp removal,” Turkelson says.

If CT colonography becomes more popular as a colorectal cancer screening test, the resulting increase in follow-up colonoscopies “could be quite substantial,” according to the report.

“Patients must already wait months for an available appointment for routine colonoscopy, and in some areas of the United States, screening colonoscopy is unavailable at all because there are not sufficient personnel to perform the test,” the ECRI authors write.

Guidelines published in 2003 by the American Gastroenterological Association and an evidence review by the Technology Assessment Center of the Blue Cross Blue Shield Associations of America in 2004 do not find enough evidence to recommend virtual colonoscopy.

CT colonography is not covered by Medicare or any of several private insurers surveyed in the ECRI report.

Colorectal cancer is the second most common cause of cancer-related death in the United States. About 145,000 cases of colorectal cancer will be diagnosed in the United States in 2005, according to the American Cancer Society. In 2000, according to the Centers for Disease Control and Prevention, only 42 percent of Americans age 50 or older had been screened, as recommended.

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