November 17, 2008
Virtual Colonoscopy — On Its Way to Being a Real Screening Tool?
By Beth W. Orenstein
Vol. 9 No. 23 P. 22
A new ACRIN study found that CT colonography is as sensitive as optical colonoscopy. Experts discuss how its use may evolve.
The results of the American College of Radiology Imaging Network (ACRIN) National CT Colonography Trial published in September in The New England Journal of Medicine confirm that CT colonography (CTC), widely known as virtual colonoscopy, is as sensitive for detecting polyps and cancers as traditional colonoscopy.
Standard colonoscopy uses a long, flexible tube with a camera to view the lining of the colon to detect cancer and precancerous polyps and has long been the gold standard for screening for colorectal cancer, the third most frequently diagnosed cancer and the second leading cause of cancer death in the United States. Optical colonoscopies are an invasive procedure performed by gastroenterologists while a patient is under sedation.
CTC employs virtual reality technology to produce a 3D visualization that permits a minimally invasive evaluation of the entire colon and rectum. While the patient must prepare for a CTC as he or she would an optical colonoscopy by cleansing the colon, the procedure requires no sedation and only takes up to 15 minutes. CTC also has a lower chance of complications such as colon perforation.
Health professionals wonder whether more people would undergo colon cancer screening if they had the option of virtual colonoscopy. The American Cancer Society recommends that adults aged 50 and older undergo a colonoscopy every 10 years, or more frequently depending on known risk factors. While the benefits of such screening have been well established, studies show that the majority of Americans aged 50 and older are not being screened for the disease. Consequently, only 37% of cases are diagnosed when the disease is localized.
“Currently, compliance is very poor,” says David Kim, MD, an assistant professor of abdominal imaging in the department of radiology at the University of Wisconsin-Madison (UW), who has extensive experience in using the virtual colonoscopy technology. “The hope is that if people are given the option of a noninvasive procedure where they could drive themselves and return to their normal activities immediately afterward, it would increase the number of people who would agree to be screened.”
If radiologists were to perform CTC, it would take some burden off gastroenterologists, Kim says. There is concern about whether the colonoscopic capacity in the United States could screen the large number of eligible individuals. “There are only about 13,000 gastroenterologists in this country, and how many can they reasonably perform?” Kim says.
On the other hand, CTC has no capability for polyp removal. If any polyps are found that need further investigation, the patient has to undergo a conventional colonoscopy to remove them, which means another prep and another medical procedure. Some may argue that this drawback limits the usefulness of CTC. However, Kim says, only eight of every 100 people need to be referred for polyp removal in the UW’s experience, which consisted of more than 5,000 screened patients. “That means for the vast majority of people, you can stop with CTC,” he says.
Kim says CTC is a cost-effective way of screening more patients and could be good news for gastroenterologists. “We’ve found that we are actually increasing their volume rather than taking away patients,” Kim says. “We believe that we are pulling people off the screening sidelines rather than having them change from screening by colonoscopy. With the additional people sent for polyp removal, numbers have gone up at UW.”
One ACRIN investigator, Judy Yee, MD, an associate professor and vice chair of radiology at the University of California, San Francisco and chief of radiology at San Francisco VA Medical Center, agrees that when a patient has no risk factors (no family history of colon cancer or polyps) the less-invasive and less-costly CTC, which has proven to be equally effective, could be offered.
“The goal is to offer a screening test that is more appealing and less invasive to people at lower risk and to send those patients who are most likely to have a lesion that would need to come out to the gastroenterologist,” she says.
Yee also points out that traditional colonoscopy only sees the inside of the colon, while CTC provides images of the entire abdomen and pelvic region. Thus, CTC could detect early cancers, including of the kidney and lung, as well as abdominal aneurysms that a traditional colonoscopy could not have located. Patients face a much higher survival rate when asymptomatic abdominal cancers are detected early by CTC, she says.
Some gastroenterologists have expressed interest in learning to read and interpret CTC. Kim and Yee believe that those who are interested, no matter their specialty, could be trained to do so. The 3D images from CTC are very similar to what the gastroenterologist sees during an optical colonoscopy, Yee says. “They mimic each other,” she explains.
However, Yee adds, gastroenterologists are at a disadvantage when it comes to interpreting the 2D portion of the exam. Unlike radiologists, she says, “Gastroenterologists are not trained to interpret 2D images, and 2D assessments are critical for proper evaluation of CTC. The big question that comes in is, can a gastroenterologist be trained to read both 2D and 3D? I think there will be a subset of gastroenterologists who would have the interest and the ability to do this, but it obviously would require additional training above and beyond what a radiologist has [to learn to interpret CTC]. I also think there would have to be fairly rigorous and ongoing quality assessments for those who are interpreting CTCs.”
