Colonoscopies may not reduce cancer deaths, study finds — but experts say you

CNN
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Colonoscopy may reduce the risk of dying of colorectal cancer by as much as 50%, but there’s a catch: It only works if you get the scan.

That’s the big takeaway message from the first randomized trial of colonoscopy, published Sunday in The New England Journal of Medicine.

Colonoscopy has been recommended as a screening test for adults age 50 and over since the mid-1990s, and some 15 million colonoscopies are performed in the United States each year. This recommendation has been based on evidence from observational studies that looked back in time to compare how often colorectal cancer is diagnosed in people who received colonoscopies versus those who did not. These studies can be subject to bias, however, so scientists look to randomized trials that blindly sort people into two groups: those who are assigned to get an intervention, and those who are not. These studies then follow both groups forward in time to see if there are differences. Those studies have been difficult to do for colon cancer, which can be slow growing and may take years to be diagnosed.

The NordICC study, which stands for Northern-European Initiative on Colon Cancer, included more 84,000 men and women ages 55 to 64 from Poland, Norway and Sweden. None had gotten a colonoscopy before. The participants were randomly invited to have a screening colonoscopy between June 2009 and June 2014, or they were followed for the study without getting screened.

In the 10 years after enrollment, the group invited to get colonoscopies had an 18% lower risk of colorectal cancers than the group that wasn’t screened. Overall, the group invited to screening also had a small reduction in their risk of death from colorectal cancer, but that difference was not statistically significant – meaning it could be simple due to chance.

The researchers say they’re going to continue to follow participants for another five years. It could be that because colon cancers can be slow-growing, more time will help refine their results and may show bigger benefits for colonoscopy screening.

Normally, those kinds of disappointing results from such a large, strong study would be considered definitive enough to change medical practice.

But there’s a big caveat in this study that limits how the results should be applied: Only 42% of the participants who were invited to get a colonoscopy went through with it.

“I think it’s just hard to know the value of a screening test when the majority of people in the screening didn’t get it done,” said Dr. William Dahut, chief scientific officer at the American Cancer Society, who was not involved in the study.

When the study authors restricted the results to the people who actually received colonoscopies – about 12,000 out of the more than 28,000 who were invited to do so – the procedure was found to be more effective. It reduced the risk of colorectal cancer by 31% and cut the risk of dying of that cancer by 50%.

Experts say it’s difficult to rely just on the results from this subgroup, however, because they can be subject to bias.

For example, in clinical trials, researchers often worry about the “healthy volunteer effect”: People who volunteer for testing may be more likely to take care of themselves by eating a healthy diet or doing other things that can’t be measured by the study that might reduce their risk.

Dr. Michael Bretthauer, a researcher on the study who leads the clinical effectiveness group at the University of Oslo in Norway, says that as a gastroenterologist, he found the results disappointing.

But as a researcher, he has to follow the science, “so I think we have to embrace it,” he said.

“And we may have oversold the message for the last 10 years or so, and we have to wind it back a little,” he said.

Bretthauer thinks of the full set of study results – including the people who didn’t get a colonoscopy – to be the minimal amount of benefit a person could expect to get, while the more narrow results – limited to the subset of people who did get colonoscopies – are the largest benefits people could expect.

Based on his results, then, he expects that screening colonoscopy probably reduces a person’s chances of colorectal cancer by 18% to 31%, and their risk of death from 0% to as much as 50%, “which is on the low end what what I think everybody thought it would be.”

Other studies have estimated larger benefits for colonoscopies, reporting that these procedures could reduce the risk of dying of colorectal cancer by as much as 68%.

There are other caveats that may limit the applicability of the study’s results.

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