People who have had a colonoscopy in the past decade are less likely to be diagnosed with advanced colon cancer than those who haven't been screened recently, according to a new study.
Researchers found less-invasive tests, known as sigmoidoscopies, were also tied to a lower risk of advanced tumours - but only those growing in one side of the colon.
It's still not clear what that means for cancer survival, according to Dr. Chyke Doubeni, who led the new study at the University of Pennsylvania's Perelman School of Medicine in Philadelphia. But while researchers wait for results from more controlled, long-term trials, the new report is more evidence that screening in general is beneficial, they said.
"There's a variety of screening tests for us to choose from," Doubeni told Reuters Health.
"This study doesn't show that colonoscopy is the only test, but it does show that colonoscopy has the advantage in being able to see the right side (of the colon)… unlike sigmoidoscopy and fecal occult blood testing."
The US Preventive Services Task Force, a government-backed panel, recommends people between age 50 and 75 get screened for colon cancer using regular fecal occult blood testing, sigmoidoscopy or colonoscopy. The suggested interval between screenings depends on the method - from blood tests every year to colonoscopy every ten years.
According to the American Cancer Society, about one in 20 people will be diagnosed with colon or rectal cancer during their lifetime.
For the new study, Doubeni and his colleagues analysed records from 474 people with advanced colon cancer and another 538 people without cancer who were the same age and covered by the same healthcare plans.
The researchers found about three per cent of people with advanced cancer had undergone colonoscopy in the past decade, compared to nine per cent of the cancer-free group. Just under 20 per cent of people with late-stage cancer had a past sigmoidoscopy, versus 34 per cent of comparisons.
That translated to a 71-per cent lower risk of advanced cancer among people with a past colonoscopy and a 50-per cent lower cancer risk after sigmoidoscopy.
The pattern held for cancers in both the left and right colon for colonoscopy. But sigmoidoscopy seemed to only prevent advanced tumours on the left side - the easier side to reach with a scope, the researchers wrote Monday in the Annals of Internal Medicine.
In another study published alongside Doubeni's report, researchers led by Dr. Beverly Green from Group Health Research Institute in Seattle found more people were up-to-date with colon cancer screening when they were reminded about screening tests at times coordinated by electronic health records.
Doubeni said his team's findings don't mean colonoscopy should be considered the only, or even best, option for cancer screening.
"It's also about whether people use it or can afford to use it," he said. Those who dread a colonoscopy so much they keep putting it off would be better served choosing a less-invasive screening technique, for example.
Dr. Nancy Baxter, a colon cancer researcher and surgeon from the University of Toronto, said researchers have expected colonoscopy to be more effective than sigmoidoscopy - the question is how much more effective, and is that worth it.
"Colonoscopy is more expensive, it's more risky, it requires you take the day off work," Baxter, who wasn't involved in the new research, told Reuters Health.
At over US$1,000 (S$1,245), colonoscopy is typically about twice as expensive as sigmoidoscopy.
The new study "emphasizes again that colon cancer screening is effective, and it's highly effective," Baxter said.
"I think what's important to see is actually flexible sigmoidoscopy does a pretty good job too, and if you don't want to go the whole way and do a colonoscopy, if that's not for you, then a flexible sigmoidoscopy is pretty good too," she said.