The findings aren't cause for alarm, or for making wholesale changes to federal screening guidelines, said Samadder. "Colonoscopy is still very good because 94 percent of the cancers were found."
But some patients may want to consider earlier and more frequent screening, he said.
The missed cancers tended to be in patients over age 65 and in those with a family history of colorectal cancer or in whom polyps had been previously found. Doctors often recommend that such patients start screening at age 40, instead of the recommended 50, and that they repeat screenings every three to five years, Samadder said.
The population-based study was published Wednesday in the journal Gastroenterology.
Researchers pulled records from 126,851 patients who underwent colonoscopies at the U. or an Intermountain Healthcare hospital between 1995 and 2009. The two health systems care for about 85 percent of Utah's population.
They identified patients who had developed cancers through the U.'s Utah Population Database, a treasure trove of medical and genealogical records, demographic data and cancer-registry information.
The next phase of research, said Samadder, will be to characterize the tumors that were missed "to see if there are any molecular features at the DNA level that makes them different."
The missed cancers were more likely to appear in the right side of the colon, at the far end of the scope'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."
Another possible factor: Cancers on the right side are often biologically different, arising from different types of polyps that are flatter and faster-growing, he said.
Colorectal cancer is the second leading cancer killer in the United States, affecting both men and women, according to the U.S. Centers for Disease Control and Prevention.
Funding for the study came from Huntsman Cancer Foundation, the National Cancer Institute, the American Society for Gastrointestinal Endoscopy and the American College of Gastroenterology.
Colon cancer decline
Colon cancer rates have dropped 30 percent in the past decade in people over age 50, largely due to routine screening, according to a report by the American Cancer Society.