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Easier Colon Cancer Test Works Well, But Colonoscopy's Still King

A big study of a colon cancer test called flexible sigmoidoscopy may provide a good example of how a cheaper, easier-on-the-patient and possibly better technology isn't always the one American doctors choose to use.

Over the past decade or so, sigmoidoscopy has been largely abandoned by the doctors in the U.S. in favor of colonoscopy to detect and prevent colon cancer. Indeed, colonoscopy largely gets the credit for a 30 percent drop in colon cancer incidence since the 1980s, much of it the past decade.

But a lot of Americans over 50 — about 40 percent — aren't getting colonoscopy exams, which require an unpleasant colon-purging ordeal beforehand and light anesthesia while the gastroenterologist snakes a tube through the 6-foot length of the colon.

By comparison, sigmoidoscopy examines only the last two feet of colon. It doesn't require such rigorous prep or anesthesia. It takes less time and costs hundreds of dollars less. And it has a lower risk of side effects, such as perforation of the colon.

The new study of sigmoidoscopy versus "usual care" in 155,000 Americans shows that those in the "scope" group had 21 percent fewer diagnoses of colorectal cancer and were 29 percent less likely to die of it.

(In absolute terms, there were 12 colorectal cancer cases in the screened group versus 15 who got usual care per 10,000 people followed for a year. The cancer death rate was 3 per 10,000 person-years among those who were scoped, versus 4 in the comparison group.)

In fact, the benefits of sigmoidoscopy were probably bigger than that because nearly half of the "usual care" group also got scoped. That diluted the difference between the two groups.

The study, paid for by the National Cancer Institute, was unveiled on Monday at the Digestive Disease Week conference in San Francisco and published online by the New England Journal of Medicine.

Now that there's gold-plated evidence that a cheaper, easier colon cancer screening test prevents cancer and saves lives, you might expect the balance might shift away from colonoscopy, at least a little bit. After all, "no randomized clinical trial proving that colonoscopy can reduce cancer mortality has yet been published," as Dr. John Inadomi of the University of Washington notes in an accompanying editorial.

But it's not that simple. Gastroenterologists in the U.S. already have a substantial investment (in all senses) in colonoscopy. Some say that scoping only the left side of the colon, as sigmoidoscopy does, is like doing mammography on only one breast. You might be missing something important.

On the other hand, 2 of 3 precancerous colon polyps arise on the left side. And it's not clear how good colonoscopy is in identifying and removing right-sided polyps, which are flatter.

These right-sided polyps might be biologically different too, perhaps giving rise to cancers that are more aggressive and less amenable to cure. That could undercut the effectiveness of colonoscopy — even if they're found and removed, it might make less difference than removing left-sided polyps.

Still, many doctors would be nervous about sigmoidoscopy's inability to find deeper polyps. Authors of the new study estimate that sigmoidoscopy found more than 1,000 tumors but missed 97.

"We missed some tumors, so I think that having colonoscopy is probably the better first test," study leader Christine Berg told Shots. "But if you don't want to have a colonoscopy and would feel more comfortable having a flexible sigmoidoscopy, you should have it."

Dr. Michael LeFevre, a professor of family practice and community medicine at the University of Missouri, thinks many people would prefer it.

"The prep is dramatically different," LeFevre says. "Compared to that gallon of liquid you have to drink the night before your colonoscopy that makes you feel like you're about to explode, with sigmoidoscopy it's typically a laxative the night before and a couple of enemas in the morning and you're ready."

And as Dr. Inadomi of the University of Washington says: "The best test is the one that gets done."

Copyright 2020 NPR. To see more, visit https://www.npr.org.

Since he joined NPR in 2000, Knox has covered a broad range of issues and events in public health, medicine, and science. His reports can be heard on NPR's Morning Edition, All Things Considered, Weekend Edition, Talk of the Nation, and newscasts.
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