A new method of performing virtual colonoscopy using a CT scan - which does not involve the dreaded laxative preparation to clear the colon the night before - may be about as effective as a standard colonoscopy at identifying the large polyps most likely to become cancerous, according to research conducted at Massachusetts General Hospital, Brigham and Women’s Hospital, and elsewhere.
If larger studies confirm the finding, the technique could eventually serve as a first-line screening tool for colon cancer, especially for the many people who avoid screening altogether.
The new technique works by using a contrast agent - a tiny amount of dye is ingested two days before the test - to highlight fecal matter in the colon, which can then be digitally erased from the scan using a software program, making it easier to see polyps.
In the study of 605 patients, published Monday in the Annals of Internal Medicine, the virtual colonoscopy was able to identify more than 90 percent of suspicious polyps that were 10 millimeters or larger, compared with 95 percent identified when the same patients later had a standard colonoscopy.
The research was partially funded by GE Healthcare, maker of the CT imaging device.
“Study participants reported an improved level of comfort with the new technique,’’ said study author Dr. Michael Zalis, a radiologist at Massachusetts General Hospital, primarily because it did not involve the diarrhea-inducing prep that keeps most people in close range of a bathroom the night before a colonoscopy. Of the study participants who expressed a preference, 62 percent said they preferred the prep-free CT procedure over the colonoscopy.
Only one in six Americans over age 50 get the American Cancer Society’s recommended screening for colon cancer: a colonoscopy every 10 years, or a flexible sigmoidoscopy, virtual colonoscopy, or barium enema imaging every five years. Surveys suggest that patients find the preparation - required for both colonoscopy and traditional virtual colonoscopy - to be the worst part of screening.
“A number of folks, who currently aren’t being screened because they fear the prep or aren’t willing to go through it again, may be willing to have screening if they’re told they don’t need to take laxatives and can still get a potent test,’’ said Dr. Durado Brooks, director of prostate and colon cancer for the American Cancer Society.
About 49,000 Americans die every year from colon cancer, added Brooks, and at least half of those deaths could be prevented if everyone followed the screening recommendations.
Virtual colonoscopy, with or without prep, has limitations. One in five patients must undergo a standard colonoscopy after the imaging to have suspicious polyps snipped off and biopsied. The CT scan also delivers a dose of radiation that while only one-fifth of the dose of an abdominal CT scan was significant enough to raise red flags at the US Preventive Services Task Force.
The government advisory group decided in 2008 not to recommend the use of virtual colonoscopy, saying the evidence was not sufficient to prove the benefits outweighed the harm, which includes a dose of radiation with every screening. The scan also images areas outside the colon, sometimes detecting lesions that warrant further testing to ascertain whether they are cancerous. This can lead to unnecessary procedures when the lesion turns out to be harmless.
Medicare does not provide coverage for the virtual colonoscopy, which Zalis said costs about one-third as much as the traditional scope screening.
Traditional colonoscopy trumps the virtual test when it comes to detecting smaller polyps of less than 10 millimeters. The new study found that virtual colonoscopy detected 70 percent of polyps under 8 millimeters, compared with 88 percent detected by a scope.
What is not clear is whether missing smaller polyps will make a difference in terms of catching growths before they turn into cancer.