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ACRIN trial shows Virtual Colonoscopy ready for widespread use

Trial results unveiled this month (October, 2007) marked the apparent end of a long road to validation for virtual colonoscopy (VC or CT colonography [CTC]), a radiology-based colon screening exam whose advocates have toiled for more than a decade to show equivalent detection sensitivity in a large screening trial compared to more invasive optical colonoscopy.

Preliminary results of the National CT Colonography Trial a study funded by the National Institutes of Health (NIH) and performed on 2,531 participants in 15 U.S. centers, yielded an impressive per-patient sensitivity of 90% for adenomatous colorectal lesions 1 cm or larger in diameter, a sensitivity on par with that of optical colonoscopy.

Reported principal investigator Dr. C. Daniel Johnson from the Mayo Clinic in Rochester, MN, only 8.3% of trial participants would have proceeded to same-day optical colonoscopy for removal of polyps 6 mm or larger, a referral rate low enough to suggest that VC will not be too expensive an alternative for routine use in colorectal cancer screening.

"Colon cancer is the second most common cancer killer in the U.S., affecting one in 18 individuals in the United States," Johnson said, adding the latest grim statistics: 145,290 new cases in 2007, and an estimated 73,470 deaths. Yet only one-third to one-half of the 70 million people eligible to screen in the U.S. ever get tested. It is hoped that the addition of a new alternative will encourage more individuals over 50 years of age to get screened, though as in optical colonoscopy, most VC patients will still have to undergo cathartic bowel cleansing before screening.

"From a patient perspective, once the colon's prepped, it's pretty easy, just two breath-holds in the prone and supine position, and then the examination is done on the image data rather than on the patient," said Johnson, a professor of radiology at the Mayo Medical School in Rochester, MN. He presented the first results of the trial at the 2007 ACRIN (American College of Radiology Imaging Network) fall meeting.

The efficacy of colorectal cancer screening has already been proved because it can detect precursor lesions in the colon long before they progress to cancer, Johnson said.A study by Dr Perry Pickhardt demonstrated 94% sensitivity and 96% sensitivity for significant colorectal lesions in more than 1,100 asymptomatic subjects in 2003, subsequent studies by Cotton et al and Rocky et al yielded sensitivities as low as 55% and 59%, respectively, for clinically significant polyps. These later results cast serious doubt on the robustness of VC as a screening method, though many radiologists maintained that poor study design and lack of training were the main problems.

"The aim of this (ACRIN) study was to evaluate the sensitivity of CT colonography for detecting participants with at least one adenoma a centimeter or larger using colonoscopy as the reference standard," Johnson said.

The 15 U.S. sites included both academic centers and private practices, which recruited 2,600 asymptomatic outpatients who were scheduled for optical colonoscopy screening. Those with symptoms suggestive of an elevated risk of colon cancer, such as blood in stool, abdominal pain, or family or personal history of colorectal polyps, were excluded from the study.

Training was an important component of the study, with VC readers obligated to have read at least 500 cases, or attend a 1.5-day training course. And all had to pass a certified exam in which they detected at least 90% of the adenomas 1 cm or larger in 50 cases, Johnson said.

"It's interesting to note that more than half of the readers had to undergo additional training in order to pass the certified exam initially, and with additional training, all the readers eventually passed," he said.

"I think we can say that CT colonography is similar to the performance of colonoscopy for large adenomas 1 cm or larger, as well as those intermediate adenomas 5-10 mm in diameter," Johnson said. "And I think it's reasonable to consider broader application of this relatively noninvasive imaging modality, which hopefully will enhance compliance with colorectal cancer screening guidelines. The prevalence of those adenomas 6 mm or larger was 8.3%; this would indicate that most patients undergoing CT colonography wouldn't need colonoscopy, sparing them the cost, risk, and inconvenience of that second test."

The contributions of reader training and advanced techniques in the success of the trial cannot be underestimated, Johnson said. The success in the past has been attributed to the primary 3D reading technique, but the results showed that primary 2D can be as accurate. In addition, the study was performed with a low-dose CT technique yielding a total dose of about 5 mSv per exam, an amount the Health Physics Society considers a risk that is either nonexistent or "too small to be measured," he said.

Johnson said he expects the information to be of interest to the health agencies that define the tests that are included in colorectal screening guidelines, and called on the primary care and gastroenterology communities to work together with radiologists to prepare for the wider implementation of CT colonography for screening.

"I think the results show a remarkable amount of thoughtful effort" on the part of the researchers, commented Dr. Elizabeth McFarland, adjunct professor of radiology at the Mallinckrodt Institute of Radiology at Washington University in St. Louis. What matters from this point forward is "how we disseminate this (modality) in a way that we maintain high standards and make it feasible for people to do out in the community," she said.

"Let's all agree that the validation phase is over, and let's work on widespread screening," said Dr. Perry Pickhardt, an associate professor of radiology at the University of Wisconsin in Madison, in a telephone interview with

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