Virtual Colonoscopy is a new method for detecting colorectal polyps and cancers. Air is insufflated into a cleansed colon, and high resolution, thinly-collimated spiral CT slices are acquired. The two dimensional slices, as well as the post-processed "fly-through" virtual colonoscopic images, are examined for polyps and tumors. Research performed at Boston University and elsewhere has shown that Virtual Colonoscopy is better able to see polyps than Barium Enema and is nearly as accurate as Conventional Colonoscopy. In addition, most patients report that the Virtual Colonoscopy technique is more comfortable than either Barium Enema or Conventional Colonoscopy.

All patients who are over the age of 50 years or patients with an increased risk of colon cancer should be screened for colon cancer every 3 5 years. Current research on Virtual Colonoscopy has focused on high risk patients over 50 years of age, with a prior history of polyps, fecal occult blood, or a family history of colon cancer. Virtual Colonoscopy has performed well in this group of patients and is being recommended in general for the exclusion of malignancy.

Studies suggest a very high sensitivity and specificity (96%) for the detection of polyps 1 cm or greater. Such polyps have significant malignant potential. Sensitivity for polyps less than 5 mm is significantly less. Although controversy exists as to the definition of a "significant" polyp with regard to size, polyps < 1 cm in size have a low probability of malignancy and the likelihood of any single lesion progressing to cancer is also small. Many leading authorities have suggested that a policy aimed at identifying and removing only polyps above a threshold size, as opposed to universal polypectomy, could result in a similar benefit in terms of mortality reduction, but at a lower risk and cost.

Virtual Colonoscopy is minimally invasive, and does not carry the low but real (1 in 1500) risk of perforation associated with Conventional Colonoscopy. It is well tolerated by patients and does not require sedation. It is capable of evaluating the colon upstream from obstructing lesions that prevent passage of an endoscope and has the additional advantage that viewing can be directed in any direction whereas the conventional colonoscope looks only in the direction it is pointing. Virtual Colonoscopy is significantly less expensive than Conventional Colonoscopy. A further advantage is that CT scan of the abdomen and pelvis is performed as part of the procedure.

The dose of ionizing radiation is far less than that of a conventional abdominal CT, and is comparable to obtaining a supine and upright plain film exam of the abdomen. Virtual Colonoscopy by CT does not provide the same information as Conventional Colonoscopy in that mucosal detail and colour is not visible which limits the characterization of lesions. In addition, the detection of small polyps is slightly inferior. As with any procedure, including Conventional Colonoscopy, there are no guarantees that all clinically significant growths will be detected as both techniques are operator dependent. It should be remembered than between 10 and 20% of all polyps, and up to 5% of colon cancers are missed, even on Conventional Colonoscopy. Virtual Colonoscopy (like the Barium Enema) is a diagnostic not therapeutic technique. All patients in whom polyps are identified would need to undergo Conventional Colonoscopy for removal.

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