SALT LAKE CITY — Not all colon cancers are recognized immediately following a colonoscopy, according to new research from the Huntsman Cancer Institute.
A Utah population-based study reveals that about 6 percent of colorectal cancers are diagnosed within three to five years after a patient gets a clean colonoscopy report.
"But that means that colonoscopies catch 94 percent of colon cancers," said Dr. Jewel Samadder, lead author of the study and a Huntsman Cancer Institute investigator. He said that "for a long time, the screening mechanism has been thought to be 100 percent effective at eliminating the risk of colon cancer, but there is a small risk within three to five years of getting a colonoscopy."
"Colonoscopy is very good, but it is not perfect," Samadder said.
The "missed" cancers are either overlooked at the time of the colonoscopy or develop rapidly before the next colonoscopy is ordered, which is typically between five and 10 years later.
The study, published online Thursday in the academic journal Gastroenterology, also helped researchers discover a number of factors associated with the missed cancers, Samadder said. He said the condition — involving possibly flatter and faster growing polyps that are unseen during a colonoscopy — arises most in patients over age 65, in patients with a family history of colorectal cancer and patients in whom polyps were found during previous colonoscopies.
The majority of missed cancers were also more likely to appear in the right side of the colon, 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."
The two-year study included results from nearly 127,000 colonoscopies performed at University of Utah Health Care and Intermountain Healthcare facilities between 1995 and 2009, as well as data from the Utah Population Database, which combines genealogical, medical and demographic data with cancer records from the Utah Cancer Registry.
The study is the largest in the U.S. to ever look at missed colon cancer diagnoses and was made possible largely because of Utah's record-keeping efforts.
Utah rates well for colonoscopy screening, with more than 60 percent of Utahns over age 50 having had some sort of procedure to determine colorectal cancer risk — slightly above the national average, according to the U.S. Centers for Disease Control and Prevention.
The American Cancer Society on Monday announced a goal to reach 80 percent screening rates by 2018.
The Huntsman discovery enhances the importance of doctors obtaining a complete medical history, accounting for older age, family history of colorectal cancer and prior history of polyps so more time can be spent examining patients with increased risk factors, Samadder said.
The American College of Gastroenterology recommends that physicians spend at least six to 10 minutes closely examining the colon lining for polyps during the procedure's withdrawal phase.
"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.
Samadder and his team are now looking into each case of the missed diagnoses to determine whether all colon cancers are the same or if they are somehow biologically different.
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