Colon Tumors and Colonoscopy

Virtual Colonoscopy

Additional prospective studies of the sensitivity and specificity of virtual colonoscopy (VC) have been reported, all of which involve radiologists blinded to the results of conventional colonoscopy (CC). The results were quite mixed. A British group reported that in 201 consecutive patients, VC detected polyps in seven of 14 patients (50 %) with polyps ≥ 1 cm, and in only four of 25 patients (16 %) whose largest polyps were 6 - 9 mm [1]. A Japanese group [2] evaluated the sensitivity of VC for right colonic polyps in 58 patients. Five of six polyps > 10 mm (83 %) were detected, but only four of eight (50 %) in the 5 - 9 mm range, and there were two flat adenomas and both were missed. A Boston group (Beth Israel Deaconess) studied 100 patients; the sensitivity for polyps ≥ 10 mm was 90 %, and 65 % in the 6 - 9 mm range, and at 0 and 24 hours patients reported similar scores for pain and discomfort [3]. The largest study yet performed comparing VC with CC was reported by the Veterans' Association Medical Center in San Francisco [4]. Three hundred patients (291 men, 96 asymptomatic) were studied, of whom 118 had no polyps found by CC. Of these, 33 of 118 had a false-positive finding of polyp on VC. In 182 patients with polyps on CC, VC detected eight of eight cancers (100 %), 90 % of 82 polyps > 1 cm, 80 % of 141 polyps 5 - 10 mm, and 59 % of 301 polyps < 5 mm [5].

The Mayo Clinic reported an initial experience in tagging stool with barium. In an unblinded study, patients required six or seven doses of 225 ml of dilute barium sulfate over 48 hours to label their stool adequately and detect polyps. The tolerance of this was not reported [6].

Overall, the results for virtual colonoscopy continue to be quite mixed, ranging from inadequate to very good. The sensitivity for polyp detection in the 6 - 9 mm range is of great importance, as I believe the threshold for polyp detection will have to be 5 mm or 6 mm. Most of these studies demonstrated inadequate sensitivity in this size range, and no group has yet equaled the 90 % sensitivity reported by Fenlon et al. last year [7]. I suspect the variable results are related to a prolonged learning curve, but the exact reasons remain to be established. The huge San Francisco study was done in an extremely high-prevalence population, in which the use of VC will likely never make sense from a cost perspective.

References

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