Why are 6% of the patients who undergo a clean colonoscopy diagnosed with colon cancer 3 to 5 years later? Researchers at the Huntsman Cancer Institute at the University of Utah speculate that such "missed cancers" may have been overlooked during the procedure, but it's more likely that they emerged rapidly between screenings. And they've identified a surprising risk factor.
For their study, published online in the journal Gastroenterology, they reviewed data from nearly 127,000 colonoscopies performed between 1995 and 2009 as well as from statewide demographic and cancer databases.
"Not only did we find that colonoscopy isn't perfect, we discovered a number of factors associated with these 'missed' cancers," says lead researcher N. Jewel Samadder, MD, MSc.
Missed cancers were most frequently seen among patients over age 65, who had a history of polyps and of colorectal cancer in the family. But they were also more likely to emerge in the right side of the colon, the end of the line for the endoscope.
"Our first thought was that perhaps doctors did not view the entire colon, or that preparation for the procedure was not complete, which would obscure their view," Dr. Samadder noted. "However, the medical records of the patients with missed cancers showed these problems were seldom present." Instead, he points to a quirk of cancer biology.
"Cancers in the right side are often biologically different than those in other parts of the colon, arising from different types of polyps," he says. "These types of polyps are flatter and faster growing, which may explain why they are not seen during colonoscopy as well as how a cancer could develop even when no polyps were visible."