Laxative-Free CT Colonography Detects Most Larger Lesions

Laurie Barclay, MD

May 15, 2012

May 15, 2012 — Laxative-free computed tomographic colonography (CTC) accurately detected adenomas 10 mm or larger, according to the results of a prospective test comparison study published in the May 14 issue of the Annals of Internal Medicine. Patient experience was better than with colonoscopy, but sensitivity was less for smaller lesions. The investigators suggest that laxative-free CTC may have a possible role as an alternate screening method.

"Colon screening by optical colonoscopy (OC) or [CTC] requires a laxative bowel preparation, which inhibits screening participation," write Michael E. Zalis, MD, associate professor of radiology at Harvard Medical School and Massachusetts General Hospital in Boston, and colleagues.

The study goals were to investigate the performance of laxative-free, computer-aided CTC in detecting adenomas 6 mm or larger and to evaluate patient experience with this technique at multiple ambulatory imaging and endoscopy centers.

When asked by Medscape Medical News for independent comment, Perry J. Pickhardt, MD, professor of radiology and chief of gastrointestinal imaging at the University of Wisconsin School of Medicine and Public Health in Madison, said, "This study provides initial validation of a laxative-free approach to virtual colonoscopy (VC; CT colonography) for the detection of large colorectal polyps (≥1 cm)."

The study sample consisted of 605 adults who were aged from 50 to 85 years and who had average to moderate risk for colon cancer. Patients followed a low-fiber diet before CTC and ingested a small dose of contrast material to label feces. A computer program electronically "cleansed" the colon by subtracting tagged feces from the colon images without affecting the size or appearance of mucosal folds and polyps.

Physicians performing OC were initially blinded to CTC findings. During colonoscope withdrawal, these findings were revealed to allow reexamination in the event of discordance between OC and CTC. The reference standard was unblinded OC.

Study outcomes included per patient sensitivity and specificity of CTC and first-pass OC for detecting adenomas that were at least 10 mm in diameter, at least 8 mm in diameter, and at least 6 mm in diameter. Other measurements included per lesion sensitivity and survey data regarding patient experience with preparation for the procedure and with the procedure itself.

For adenomas 10 mm or larger, per patient sensitivity of CTC was 0.91 (95% confidence interval [CI], 0.71 - 0.99) compared with 0.95 (95% CI, 0.77 - 1.00) for OC. Specificity was 0.85 for CTC (95% CI, 0.82 - 0.88) and 0.89 for OC (95% CI, 0.86 - 0.91).

However, sensitivity of CTC was worse for smaller adenomas, with a sensitivity of 0.70 (95% CI, 0.53 - 0.83) for adenomas at least 8 mm in diameter and 0.59 (95% CI, 0.47 - 0.70) for those at least 6 mm in diameter. For OC, sensitivity was 0.88 (95% CI, 0.73 - 0.96) for adenomas 8 mm or larger and 0.76 (95% CI, 0.64 - 0.85) for those 6 mm or larger.

Specificity of CTC was also less than that of OC in this size range, at 0.86 vs 0.91 for adenomas 8 mm or larger and 0.88 vs 0.94 for adenomas 6 mm or larger (P = .02).

Participants reported a better experience in terms of comfort and difficulty of examination preparation with CTC than OC.

CTC "was accurate in detecting adenomas 10 mm or larger but less so for smaller lesions," the study authors conclude. "Patient experience was better with laxative-free CTC. These results suggest a possible role for laxative-free CTC as an alternate screening method."

Limitations and Implications

The study authors note that limitations of this study include the use of only 3 CTC readers and a lack of independent validation of the survey instrument.

"Potential limitations include lower accuracy for small polyps (6 - 9 mm), which are of much less clinical significance, and inability to perform same-day polypectomy (as we currently offer with our cathartic-prep CTC approach)," Dr. Pickhardt said.

In a commentary published in the June 2007 issue of the Mayo Clinic Proceedings, Dr. Pickhardt discussed other overlooked disadvantages to a laxative/cathartic-free approach to VC/CTC. Currently used laxative-free regimens are not "prepless," because they involve a fairly rigorous regimen of dietary restriction and ingestion of various oral contrast agents. Reduced accuracy of VC/CTC could result in missed lesions, as well as in overuse of colonoscopy.

In addition, the greatest aversion to cathartic preparation appears to be from those who have already been screened, and evidence to date suggests that cathartic preparations have a favorable safety profile.

"Laxative-free CTC can be considered as an alternative screening option for individuals unwilling to undergo a cathartic preparation," Dr. Pickhardt said. "CTC (or colonoscopy) with a cathartic prep would remain the preferred front-line strategies, however."

"Further improvements in the laxative-free preps may be needed to allow for primary [3-dimensional] fly-through at CTC, which is necessary to achieve equivalent sensitivity to colonoscopy for detection of relevant polyps," Dr. Pickhardt concluded. "Further independent validation of laxative-free CTC may be called for."

GE Healthcare and the American Cancer Society funded this study Dr. Zalis and 3 coauthors are coinventors of electronic cleansing and computer-aided detection software patents assigned to their home institution, without associated royalties. Some of the study authors report various financial relationships with GE Healthcare, the American Cancer Society, and/or the National Institutes of Health. Complete disclosures of the study authors can be viewed at on the journal's Web site. Dr. Pickhardt is a consultant to Viatronix, Medicsight, Bracco, and Check-Cap and is co-founder of VirtuoCTC.

Ann Intern Med. 2012;156:692-702. Abstract