Computed tomographic colonography (CTC), also known as virtual colonoscopy, is entering a new era. With the recent publication of the successful American College of Radiology Imaging Network National CT Colonography Screening Trial, CTC is now widely recognized as a highly sensitive and specific test for identifying polyps in the colon . Attention is now turning toward reimbursement, training, and dissemination of the technique into community practices. Negative issues such as the costs of workup for extracolonic findings and the potential harms of radiation dose are now under investigation. But there is one “elephant in the room” that is mentioned less often: the poor patient acceptance of the bowel preparation common to both optical and virtual colonoscopy .
In this issue of Academic Radiology , Nagata et al report on a prospective comparison of CTC in high-risk patients using two different bowel preparations, a full standard laxative preparation and a “minimum preparation” with reduced laxative administration. The subjects were a consecutive series of 101 patients who were alternately assigned to either preparation. Within 7 days, the minimum-preparation patients also underwent optical colonoscopy after full bowel preparation and hence received both preparations. Both preparations included the tagging of residual fluid and feces with inexpensive sodium diatrizoate. The results of a questionnaire given to the patients indicated a strong preference and high tolerance for minimum-preparation CTC over colonoscopy. Both bowel preparations led to high sensitivity for detecting polyps ≥6 mm (97% for full preparation and 88% for minimum preparation), but specificity was much lower for the minimum-preparation group (92% vs 68%, respectively). The quality of fecal tagging was also poorer in the minimum-preparation group. Nevertheless, Nagata et al conclude that because of its high sensitivity and patient acceptance, patients should be offered the minimum-laxative version of CTC as an alternative to full-laxative preparation if they are willing to accept the decrease in specificity.
Bowel preparation has been an important subject of research on CTC over the past decade. The earliest preps mimicked those used in barium enema and colonoscopy, typically a vigorous cathartic cleansing with oral bisacodyl tablets and 2 to 4 L of polyethylene glycol (PEG) . Researchers soon began exploring other bowel preparations. An early successful choice was a switch to oral sodium phosphate preps, which were shown to leave less residual fluid in the colon . Soon thereafter, oral contrast agents containing iodine and/or barium were added to the sodium phosphate preparation and shown in a seminal multi-institutional clinical trial to lead to high sensitivity and patient acceptance of CTC . The oral contrast agents improved sensitivity for polyp detection by tagging residual fecal matter and colonic fluid. At about the same time, researchers began investigations of CTC without cathartic preparation or with less vigorous cleansing using dietary modification and the oral administration of various combinations of magnesium citrate, bisacodyl, senna, diatrizoate meglumine, and barium .
The laxative-free and reduced-cleansing bowel preps for CTC made it more challenging to interpret the images, because polyps were hard to see in the presence of incompletely tagged stool and residual fluid. Subsequently, researchers developed “electronic cleansing” to identify and remove residual fluid and/or fecal matter from the images .
Despite the advances in electronic cleansing and the hope that laxative-free or minimum-preparation CTC will be successful, full preparation is considered the clinical standard now that the American College of Radiology Imaging Network trial has proved its efficacy. Nagata et al have challenged the need for full prep with CTC by showing high sensitivity for polyp detection with minimum-preparation CTC. Moreover, because Nagata et al did not use electronic cleansing, whether electronic cleansing would have improved the results is unknown. Another factor to consider is the patient population. The subjects in Nagata et al’s study were relatively young, with a mean age of about 55 years. Older patients may not cleanse as well and could have poorer sensitivity for polyp detection.
However, a problem remains: the 24% decrease in specificity in the minimum-preparation group compared with the full-preparation group. Thus, about one quarter more of the subjects without polyps would be referred for unnecessary optical colonoscopy on the basis of false-positive findings on minimum-preparation CTC. Would patients accept such a high false-positive rate if they were told about it beforehand? Would physicians be willing to refer patients for an examination in which about one third of normal subjects (on the basis of 68% specificity) need to undergo two tests (optical and virtual colonoscopy) instead of just one? Not only would patients undergo two tests, but they would undergo two expensive tests as well as full cathartic preparation. It seems highly unlikely that medical advisory bodies and payers would look favorably on this situation.
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