TORONTO - The risk of getting cancer in the upper colon is greater than in the lower colon in the years after a negative colonoscopy, a new study suggests.
The research from Ontario's Institute for Clinical Evaluative Sciences involved examining patient records for more than 110,000 people age 50 to 80 who had a negative colonoscopy in the mid-1990s, and following them for 14 years.
Dr. Linda Rabeneck, a gastroenterologist and the lead ICES investigator, embarked on the study because she and colleagues, based on their observations in practice, had the impression that having a negative colonoscopy doesn't mean you never get colon cancer.
"We were particularly wondering whether the protective benefit, if you will, of having a negative colonoscopy was equal in the lower colon compared with the upper colon," said Rabeneck, a professor of medicine at the University of Toronto.
In the followup period, colorectal cancer was diagnosed in 1,461 people.
Researchers found the risk of colorectal cancer in the lower colon was reduced when patients were compared with the general population who hadn't had a colonoscopy. A reduction in the incidence of cancer in the upper colon was mainly seen in the second part of the 14-year period.
The study was published Thursday in the journal Clinical Gastroenterology and Hepatology, along with a commentary by Dr. Robert Sandler of the University of North Carolina at Chapel Hill.
"I think it's an important study, and it's important for two reasons," he said in an interview.
"It once again shows that colonoscopy can keep people from getting cancer," he said.
"At the same time I think the study shows that colonoscopy is not perfect."
There are a number of reasons colonoscopy wasn't as accurate, perhaps, for the upper part of the colon, he said.
"It could be that some lesions were missed. Sometimes polyps in the proximal (upper) colon are flat and they're difficult to see," he said.
"Another reason is it's hard to clean out the colon perfectly for a colonoscopy, and the preparation might be sub-optimal in the proximal colon."
"Another reason is perhaps the tumours in the proximal colon are different than ones in the distal (lower) colon and grow more rapidly -- although the test was negative, they developed a short time after the colonoscopy."
Rabeneck said the findings suggest that doctors performing colonoscopies need to take an extra careful look at the upper colon.
"Technically, it's the most challenging part to reach, and maybe for technical reasons the examination of that part of the colon isn't as well done, or carefully done, as the examination of the lower part of the colon," she said.
And perhaps using different types of light, rather than white light, would help physicians see better in the upper colon, she suggested.
Rabeneck also said patients need to follow instructions for colonoscopy preparation to the letter.
And she agreed with Sandler that the findings might indicate there are differences in the biology of tumours in the upper colon, and this is something that calls for further research.
Although the study population was large, the research comes with some qualifiers.
"We don't know, for instance, if they had a family history of the disease, and we don't know whether the colonoscopy was being done for screening or for diagnostic reasons," Rabeneck said.
"Whether these findings translate into other populations is a question."
Sandler said it's very important that people get screened for colon cancer.
"All these studies, I think, help to raise people's awareness of the fact that we can prevent people from dying."
Ontario's colorectal screening program involves people over 50 who have no family history of the disease doing a simple at-home stool test to test for blood. If the test is positive, then they need a colonoscopy, Rabeneck said.
In addition, those with a close relative with colon cancer should have a colonoscopy.
