Current Clinical Results of Virtual Colonoscopy
Ultimately, virtual colonoscopy may assist other current options in enabling widespread screening of the entire colon to be performed in a time-efficient manner. Colonoscopy is the current gold standard for evaluation of the colon, allowing visualization of the entire colon in most patients with the added benefit of biopsy and removal of polyps. But as stated above, there are several barriers to widespread colonoscopy screening, including cost, need for sedation, potential complications of the procedure, and importantly manpower issues.
Within the gastroenterology community there are differing opinions as to the current role of virtual colonoscopy. It has been suggested that while virtual colonoscopy is an exciting imaging technique that has promise, it is not yet ready to be recommended for general screening (40, 53). What are the data comparing virtual and conventional colonoscopy?
Many clinical studies evaluating virtual colonoscopy have demonstrated a success rate of over 90 percent in detecting colorectal polyps 1 cm or more in diameter when compared with conventional colonoscopy (1). These results compare favorably with studies that have evaluated double-contrast barium enema with colonoscopy in detecting lesions of this size (22). A study published in the New England Journal of Medicine of 100 patients undergoing back-to-back virtual and conventional colonoscopy showed a sensitivity of 100 percent for colorectal cancer, 91 percent for polyps 10 mm or more in diameter and 82 percent of polyps that were between 6 and 9 mm in diameter (55). However, not all studies have demonstrated such a high success rate for detecting the 10 mm polyps. In a group of 180 patients, Fletcher (33) showed an 85-percent detection rate for polyps measuring 10 mm or wider. In 1997, a study by Hara (27) showed a 75-percent sensitivity for detecting polyps in this range. In 2001, a follow-up study by Hara (42) showed improved sensitivity ranging to 80-89 percent for the 10-mm polyp. These data suggest that like most new techniques there is a learning curve and clearly there is one with virtual colonoscopy.
For small polyps (5 mm and less in diameter), the sensitivity for detection is lower. The clinical significance of these small (<5mm) raised polyps is questionable. Many represent benign polyps or normal elevations of the mucous membrane of the colon (3). However, some will represent small cancers. These lesions are difficult to detect with virtual colonoscopy. What should an appropriate interval follow-up be in a patient with a normal interpretation at virtual colonoscopy? What if a small 3-mm filling defect is detected at virtual colonoscopy? Should this patient undergo traditional colonoscopy? These are difficult questions, somewhat related to patient age and underlying health status, but clearly they deserve further attention.
Perhaps of more concern than the small raised polyp is the truly flat adenoma (cancerous tissue) that is almost impossible to detect with virtual colonoscopy (55). A previous report pointed out the difficulty in detecting these lesions with virtual colonoscopy. In our experience, they are very difficult to see and even in retrospect are usually not detected. However, these lesions appear to be relatively rare in western populations. The ability of virtual colonoscopy to detect the vast majority of clinically significant lesions is still important. It should be pointed out that even traditional colonoscopy has limitations in its ability to detect all colorectal polyps (56)