Even if a colon screening exam were 100% accurate, it would be useless if no one showed up to use it. In fact, fewer than half of adults over 50 ever getting screened for colorectal polyps and cancer, so researchers are eager to find new ways to get the rest into their doctors' offices.
Some see virtual colonoscopy (VC, or CT colonography [CTC]) combined with a gentler bowel prep as an important way to screen the uncleansed masses. There is evidence an easier exam might work. Patient surveys have found that rigorous bowel cleansing with laxatives is the single most unpleasant part of the exam, be it optical colonoscopy or VC.
And unlike conventional colonoscopy, VC is in a position to make things easier for the patient. Providers are experimenting with gentler regimens that produce less discomfort and mark the residual stool, most commonly with a barium sulfate suspension. Iodine-based oral agents are also gaining popularity for their fluid-tagging properties. The technical goal is to create reliable and reproducible CT intensity differences among fluid, stool, and polyps or cancer, thereby rendering lesions more conspicuous.
The optimal fecal and fluid tagging regimen isn't fully established, though several studies have produced answers. The latest of these, published online before print in European Radiology, sought to find the optimal formula for reduced-laxative tagging.
"The ideal tagging regimen remains controversial but must be safe, effective, simple, and well-tolerated," wrote Dr. Stuart Taylor, Dr. Andrew Slater, Dr. David Burling, and colleagues from University College, St. Marks, John Radcliffe and College Hospitals in the U.K. "Previous work has shown adequate tagging may be achieved using low volumes of 40% w/v (weight to volume) barium, although the ideal volume and dosing regimen has not been fully established. Furthermore, efficacy for fluid tagging has been question, with some investigators preferring iodine-based contrast either alone or in combination, claiming a more homogeneous fluid opacification better suited to digital subtraction" (European Radiology, April 3, 2007)
Their study looked prospectively at 95 patients referred for colonoscopy "for symptoms suggestive of colorectal neoplasia (change in bowel habit, rectal bleeding, unexplained weight loss, or palpable abdominal mass), offering them a double exam instead: virtual colonoscopy in the morning and optical colonoscopy later on the same day.
Patients were randomized to one of four reduced-laxative regimens using barium-based fecal tagging (20 mL of 40% w/v barium sulfate, Tagitol V, E-Z-EM, Lake Success, NY): regimen A consisting of four doses, B with three doses, C with three doses plus 22 mL of 2.1% barium sulphate, or D consisting of three doses plus 15 mL of meglumine amidotrizoate.
All patients followed the same reduced laxative regimen, which including a low-residue diet two days before VC, avoiding fatty food, milk, and vegetables. The day before the exam they were given a low-residue meal kit (Nutraprep, E-Z-EM), also ingesting 13 g of senna granules (Reckitt Benckiser Healthcare, Hull, U.K.) in the morning and 18 g of magnesium citrate (LoSo Prep, E-Z-EM) in the evening.
Following automated insufflation with CO2 (ProtoCO2l, E-Z-EM) patients were scanned prone and supine at 120 kVp and 50 mAs with either four-slice MDCT (LightSpeed Plus, GE Healthcare, Chalfont St. Giles, U.K.) with 2.5-mm collimation and 25-mm reconstruction intervals, or 64-slice MDCT (Somatom Sensation 64, Siemens Medical Solutions, Erlangen, Germany) with 0.6-mm collimation.
Two radiologists experienced in VC interpretation graded residual stool, tagging efficacy, fluid, and bowel preparation, as well as the findings, on a dedicated workstation (Vitrea 3.8, Vital Images, Minnetonka, MN).
After VC, patients were injected with an additional 18 g of magnesium citrate to adequately cleanse the colon for optical colonoscopy, performed with segmental unblinding.
Immediately thereafter the patients were handed a questionnaire on their experience with the reduced laxative regimen; these results were then compared to historical data on similar patients who had undergone a full bowel prep. A week later the researchers mailed out a follow-up questionnaire asking about the limited prep, and whether they preferred it over the full prep for colonoscopy that followed later in the day.
