There’s no question that colonoscopy can save lives by detecting colon cancer early, but there are a couple of very good reasons why so few people actually get screened — namely, to get the test, you have to clean out your colon with a laxative and endure having a scope inserted through your rectum.
Virtual colonoscopy, which uses CT scanning technology to take a non-invasive picture of the colon, is an attractive alternative to getting probed, but it still requires a pre-test laxative cleansing the night before the exam. But now researchers at the Massachusetts General Hospital (MGH) say they have developed computer software that can digitally “cleanse” a virtual colonoscopy test of feces and leave behind only a relatively accurate picture of the colon wall and any potentially cancerous growths. No laxative, no probing — and a high degree of accuracy.
The laxative-free approach resulted in detection of 91% of patients who had one or more of the riskiest lesions that were 10 mm or larger, according to Dr. Michael Zalis, director of CT colonography at MGH and lead author of the study, which was published in the Annals of Internal Medicine. “The level of performance was very comparable to CT [virtual] colonography as well optical [scope-based] colonoscopy,” he says.
The accuracy of the laxative-free exam is a promising sign that screening people may eventually not require the invasive, uncomfortable standard colonoscopy procedure. The participants in the study were not at especially high risk of developing colon cancer; they agreed to undergo a virtual colonoscopy using the laxative-free method, as well as a standard, invasive colonoscopy several weeks later, and the researchers then compared the number of polyps labelled by each method. To prepare for the laxative-free screen, 605 men and women between ages 50 and 85 ate a low-fiber diet for two days prior to coming in for the virtual colonoscopy, and at the same time drank small amounts (about the size of a condiment packet) of contrast agent dissolved in their food or drink three times a day. The contrast agent then selectively tagged only digested food or drink and feces in the stomach or colon, leaving all other tissues — and possible polyps — untouched. The CT scan itself took about 15 to 20 minutes, and patients were not sedated and could return to work after the test (their only complaint: a feeling of fullness from having air pumped into the colon). The standard colonoscopy that occurred several weeks later required patients to fast the night before the test and drink a laxative that flushed out the colon over a period of several hours.
Following the laxative-free CT-based colonoscopy, Zalis and his colleagues then applied software that they developed to analyze the scan and digitally remove the tagged elements, leaving behind a “cleansed” colon for radiologists to examine for suspicious lesions.
“We are really very strongly encouraged to achieve this level of performance this early in the study of the laxative-free technique,” he says. “The fact that we could achieve similar performance to CT colonoscopy without a laxative is a real bonus and addresses an important barrier to participation in screening.”
Of course, even a laxative-free virtual colonoscopy has one disadvantage compared to the more invasive test: because the scope-based exam also includes instruments that allow doctors to remove suspicious growths as they see them, it serves as a screen and a treatment combined into one. If physicians see lesions on a virtual colonoscopy, the patient then needs to return for a scope-based colonoscopy to remove the lesions. But, says Zalis, the percentage of patients who require such treatment is generally small; expert groups recommend that only lesions that are 6 mm or larger need to be removed. About 85% of people getting screened for colon cancer have polyps that are smaller and not likely to be aggressive enough to warrant any action. These people can rely on virtual colonoscopy screening every five to 10 years and don’t need to go through the extensive preparation and expose themselves to the small risk of infection and perforation that probe-based colonoscopy carries.
Now, Zalis hopes that other research groups will validate his results. That would justify making the strategy available to more patients. Minus the laxative and discomfort of being probed with a scope, more people just might be willing to get screened.