A new government analysis of CT scanning to detect pre-cancerous colon polyps says giving the imaging procedure once every five years to healthy older adults would wind up costing more and be no more effective than giving them a colonoscopy once a decade.

The Agency for Healthcare Quality and Research report, released earlier this week, said the cost of the test would have to be cut to $108 to $205 per scan to compete with other colorectal cancer screening technologies, which include not just colonoscopies but annual fecal blood tests. The average cost of lower body CT scans was $644, according to the report, while colonoscopies cost $979.

The report sets the stage for next week's meeting of the Medicare Evidence Development and Coverage Advisory Committee, which will recommend whether the nation's financially troubled senior citizen health care program should pay for the pricey technology.

The meeting could get heated as radiologists and gastroenterologists square off over who should get first crack at aging baby boomers worried about contracting this second-leading cause of cancer. Colon cancer strikes an estimated 149,000 Americans each year and causes about 50,000 deaths.

Earlier this year, the American Cancer Society added CT scanning and DNA stool testing -- another technology that is far more expensive than existing technologies -- to its list of recommended cancer screening tests. The move set off grumbling among many prevention experts, and the U.S. Preventive Services Task Force refused to endorse CT scanning or DNA stool testing when it released its latest colon cancer screening guidelines last month.

Why wasn't it cost effective, according to this latest report? Simple. If the scan spots a polyp -- most of which are benign -- the patient will be scheduled for a colonoscopy anyway to have them removed.

"At first it may seem surprising that CT colonography, based on the best evidence available to date, was not cost-effective when compared with the other (colorectal cancer) screening tests," the report said. "However, the strategy of CT colonography screening is not a single test but a two-step procedure with those with 6 mm or larger polyps referred to optical colonoscopy."

The report also noted that two major claims for using CT scans instead of colonoscopies -- that it is less risky and would trigger greater compliance (only about half the eligible population gets existing screening tests) -- are unproven and based on questionable assumptions. "The long-terms effects of cumulative exposure to radiation that would be associated with interval screening with CT colonography are unknown," the report said. "In addition, concern for radiation risk on part of patient or physician could affect willingness to adhere to CTC screening.

The report was also skeptical about claims that people would be more willing to undergo scans because they don't entail the pre-colonoscopy bowel cleansing ritual (as someone who has already gotten two colonoscopies, I can assure you that it isn't fun). "The patient impression is often that CT colonography is 'virtual' and non-invasive . . . (but preparations for the procedure) involve both dietary restriction over a number of days and ingestion of various oral contrast agent(s)."

The AHRQ draft assessment, although negative, may have been too optimistic. It relied on the benchmark study of CT screening for colon cancer conducted by Perry Pickhardt of the University of Wisconsin. An article in the latest issue of Health Affairs pointed out that Pickhardt's study recorded an almost perfect record in identifying polyps because the trial "included an especially meticulous bowel preparation" and the images were "interpreted by well-trained top academic specialists."

When the technology was tested in community settings, the results were "markedly inferior," Steven Pearson, president of the Institute for Clinical and Economic Review at Massachusetts General Hospital and lead author of the article, wrote. "Faced with significant variation in diagnostic accuracy across these major studies, no public or private insurer moved to cover CTC for screening purposes."

Medicare operates under a legislative standard that says it must pay for health care that is "reasonable and necessary." There is no doubt that CT scans for colonoscopy can be an effective screening tool. But at current prices, this latest report suggests it is the more expensive alternative.

In the past, Medicare has said that "too expensive" is not a reason to dismiss a new technology as neither reasonable nor necessary, even when there are less expensive and equally effective alternatives available. It will be interesting to see if that definition still holds as the nation enters a new, more cost-conscious era.