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Am Fam Physician. 2004;69(9):2197-2198

Although colon cancer is the second leading cause of cancer mortality, only one half of adults at average risk have been screened. Virtual colonoscopy, which uses computed tomographic (CT) scanning to screen the whole colon for cancer-causing polyps, has been offered as a less invasive alternative to endoscopic examinations. However, earlier studies of patients at average risk revealed sensitivities as low as 48 percent for detection of polyps 1 cm or larger. Pickhardt and colleagues report on a large trial comparing same-day virtual and endoscopic colonoscopy for colon cancer screening in an average-risk patient cohort.

A total of 1,253 patients were enrolled, of whom 1,233 underwent both virtual and endoscopic colonoscopies. Six patients were excluded because of inadequate bowel preparation, six others did not have successful CT scans of the entire colon, and eight had incomplete endoscopic colon examinations. Virtual colonoscopy involved insertion of a rectal catheter and air insufflation of the colon, followed by CT scanning of the abdomen. This procedure took an average of 14.1 minutes to complete.

Two aspects of the CT protocol in this trial differed from previous studies. First, patients were given 500 mL of a barium solution to “tag” any remaining solid stool and 120 mL of an opacifying solution to tag any remaining intraluminal fluid. With the residual stool and fluid tagged, the scanning software was able to remove them from the CT images of the bowel. Second, this trial used three-dimensional CT image reconstructions as the primary screening modality, whereas previous studies had relied for the most part on standard two-dimensional CT images for polyp detection.

Experienced colonoscopists performed endoscopic colonoscopy after CT scanning was completed. Each colonic segment was first examined without knowledge of the virtual colonoscopy results and then re-examined if any polyp identified on CT scanning was missed by the initial endoscopic examination.

After virtual and endoscopic colonoscopies were completed, adenomatous polyps 6 mm or larger were detected in 13.6 percent of patients, and polyps 10 mm or larger were found in 3.9 percent. Virtual colonoscopy had a sensitivity of 88.7 percent for polyps 6 mm or larger and 93.8 percent for polyps 10 mm or larger compared with sensitivity rates of 92.3 and 87.5 percent, respectively, for endoscopic examination.

Compared with the reference standard of endoscopic colonoscopy and histopathologic examination of all resected polyps, the specificity of virtual colonoscopy was 79.6 percent for polyps 6 mm or larger and 96.0 percent for polyps 10 mm or larger. Virtual colonoscopy detected 55 polyps that were 5 mm or larger and were not seen on the initial endoscopic examination out of a total of 1,310 detected polyps (4.2 percent). Two malignant polyps were found overall. Both were seen on virtual colonoscopy, and one was missed on the initial endoscopic examination.

Virtual colonoscopy without sedation was rated by 54.3 percent of patients as the more uncomfortable examination, while the sedated endoscopic examination was rated as more uncomfortable by 38.1 percent; 7.6 percent of patients were undecided.

The authors conclude that the sensitivity and specificity of three-dimensional virtual colonoscopy for polyp detection are comparable to that of endoscopic colonoscopy.

editor’s note: This is the first large trial of virtual colonoscopy in an average-risk population demonstrating a diagnostic accuracy equivalent to that of endoscopic colonoscopy and providing an additional safety margin by detecting some polyps that were missed on endoscopic examination. If this accuracy can be confirmed in similar studies and cost-effectiveness can be demonstrated, the door may be opened for more widespread use of virtual colonoscopy. Of course, polyps cannot be removed during CT scanning. Using a size cutoff for polyp removal of 10 mm or larger, the authors of this trial calculated that one in 13 patients would have required an endoscopic examination subsequent to the virtual colonoscopy. Even with a conservative polyp size cutoff of 6 mm or larger, more than two thirds of patients would have avoided the need for endoscopic colonoscopy. If screening centers could duplicate the efficiency of the protocol used by these authors, so that all patients with polyps detected on virtual colonoscopy could have them removed endoscopically on the same day, taking advantage of the same bowel preparation, virtual colonoscopy might have an even greater appeal as an initial step in colon cancer screening.—B.Z.

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