THURSDAY, March 20, 2014 (HealthDay News) -- Colorectal cancer is missed in about 6 percent of colonoscopies, according to a new study.
"Not only did we find that colonoscopy isn't perfect, we discovered a number of factors associated with these 'missed' cancers," study lead author Dr. N. Jewel Samadder, of the Huntsman Cancer Institute at the University of Utah, said in a university news release.
"They tended to appear in patients over the age of 65, patients with a family history of colorectal cancer and patients in whom polyps were previously found," he said.
A colonoscopy involves threading a thin tube with a camera at the end into the rectum to check for any signs of cancer or precancerous polyps in the bowel.
Researchers analyzed data from patients who underwent colonoscopies in Utah between 1995 and 2009. About 6 percent of the patients who got a clean bill of health after their colonoscopy were diagnosed with colorectal cancer within three to five years.
These cancers could have been overlooked during colonoscopy or might have developed rapidly between colonoscopies, according to the study, which was published online March 20 in the journal Gastroenterology.
Colonoscopies to screen for colorectal cancer should be done every 10 years for people 50 and older in the general population, and every five years for those at increased risk, according to experts.
The researchers also found that many of the missed cancers occurred on the right side of the colon, which is at the far end of the colonoscope's reach.
"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," Samadder said. "However, the medical records of the patients with missed cancers showed these problems were seldom present."
Cancers on the right side of the colon are often biologically different than those in other parts of the colon, arising from different types of polyps, Samadder said. "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," he said.
The rate of colorectal cancers missed during colonoscopies in this study is slightly lower than what similar studies found in Canada and Germany.
"Only by understanding the limitations of colonoscopy can we improve its use and ability to detect polyps and thereby reduce the burden of colorectal cancer," Samadder said.
The U.S. National Cancer Institute has more about colorectal cancer screening.