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Sensitivity of CT Colonography for Screening Large Colorectal Lesions

In this study reported in the New England Journal of Medicine, 2531 asymptomatic adults underwent both colonoscopy and CT colonography. Most were white, middle aged; fewer than 10% had a FHx of polyps or cancer. Compared with colonoscopy, CT had a sensitivity of 0.9, correctly identifying 90% of the larger adenomas and cancers (over 10 mm diameter) detected by colonoscopy. Sensitivity for adenomas 6-9 mm was 0.78. The downside: about 17% had what looked like a large polyp on CT; however, 3/4 of them turned out to be false alarms. Other factors for physicians and patients to bear in mind: consider the radiation dose incurred with repeated CT scanning, and the fact that positive scans must be confirmed with a diagnostic colonoscopy.

CT colonography is a sensitive screening tool for larger colorectal lesions

N Engl J Med 2008;359:1207-17 [NEJM abstract]

N Engl J Med 2008;359:1285-7 [NEJM Commentary Medline Citation]

Computed tomography (CT) of the colon is a non invasive tool for screening asymptomatic individuals for colorectal cancers and adenomas. Compared with colonoscopy, CT had a sensitivity of 0.9 (95% CI 0.84 to 0.96) in the most recent and largest prospective study to compare the two. In other words, the less invasive option correctly identified 90% of the larger adenomas and cancers (>=10 mm in diameter) detected by colonoscopy. Sensitivity for smaller adenomas (>=6 mm) was 0.78 (0.71 to 0.85).

So CT is good at finding the bigger and most clinically important lesions, but there is a downside, says an editorial (pp 1285-7). About 17% of the participants had what looked like a large polyp on their CT scan: three quarters of them turned out to be false alarms. CT also tends to pick up extracolonic lesions that may not be clinically relevant, triggering further investigations and incurring costs. Patients choosing between the two must bear in mind the radiation dose associated with repeated screening scans, and the fact that a positive scan must be confirmed with a diagnostic colonoscopy.

In this study, 2531 asymptomatic adults had both screening tests. Most were white and middle aged. Fewer than one in 10 had a family history of colorectal polyps or cancer.

© 2008 BMJ Publishing Group Ltd.

Related Resources from Epocrates:
Epocrates Online Diseases: Colorectal cancer monograph