Considering a Colon Scan

A thumbs up for virtual colonoscopy expands the options

Revealed by CT scan, a colon polyp appears in 3-D.

Revealed by CT scan, a colon polyp appears in 3-D.

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It's a scary pair of statistics: Nearly 150,000 Americans are expected to be diagnosed with colorectal cancer this year, and almost 50,000 are likely to die from it. Screening tests can nip precancerous polyps in the bud, saving lives. But for whatever reason—the ick factor, inconvenience, fear of the results—less than half the eligible population gets any kind of screening. Now that two more options have gotten the stamp of approval from a coalition of medical groups including the American Cancer Society, doctors hope to see a jump in screening rates. The latest guidelines, issued earlier this month, endorse a new type of annual stool test that looks for DNA shed from tumors as well as the so-called virtual colonoscopy, which is actually a CT scan of the lower part of the digestive tract. (The new guidelines are for average-risk Americans age 50 and over; if you have a family history of colon cancer, a personal history of polyps, or other risk factors, check with a doctor.)

With the new imprimatur, virtual colonoscopy is likely to get a higher profile. By now, everyone knows what a traditional colonoscopy is, and swapping that for a procedure that is less invasive, doesn't require sedation, and carries almost no risk of serious complications sounds like a no-brainer. Both tests share a common—and important—quality: Rather than screening only for the earliest signs of cancer, like the handful of stool tests, they identify polyps that might turn cancerous years down the line. "Colon cancer is somewhat unique in that it can be prevented," says David Weinberg, a gastroenterologist who chairs the department of medicine at the Fox Chase Cancer Center in Philadelphia. But as promising as virtual colonoscopy is, no single test is perfect for everyone. Here's what you need to know:

It requires a prep. Those who have never had a regular colonoscopy fear the long tube. Those who have say the worst part is actually the bowel-cleansing prep undertaken the day before. The laxatives and clear liquid diet are also required for a virtual colonoscopy. And while there's no long tube, there is a short one inserted into the rectum to pump air into the bowel.

It may miss some smaller polyps. Virtual colonoscopy is just as good as the regular kind for detecting the mushroomlike polyps of 1 centimeter or larger that are most likely to turn cancerous, according to a study published last fall in the New England Journal of Medicine. But "when you get to the intermediate size of 6 to 9 millimeters, [detection] drops off modestly, and below 6 millimeters, it drops off sharply," says Sidney Winawer, an attending gastroenterologist at Memorial Sloan-Kettering Cancer Center and a coauthor of the new guidelines. There is only a tiny chance that those smaller polyps will become cancerous in the years before your next colonoscopy, however, so the risk of missing them isn't huge. Flat or depressed lesions can also go undetected by virtual colonoscopy, according to researchers who recently reported in the Journal of the American Medical Association that those growths may be more common and more dangerous than previously thought.

You will need it more often. The ideal time interval between virtual tests hasn't been determined, so the new guidelines say that if no polyp larger than 5 millimeters is found, it's "reasonable" to wait five years until your next one. That compares to 10 years in between negative colonoscopies. Each CT scan exposes you to a small amount of radiation, but it isn't likely to be a health hazard—unless perhaps you're also exposed to radiation for some other reason.

If midsize polyps are found, you have to consider a follow-up procedure. During regular colonoscopy, doctors tend to snip out any polyp they see, using an appendage built into their scope. That's not possible during a virtual test, so removal requires follow-up with a conventional colonoscopy. Deciding whether one is needed can be tough, since it's not clear what to do with polyps between 6 and 9 millimeters, which may be safe to monitor over time. Most of them are not going to develop into cancer, says Robert Smith, director of cancer screening at the American Cancer Society. "But right now we're leaning toward playing it safe," he says, which means a referral to a gastroenterologist. At the University of Wisconsin-Madison, where patients are offered their choice of virtual or regular colonoscopy, finding midsize polyps means you can either get a regular colonoscopy or come back for another CT scan in one or two years (the recommended interval depends on the size of the polyps), to see if they've grown, says David Kim, a UW radiologist who performs the scans and coauthored last year's NEJM study.