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3-D Virtual Colonoscopy: A Breakthrough for Colon Cancer Screening?

The results of this study appear to be a breakthrough in the performance of virtual colonoscopy, providing technical direction for virtual colonoscopists to follow and to develop further.

Results from previous studies of computed tomographic (CT) virtual colonoscopy for colon cancer screening have varied widely, and recent results in populations with low prevalences of colorectal cancer have been poor (Journal Watch Gastroenterology Sep 9 2003 ). In this study, investigators at 3 military hospitals performed same-day virtual and conventional screening colonoscopy in 1233 asymptomatic, average-risk adults.

Whereas most previous researchers used 2-dimensional techniques, these radiologists relied on 3-D fly-through images for initial detection of polyps with virtual colonoscopy; in addition, barium contrast material was used during bowel preparation to tag residual fluid and stool. To verify inconsistencies between the 2 techniques, colonoscopists were unblinded to the results of virtual colonoscopy after they completed blinded conventional colonoscopy evaluation of each segment of the colon and, then, were allowed to reexamine that segment with conventional colonoscopy to resolve discrepancies.

In per-patient analyses and compared with unblinded conventional colonoscopy, virtual colonoscopy and (blinded) conventional colonoscopy had sensitivities of 93.8% and 87.5%, respectively, for detecting polyps of 10 mm or larger in diameter; 93.9% and 91.5% for polyps of 8 mm or larger; and 88.7% and 92.3% for polyps of 6 mm or larger. Two malignant adenomas were identified; both were detected on virtual colonoscopy, and 1 was missed on conventional colonoscopy before unblinding. The specificity of virtual colonoscopy for detecting adenomas was 96.0% for polyps of 10 mm or larger, 92.2% for polyps of 8 mm or larger, and 79.6% for polyps of 6 mm or larger. Interobserver agreement on virtual colonoscopic studies was good (range, 97.6% to 99.6%). At a polyp-diameter cutoff of 10 mm as detected by virtual colonoscopy, 7.5% of patients would have been referred for polypectomy. At cutoffs of 8 mm and 6 mm, 13.5% and 29.7%, respectively, of patients who underwent virtual colonoscopy would have been referred for conventional colonoscopy.

More patients reported greater discomfort with virtual colonoscopy than with conventional colonoscopy (54% vs. 38%; P<0.001), and 8.2% rated the discomfort associated with virtual colonoscopy as severe. However, 68% rated virtual colonoscopy as more acceptable than conventional colonoscopy, 24% rated conventional colonoscopy as more acceptable (P<0.001), and 8% were undecided. For future screening, 50% preferred virtual colonoscopy, 41% preferred conventional colonoscopy, and 9% were undecided.

Comment: This is the largest prospective study of virtual colonoscopy to date. The results are remarkably good, and the study design was outstanding. Although it is not completely clear why these investigators were able to overcome previous impediments created by high interobserver variation and a low prevalence of colorectal neoplasia, fundamental reliance on 3-D imaging, use of multidetector scanners, and electronic subtraction of labeled stool and fluid apparently contributed.

Although this study should be repeated to verify the results, its findings appear to be a breakthrough in the performance of virtual colonoscopy: It provides technical direction for virtual colonoscopists to follow, to develop further, and to refine. The current virtual colonoscopy approach of using 2-D imaging (with 3-D imaging used primarily to resolve uncertainties) should be replaced with the approach used in this study.

— Douglas K. Rex, MD

Published in Journal Watch Gastroenterology December 16, 2003

Citation(s):

Pickhardt PJ et al. Computed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults. N Engl J Med 2003 Dec 4; 349:2191-200.

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Morrin MM and LaMont JT. Screening virtual colonoscopy -- Ready for prime time? N Engl J Med 2003 Dec 4; 349:2261-4.

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