All people over 50 are encouraged to undergo a regular colonoscopy once every 10 years, or either a barium enema or a flexible sigmoidoscopy (with or without a yearly blood test) once every five years. The ACS does not deem a digital rectal exam to be a sufficient means of screening.
To compare some of these options, Pickhardt and his colleagues developed a mathematical model involving 100,000 patients with an average risk for colorectal cancer. They noted that 75 percent of the American population is subject to such risk.
In the computer simulation, all of these "patients" (over the age of 50) were screened for colon cancer once every decade for three decades, using either a standard colonoscopy, a flexible sigmoidoscopy, a virtual colonoscopy, or a combination thereof.
Polyp searches were based on one of two thresholds: those measuring 6 millimeters in diameter and up, and lesions of any size.
The model indicated that 2,940 patients would ultimately go on to develop colorectal cancer.
The simulation also revealed that flexible sigmoidoscopy screenings reduced the rate of cancer by just over 31 percent, while traditional colonoscopy reduced the rate by just over 40 percent.
Virtual colonoscopies were only slightly less efficient than the traditional method -- achieving an almost 38 percent reduction when polyps of all sizes were considered. The prevention rate dropped slightly, to 36.5 percent, when screenings focused only on polyps 6 millimeters and up.
Virtual colonoscopy also had the added benefit of dramatically reducing the need for unnecessary polyp removal. Nearly 78 percent fewer patients went on to have an invasive polyp removal after a virtual screening compared with patients who underwent a regular colonoscopy.
And when virtual screenings focused solely on lesions 6 millimeters and up nearly 12,900 additional unwarranted polyp removals were avoided.
In terms of both preventing cancer and minimizing cost, the use of any screening method was better than no screening at all, the study found. However, virtual colonoscopy with a 6-millimeter polyp diameter threshold was by far the most cost-effective approach: Costs were less than half that of traditional colonoscopy when broken down by year of life saved. Even with no polyp size threshold, virtual colonoscopy still came in at more than 20 percent cheaper.
However, Pickhardt noted that the high-tech procedure is not yet widely available.
"It's definitely ready for primetime, but people don't have it everywhere yet," he said. "The technology is getting better and better, and I can tell you that it will continue to improve. But most radiology practices aren't going to invest in the necessary software, because insurances aren't generally paying for it yet."
The ACS's director of cancer screening, Robert Smith, said his organization is taking a wait-and-see approach toward virtual colonoscopy. However, he believes the technology holds great promise.
"No organization recommends a virtual colonoscopy at this point in time, because the procedure is still regarded as experimental," he remarked. "It's still a work in progress. But I will say that as the evidence is accumulating, it appears as if it is meeting performance characteristics that -- if done well-- would make it an additional alternative to the currently recommended screening tests."
"Meanwhile," he added, "as always, the best screening for colon cancer is the one that you're willing to get, and the one that is done well."
For more about colorectal cancer screening options, visit the American Cancer Society.
SOURCES: Perry J. Pickhardt, M.D., radiologist and associate professor, school of medicine and public health, University of Wisconsin, Madison; Robert Smith, Ph.D., director of cancer screening, American Cancer Society, Atlanta; June 1, 2007, Cancer