Colon Cancer Screening (cont.)

Virtual colonoscopy

Virtual colonoscopy is a new technique that uses CT to construct virtual images of the colon that are similar to the views of the colon obtained at colonoscopy. The virtual colonoscopic images are produced by computerized manipulation of two-dimensional images obtained by a CT scanner rather than direct observation through the colonoscope. The colon is cleaned-out using laxatives the day prior to the virtual colonoscopy examination. A tube then is inserted into the anus and is used to inject air into the colon. The CT scans then are performed, and the scans are analyzed and manipulated to form a virtual image of the colon.

Properly performed virtual colonoscopy can be very good. It can even find polyps "hiding "behind folds that occasionally are missed by colonoscopy. Nevertheless, virtual colonoscopy has several limitations. They are:

1. Virtual colonoscopy cannot find small polyps (less than 5 mm in size) that are easily seen at colonoscopy.
   
   
2. Virtual colonoscopy is not as accurate as colonoscopy at finding flat cancers or premalignant lesions that are not protruding, that is, are not polyp-like.
   
   
3. Small pieces of stool can look like polyps on virtual colonoscopy and lead to a diagnosis of polyps when there are none.
   
   
4. Virtual colonoscopy cannot remove polyps. Thirty to forty percent of people have colon polyps. If polyps are found by virtual colonoscopy, then colonoscopy must be done to remove the polyps, and, therefore, many individuals having virtual colonoscopy will have to undergo a second procedure, colonoscopy.
   
   
5. There have not been studies to compare the discomfort levels of colonoscopy versus virtual colonoscopy, and comparisons will be difficult to do. The discomfort of colonoscopy is from the insertion of the colonoscope and air insufflation. The discomfort of virtual colonoscopy is from air insufflations. Patients' perceptions of discomfort from both procedures are highly variable. What makes the discomfort difficult to compare is that patients undergoing colonoscopy usually are sedated intravenously, while patients undergoing virtual colonoscopy are not sedated. As a result, patients may actually find colonoscopy more comfortable than virtual colonoscopy. On the other hand, sedation increases the risk of complications from colonoscopy.

Because of these limitations, virtual colonoscopy has not replaced colonoscopy as the primary screening tool for individuals at either normal or high risk for polyps or colon cancer. It is currently a good option for individuals who cannot or will not undergo colonoscopy