Adenomatous polyp obscured by small-caliber rectal catheter at low-dose CT colonography: A rare diagnostic pitfall

Obscuration of rectal lesions by the rectal catheter or its retention balloon at air-contrast barium enema represents a known diagnostic pitfall. By comparison, the preferred rectal catheter used for gaseous distention of the colon at CT colonography (CTC) is smaller in caliber and does not necessarily require a retention balloon. If a balloon is used, it is generally much smaller than those used for barium enemas. Furthermore, a significant rectal polyp is less likely to be missed at CTC, not only because a less obtrusive catheter is used, but also because both supine and prone scans are obtained and both cross-sectional and volume-rendered displays are evaluated. We report a case of a 10-mm rectal tubulovillous adenoma that was missed prospectively at CTC due to inopportune positioning of the rectal catheter. This false-negative CTC finding may have been avoided by less advanced placement of the catheter tip beyond the anal verge.