Kim agrees: “There are aspects of the CTC that look like endoscopies, and that’s why people call it virtual colonoscopy. But these views are simply postprocessed data that help you detect a possible polyp.” He explains that the true core of interpreting the exam involves taking the list of possible polyps that are seen on the endoluminal views and examining them on the 2D images to determine what truly represents a polyp. “This requires a skill set that you use for CT,” he says. “If you don’t have this cross-sectional skill set, poor quality interpretations result. Gastroenterologists can certainly learn these radiology skills, but it requires dedicated effort and training. It is somewhat naïve to think you can read CT colonography without cross-sectional skills.”
A Gastroenterologist’s View
Paul J. Limburg, MD, MPH, of the Mayo Clinic in Rochester, Minn., is a gastroenterologist and coauthor of the ACRIN study. Theoretically, he says, if CTC can be done efficiently and effectively and with the appropriate quality assurances “just like we have for colonoscopies, it should provide for greater access for colorectal cancer screening, and that is a positive.”
However, he says, while little data are available, the option of CTC does not seem to change the number of people who seek screening, whether virtually or traditionally. “People have argued both sides of that,” he says, “but a study done at the University of Wisconsin in Madison found that when CTC and optical colonoscopy were both available to patients, meaning that insurance coverage was not a differential barrier, the number of [optical] colonoscopies performed did not significantly change up or down.”
Limburg says it’s hard to predict what may happen should CTC become more widely available. “The truth is that the jury is still out,” he says.
Limburg also says that while CTC has the ability to detect additional findings, the downside is that “sometimes you pick up things that are of uncertain clinical significance and they require additional follow-up, and that can be problematic from a resource utilization standpoint.”
Still, he says, CTC may represent an attractive screening option for many patients and providers. “I do think that CTC surpasses the threshold set by the American Cancer Society and other organizations for use as a primary screening tool.”
The goal, Limburg says, “should be to shift our way of thinking and say, ‘All these different options have some advantages and disadvantages. How can we apply them and integrate them with the ultimate goal being to meet patients’ needs?’”
The radiologists believe that reimbursement will eventually decide how quickly, how often, and by whom CTC is performed in place of or in addition to optical colonscopies. Currently, CTC is not reimbursed as a screening exam, but that possibility is pending on several fronts, Kim says.
The most promising is that the Centers for Medicare & Medicaid Services has initiated a national coverage analysis for CTC, Kim says. It has helped that CTC has been endorsed by the ACR and the American Gastroenterological Association. “These are major societies, and they have all endorsed it to be reimbursed,” he says. The analysis is expected to end sometime this year, and the determination should be made in the spring. “Hopefully it will be positive and screening CTC will be reimbursed within a year or two.”
A bill also has been introduced in Congress that would require third-party payers to reimburse for CTC. However, Kim says, “That bill in an election year is unlikely to go anywhere.”
Still another possibility is changing the coding of the exam from a category 3 to a category 1, which would make it eligible for reimbursement, Kim says. That, too, is a possibility not likely to happen soon, he says.
The radiologists also believe that when researchers are able to devise a CTC test that does not require colon preparation, patients will be more inclined to choose that screening option. A no-prep CTC is not that far off, they say.
Yee says that unlike breast cancer screening, which is meant to find lesions early in their most treatable stage, screening for colon cancer can actually prevent the disease. Most colon cancers develop from polyps, she says, and screening to find and remove these polyps will save lives.
The ACRIN trial, sponsored by the National Cancer Institute, enrolled more than 2,500 patients at 15 sites nationwide. It is the largest multicenter study to compare the accuracy of state-of-the-art CTC with conventional colonoscopy.
The CTC findings were evaluated using standard colonoscopy as the reference standard. CTC was found to be highly accurate for the detection of intermediate and large polyps. Ninety percent of the polyps that were 1 centimeter or larger were detected by CTC, a sensitivity that is equal to optical colonoscopy. Even polyps as small as 0.5 centimeters were detected by CTC with a high degree of accuracy.
Participants were aged at least 50, scheduled for a screening colonoscopy, and had not undergone a colonoscopy in the previous five years. Each participant had a CTC exam followed by a colonoscopy, with 99% of both accomplished on the same day.
— Beth W. Orenstein is a freelance medical writer and a regular contributor to Radiology Today. She writes from her home in Northampton, Pa.