A less invasive method of screening for colon cancer could become more common, if results from a newly published clinical trial are borne out by further research. Doctors from the National Naval Medical Center report in the New England Journal of Medicine (Vol. 349, No. 23: 2191-2200) that virtual colonoscopy using computed tomography (CT) scans was just as effective as traditional optical colonoscopy at finding precancerous polyps in people at average risk of colon cancer.

"The study should be taken as evidence that this technology is maturing and has the potential to become a useful screening tool for average risk adults," said Robert Smith, PhD, director of screening for the American Cancer Society.

ACS does not yet recommend virtual colonoscopy as a screening test for this population, however, because there's still not enough evidence it works as well as traditional methods. But Smith said these new findings would be "evaluated very carefully" the next time colon cancer screening guidelines are updated.

Colorectal cancer is the second leading cause of cancer death in the United States, killing about 57,000 people each year. Yet this disease is highly preventable. Screening tests like colonoscopy and sigmoidoscopy can find polyps long before they become dangerous so that they can be removed.

The American Cancer Society recommends colon cancer screening begin at age 50 for adults who are at average risk of the disease, and younger for people with a family history or other risk factors. Screening options include a fecal occult blood test every year; flexible sigmoidoscopy every five years; flexible sigmoidoscopy every five years plus annual FOBT (this combination is preferred over either option alone); double-contrast barium enema every five years; or colonoscopy every 10 years.

But only about half of Americans who should get screened actually do get screened.

Increasing the number of people who get screened for colon cancer is an important goal of public health officials. Virtual colonoscopy is an attractive option, because unlike traditional colonoscopy, it requires no sedatives or recovery time, and takes less time to complete. In virtual colonoscopy, the colon is filled with air while a series of x-rays are taken, which a computer then assembles into a two- or three-dimensional image of the colon.

However, previous studies of this technique have not always shown that it works as well as conventional methods, especially in people who might not have many polyps – people at average risk of colon cancer who would typically undergo screening.

Lead researcher Perry Pickhardt, MD, and colleagues recruited more than 1,200 people aged 40-79 with no symptoms of colorectal cancer. The participants underwent the standard preparation for both tests, which includes laxatives and sticking to a clear-liquid diet to clear the colon, and drinking a solution of barium and other chemicals before the procedures.

The participants first were given virtual colonoscopy, then the traditional optical colonoscopy with an endoscope. After each segment of the conventional procedure was completed, the researchers revealed the results of the virtual CT to the technician, so that they could be compared.

Virtual colonoscopy was just as accurate as optical colonoscopy, and even found a few polyps that were missed during the traditional procedure.

The researchers attribute their better results to a more refined technique for virtual colonoscopy than was used in previous studies. In this study, the researchers used three-dimensional models of the colon instead of two-dimensional models, so they were able to get a better view of potential problem areas. They also used a more advanced type of CT scanner, and software that allowed them to "erase" any stray stool and fluid from the images, giving them a clearer view of the colon. The doctors reading the scans were also highly experienced.

Pickhardt and other experts acknowledge that there are still issues that must be resolved before virtual colonoscopy could become part of routine colon cancer screening.

For instance, virtual colonoscopy allows doctors only to view polyps; patients must still have the traditional procedure if a polyp is found and needs to be removed. There are not yet any guidelines on how large a polyp should be before a virtual colonoscopy patient is referred for traditional colonoscopy and polyp removal.

Another problem is that the CT techniques and software used in this study are not yet commonly available in screening centers. The experience of the doctors may also be a factor – less experienced doctors may not get the same good results as seen in this study.

Cost is also an issue. It is not clear how much more or less this type of advanced CT screening would cost, compared to traditional colonoscopy, or whether insurance companies would cover this procedure as a screening method.

Despite these questions, Pickhardt calls virtual colonoscopy a "promising tool" for colorectal cancer screening. Writing in an editorial accompanying the study, two experts from Harvard Medical School agree. They say that if questions about follow-up can be resolved, and good results are seen in future studies, virtual colonoscopy will be "ready for prime time."