VC's sensitivity for the detection of two cancers was 100%, 81% for 21 polyps 6 mm or larger, and 32% for polyps 5 mm and smaller, with just four false positives reported in the cohort for polyps 6 mm and larger, the group reported.
The results showed high tagging quality across all four tagging regimens (mean 3.7-4.5) with no significant differences among them. Still, the Hounsfield unit values of layered tagged fluid was significantly higher for regimens C and D than for A or B (p = 0.0002).
In all 89 of the 95 (94%) of patients completed the questionnaires. Compared to demographically similar historical controls, the reduced prep experience was not significantly different from previous full-prep responses, except for sleep disturbance, which was actually worse after reduced preparation (p = 0.001).
Responses to the follow-up questionnaire a week later found that the extra prep required was "no problem" for most (51/69, 74%) patients. Still, 61% called the reduced prep "better" or "much better" than the full preparation required for colonoscopy.
"In accordance with previous studies, we found patients in general preferred reduced preparation to the full purgation required for colonoscopy, although this (61%) preference was much less than expected," Taylor and colleagues, suggesting that the reduced prep could be further reduced.
Because the combination of 13 g of senna and 18 g of magnesium citrate "clearly produces relatively strong purgation ... it would therefore seem reasonable to study reduced laxation further, perhaps omitting the senna and/or reducing the dose of magnesium citrate," they wrote.
In fact, the relatively "harsh" reduced laxative regimen could be considered a weakness of the study, the researchers noted, inasmuch as good results have been reported without the use of any laxatives whatsoever.
Further, it is assumed that because cathartic prep is often cited by patients as the worst part of any colonic exam, compliance with exams would improve if the bowel prep could be made less rigorous, they wrote.
"However, this assumption has not been proven in prospective trials, and we cannot extrapolate the preferences we found into increased compliance with CTC," the authors cautioned. "Indeed it could be argued the laxative regimen we used would have relatively little impact on compliance in a screening setting, given the side-effect profile."
As in previous studies, the addition of iodinated oral contrast in regimen D -- 15 mL of meglumine amidotrizoate -- yielded a significantly better preparation, "possibly due to a 'washing effect,' they wrote. "Although we used a small dose, meglumine amidotrizoate is also known to have a laxative effect that may also have added to the superior cleansing," the researchers noted.
The fact that stool tagging was good in all four groups "suggests barium-based tagging can be simplified to a one-day regimen only," they wrote. "Similarly all regimens produced an average tagged fluid density of 500 HU." The addition of a morning dose of 2.1% barium sulphate or meglumine amidotrizoate "significantly increased the attenuation of the nondependent fluid layer," they added.
Also of note, Zalis et al showed suboptimal electronic subtraction when high-density barium was used as the sole tagging agent, so the choice of tagging regimen can be expected to have increasing implications for software used to aid polyp detection. "It is interesting to speculate whether manipulation of fluid-tagging could optimize subtraction," Taylor and colleagues wrote.
The researchers concluded that a combination of reduced laxatives and tagging based on 40% barium sulphate a day before VC produces acceptable diagnostic accuracy.
"Three doses of 20 mL 40% w/v barium sulfate are as effective as more complex regimens, but fluid tagging can be manipulated by the addition of dilute barium or meglumine amidotrizoate on the morning of CTC, the latter also reducing the volume of residual stool," they concluded.
By Eric Barnes
AuntMinnie.com staff writer
April 18, 2007
Related Reading
New developments improve VC -- and colonoscopy, April 6, 2007
Prepless VC a gentle alternative for elderly, March 13, 2007
MR colonography disappoints in reduced-prep protocol, March 6, 2007
Most Americans still skip colon cancer screening, March 8, 2006
Iodine tagging regimen yields best VC results, January 27, 2005
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