Alternative bowel preps prove useful for CT colonography

Noncathartic bowel preparation is not an obstacle to accurate CT colonography (CTC). Preparations such as fecal tagging, electronic cleansing, and the use of stool-subtraction algorithms produce results comparable to those obtained with conventional bowel-cleansing preparations, according to studies presented at the 2006 RSNA meeting.

The prospect of conventional bowel preparation often discourages patients from undergoing colorectal cancer screening. Removing that obstacle may improve compliance with colorectal screening guidelines, according to Dr. C. Daniel Johnson, a radiologist at the Mayo Clinic in Rochester, MN.

Johnson compared CTC and optical colonoscopy exams performed on 87 adenomatous polyps of at least 1 cm. Patients were administered barium with meals for two days before the CTC, but did not face any dietary restrictions or cathartic bowel preparations. The results of the CTC exams were evaluated by three experienced radiologists who were blinded to the findings of the colonoscopy. Johnson found sensitivity for these adenomas ranged from 81% to 89%, while specificity ranged from 88% to 100%.

A similar study presented by Sebastiaan Jensch of the Academic Medical Center in Amsterdam reviewed 168 patients who were administered barium sulfate and diatrizoic acid with meals for two days before their CTC exam, along with bisacodyl for stool softening. No cathartic bowel preparation was undertaken. This limited bowel preparation produced a per-patient sensitivity of 93% for polyps of at least 6 mm and 94% for polyps of at least 10 mm. Specificity ranged between 83% and 84% for polyps of at least 6 mm and between 96% and 97% for polyps of at least10 mm.

Both studies found that false positive results can be a problem with CTC, but that rate can be influenced by a variety of tools.

For example, Johnson compared the sensitivity and specificity of studies when stool-subtraction algorithms were applied to the CTC. In polyps of 1 cm in size or larger, he found that stool subtraction caused four false positives, reducing specificity from 96 % to about 89%. In intermediate lesions, between 5 mm and 1 cm in size, stool subtraction increased sensitivity to about 87%, but reduced specificity to about 77%.

Electronic colon cleansing can be beneficial for patients with substantial fluid retention, improving sensitivity and reducing the number of false positives, according to a study presented by Markus Juchems from University Hospitals of Ulm in Germany.

In a study of 79 patients who underwent CTC, Juchems found that sensitivity increased from about 50% without electronic cleansing to about 60% when electronic colon cleansing was performed. The results were even more significant for polyps of at least 10 mm in size, with sensitivity increasing from 66% to 83% when electronic cleansing was performed. In addition, the false positive rate dropped from 51% to 38%.