Virtual Colonoscopy matches gold standard for diagnosis

CT colonography matches gold standard in eagerly awaited ACRIN trial results

Tens of millions of people eligible for colon cancer screening every year shy away from stool tests and the invasive colonoscopy exam. They could soon have another viable option: CT colonography.

CTC is comparable to the colonoscopy gold standard for intermediate to large adenomas, according to the eagerly awaited results of the American College of Radiology Imaging Network screening trial of more than 2500 patients.

The virtual imaging study had 90% sensitivity and 86% specificity for adenomas 1 cm or larger, according to the results released at the ACRIN 2007 fall meeting in Arlington, VA, on Sept. 28. Performance characteristics remained high in smaller sized polyps, with 84% sensitivity in lesions 7 mm or bigger. Specificity remained high (86% to 89%) across all relevant lesion sizes, said principal investigator Dr. C. Daniel Johnson.

CTC has proven efficacious in some leading centers for some time now, but patchy reimbursement and other obstacles have held the technique back. The results of a major study such as the National CT Colonography Trial (ACRIN 6664) could turn that situation around.

“It is reasonable to consider broader application of this relatively noninvasive imaging modality. Hopefully, this will enhance compliance with colorectal cancer screening guidelines,” said Johnson, a professor of radiology at the Mayo Clinic, Rochester, MN.

In the U.S., colorectal cancer is the third most common cancer and third leading cause of cancer death. Cancers start as polyps and are very slow-growing. Screening is recommended for men and women over 50, but fewer than 40% of the 70 million screening candidates actually comply. Consequently, only 37% of cases are diagnosed when disease is localized, which has a negative impact on treatment options and survival rates.

The ACRIN trial involved 15 centers and 2531 asymptomatic patients over age 50. Researchers used mechanical insufflation with CO2, stool tagging, and fluid tagging. Several multislice CT scanners were used, but all had at least 16 detector rows. Fifteen radiologist readers were randomly assigned to perform primary interpretations with either 2D or 3D software from a range of vendors. In almost all cases, colonoscopy was performed immediately after the CTC.

Overall, 547 polyps were detected in 390 patients. About two-thirds of the polyps detected were adenomatous. Mean polyp size was 8.9 mm, and 128 polyps one cm or larger were found in 109 patients, equivalent to 4.3% of the study group. Prevalence of lesions 6 mm or larger was low at 8.3%. Seven cancers over the size of 5 mm were detected.

“Results indicate most patients undergoing CTC would not need subsequent colonoscopy, sparing them cost, risk, and inconvenience of needing a second test,” Johnson said.

Researchers found the type of scanner or software did not influence outcomes. Nor did the type of primary interpretation: 2D or 3D. Success of research conducted by the University of Wisconsin’s Dr. Perry Pickhardt has been attributed to the primary 3D fly-through technique, but in fact, the 2D reads were as effective as primary 3D reads and required six minutes less interpretation time, Johnson said.

In releasing the results, Johnson attributed the trial’s success in part to reader training and certification. Participant radiologists had to either read 500 studies or undergo 1.5 days of training, including at least 50 cases. They also had to pass a certifying exam, which required detection of 90% of abnormal lesions 1 cm or larger. More than half of readers did not initially pass and needed additional training before they could become certified. Significant differences in performance were not demonstrated in the study.

“We think quality and competency standards need to be addressed soon so that good care is provided to patients,” Johnson said.

Other probable success factors were the use of stool and fluid tagging, mechanical insufflation, modern imaging techniques, and a very compliant patient group, he said.

Johnson stressed the need for multidisciplinary cooperation.

“Radiologists, primary-care doctors, and colonoscopists need to work together so those who do have lesions identified on CTC can go ahead and have the colonoscopy in the same day, obviating the need for a second bowel preparation,” he said.