Virtual relief?

Patients dread colonscopies. So what are the arguments for a less-invasive approach that uses X-ray technology?

In the pantheon of medical screening tests, patients reserve a particular place of dread for the colonoscopy. Faces blanch. Eyes narrow. A root canal suddenly seems appealing.

"Americans in particular are just very squeamish about dealing with colorectal issues," said Dr. Sapna Syngal, a gastrointestinal cancer specialist at Dana-Farber/Brigham and Women's Cancer Center. Not only do they avoid the screening, they avoid the topic. "You wouldn't talk about it at the cocktail party," she said.

Unless, of course, it's to tell your middle-age pals that they could avoid some of the poking and probing of a traditional colonoscopy by opting for a virtual version of the test - a high-tech X-ray of the belly that is designed to provide identical information.

After years of study, virtual colonoscopies appear closer than ever to widespread adoption, with a federal advisory panel scheduled later this month to review evidence on the test, a review that could prove pivotal as health plans decide whether to pay for the screening.

There's widespread agreement that virtual colonoscopies track down the most dangerous polyps - the harbingers of cancer - just as well as the more-invasive test.

"I may get tarred and feathered for this, but I'm actually pretty enthusiastic about the potential" of virtual colonoscopy, said Dr. Patrick Lynch, a gastroenterologist at M.D. Anderson Cancer Center in Houston. "It has tremendous promise as a screening tool, so long as a few ifs, ands, or buts are taken into account."

Two major concerns linger. One is the exposure to radiation that comes with any X-ray. Another is whether the extra images generated by the virtual test - pictures of the kidney, liver, and pancreas - could lead to unnecessary testing.

But if the virtual colonoscopy prompts more people to be screened, those concerns may be offset. Colon cancer kills more Americans (an estimated 50,000 this year) than any other form of the disease, except for lung cancer. Yet barely half of the patients who doctors say should be screened - essentially, anyone 50 or older - submit to testing.

"We have a huge untapped, unscreened source of patients," said Dr. Judy Yee, a radiologist at the University of California, San Francisco. "We have to keep our eye on the ball, which is preventing colon cancer in Americans."

All screening methods for colon cancer, including comparatively primitive stool tests, share the same premise: find and remove polyps before they turn malignant.

There has never been a gold-standard study proving that colonoscopy extends lives; that would require not testing a comparison group, which some see as unethical. But "it has been very strongly demonstrated that if you take out polyps, you reduce the risk of colon cancer," said Dr. David Forcione, a Massachusetts General Hospital gastroenterologist.

For decades, that has meant that as Americans crest 50, they're offered two things: an AARP membership and a colonoscopy.

Before the colon cancer test, patients quaff a drink designed to empty the intestines. For many, the worst part is drinking the foul fluid, and the diarrhea that follows.

To perform the test, a doctor snakes a thin, flexible tube through the twisting highways of the colon. A Lilliputian camera is attached to the end of the tube to look for evidence of cancer or polyps. Anything that can become cancerous is removed.

Given the contorted geography of the intestines, the tube can perforate the colon. Studies estimate that happens to as many as 2 of every 1,000 patients.

"I think that's another reason people are hesitant about having a colonoscopy," Syngal said.

Another reason: the sedation given to patients to keep them still. "Sedation is not trivial," said Dr. Carl Jaffe, of the National Cancer Institute - in part because it means groggy patients need help getting home.

Given those drawbacks, there was considerable interest in developing an alternative. Enter the virtual colonoscopy, known in medical lingo as computed tomographic (CT) colonography.

One recent afternoon, Dr. Michael Zalis sat in a darkened suite at Mass. General packed with dozens of flickering computer monitors. With hands that navigate the computer like a piano keyboard, Zalis, a radiologist, reviewed images from a recent virtual colonoscopy.

First, two-dimensional black-and-white snapshots of the patient's mid-section appear. Then, a computer program assembles those images into a three-dimensional "ride" through the flesh-hued cave of the intestines, as Zalis looks for signs of trouble.

Nothing, nothing, nothing - until he reaches one spot where there's a glowing green blob.

Could it be a polyp? By rotating the image, he determines it is simply fecal matter.

If something suspicious does appear on the scan - and specialists said that happens in about 10 to 15 percent of cases - the patient would need a regular colonoscopy to confirm the result and, potentially, remove polyps. That's not such a big deal if it can all be done on the same day. But if not, the patient has to endure another bowel-cleansing episode.

"This may seem minor, but if you put yourself in the position of the patient, it's not easy," said Lynch, the Houston gastroenterologist.

That inconvenience aside, the virtual test is nearly as reliable as the traditional method in detecting the polyps most likely to threaten the health of patients, researchers reported in the New England Journal of Medicine.

But the test's accuracy diminished with the size of polyps. "And the question you can argue over and over again is, are these small polyps significant? Do they need to be taken out?" Forcione said. There is, specialists said, no agreement on those points.

An independent panel of scientists that reviews evidence on disease prevention said last month that it remains undecided on whether virtual colonoscopy should be used routinely. Specifically, the US Preventive Services Task Force said it could not find evidence to erase worries about the potential for unnecessary testing and radiation exposure.

Zalis dismissed the first concern; radiologists usually can distinguish what's worrisome on other organs, he said, meaning few patients require further review. As for radiation, specialists said exposure is less of a concern for adult patients. And the radiation amount is so slight, said Zalis, as to pose "zero harm."

Ron Dow was 71 when he signed up for his first colon screening. When researchers at Mayo Clinic's Arizona outpost asked if he would like one of the newfangled tests, he said sure. The scan showed something suspicious. A traditional test was done and, later, a cancerous growth was removed.

All things considered, he preferred the virtual colonoscopy. "If nothing shows up, you're not going to go any further - it can end right there," he said.

Stephen Smith can be reached at stsmith@globe.com.