The findings of a big European study published in the New England Journal of Medicine this week seemed to cast doubt on just how beneficial a colonoscopy is in preventing colorectal cancer, which is a leading cause of cancer deaths in the U.S. The results have generated a lot of controversy and buzzy headlines in the popular press — such as "Screening Procedure Fails to Prevent Colon Cancer Deaths in Large Study."

But that's not the whole story.

A colonoscopy is a widely recommended tool for cancer screening that involves putting a scope into the colon to look for potentially cancerous growths, called polyps, and cutting them out. Sometimes these slow-growing polyps — or adenomas — would have turned into cancer, so by looking periodically and removing any polyps, the procedure serves as both a screening tool for cancer and an intervention to prevent a tumor from developing in the first place.

Research going back more than a decade has shown that colonoscopies can save lives; A 2018 study from Kaiser Permanente, for example, found a 67% reduction in cancer deaths among people who got a screening colonoscopy.

In contrast, the topline findings of this week's NEJM study point to a mere 18% reduction in colorectal cancer among thousands of men and women in Europe who were 'invited' to get a colonoscopy. And, as some media reports pointed out, the reduction in deaths was too small to be considered statistically significant. Sounds pretty disappointing, right?

A hitch in the study

But here's the bigger picture: It turns out that more than half of the research participants who were 'invited' to get a colonoscopy never showed up for the procedure.

"A colonoscopy will only work if a patient gets one," says Bret Petersen, a gastroenterologist at Mayo Clinic and president of the American Society for Gastrointestinal Endoscopy, a leading group of GI doctors. Petersen says it's important to focus on the outcomes of the people who actually underwent the procedure, which was about 42% of participants who all lived in European countries, including Norway and Poland.

Among this group — people who actually got a screening colonoscopy — the risk of developing colon cancer decreased by about 31%. "And deaths were importantly decreased by a significant proportion — about 50%," Petersen says. He pushes back against the suggestion that the study published this week calls into question the effectiveness of colonoscopies. "To the contrary, I think we have no data from this study to suggest it's less valuable," Petersen says. "Based on available studies today, colonoscopy is still the gold standard in detecting and preventing colorectal cancer," he says. Petersen's group, ASGE, released a statement that doubles down on the assertion that a colonoscopy "is still the best and most proven way for patients to be screened for colorectal cancer."

American Cancer Society: 'This result points to the value of continued screening'

The American Cancer Society has weighed in on the study, too, also pointing to the high number of participants who didn't get the procedure. "It's hard to know the value of a screening test when the majority of people in the study didn't get it done," says Dr. William Dahut, Chief Scientific Officer at ACS. He highlights the 31% reduction in risk among those who were screened.

"This result points to the value of continued screening," Dahut says. The ACS also says it's important to consider that participants in the study were screened sometime between 2009 to 2014, so some got their colonoscopy as recently as 8 years ago. "The time from polyps to cancer to mortality is almost always greater than this — so a much longer follow-up is needed," an ACS statement concludes. Over time, the reduction in cancer or deaths could be greater.

It's not likely that the controversy created by this study will lead to changes in U.S. screening recommendations. "Preventive cancer screenings are the best and most trusted way to save lives," says Karen Knudsen, CEO of the American Cancer Society. The ACS recommends colorectal cancer screening for adults 45 and older. "There's no reason to change that direction," says Knudsen.

Some doctors have been quick to point out that the methods used by endoscopists (doctors performing the procedure) have improved, compared to 2009 when the European study began.

"The detection rate for polyps is much higher than it was 10 to 15 years ago," says Douglas Corley, a research scientist and gastroenterologist at Kaiser Permanente. He says the equipment is better and the preparation methods have also improved (patients are required to fast and drink a specially formulated laxative drink to help clear out the GI tract ahead of the procedure). In addition, "the physician's skill at detecting and removing polyps is better," Corley says. So, if a study were beginning now, "the benefit that we would expect to find now would be higher."

Differences between the U.S. and Europe

In addition, colonoscopies are not nearly as common in the European countries where the study was performed, and Bret Petersen says some of the doctors performing the tests did not identify the number of polyps that would be considered an acceptable rate in the U.S. "Nearly 30% of the endoscopists who were included in the NordICC trial did not meet the adenoma detection rate," Petersen says. He says this calls into question whether they were actually "just missing some of the lesions that might have otherwise been detected and thereby removed."

The lead author of the study says he's aware of all the criticism of his paper. But he pushes back against the idea that endoscopists didn't find the expected levels of polyps. "In the two countries that contributed the highest number of participants, which is Norway and Poland, the detection rate, which is the quality metric for finding polyps, was 30%, which is well above the current threshold for good quality," says Dr. Michael Bretthauer of the University of Oslo in Norway. The detection rate was much lower in Sweden, which contributed fewer participants to the study, because, he says, not as many people in Sweden have polyps. "So I don't think that that argument is valid."

Bretthauer says the paper may be getting so much attention because it challenges commonly held assumptions in the U.S. about just how protective colonoscopies can be. "I think our findings suggest that colonoscopy is not the magic bullet against colorectal cancer," Bretthauer says. But, he notes that, with up to a 50% reduction in mortality risk, that's still more beneficial than almost any other cancer screening tool.

Another challenge of the new study from Europe is that it wasn't designed to answer the questions that a lot of people have when they try to evaluate screening options. "There are different methods for colon cancer screening," Kaiser Permanente's Corley explains. Because the European researchers only assessed colonoscopies, their study doesn't offer any direct comparisons with the increasingly popular alternative to once a decade colonoscopy: stool-based home tests that are given more frequently.

Alternative screening tests for colon cancer

The U.S. Preventive Services task force recommends any of several different types of colorectal screening methods, including colonoscopy, a sigmoidoscopy (a less invasive scoping to evaluate part of the colon) or stool-based tests, typically taken at home and mailed back to a lab, that look for blood or abnormal cells in stool samples.

"It's not clear that any one of these methods is better than another at decreasing deaths from colon cancer," Corley says.

A fecal immunochemical test, called FIT, can detect small amounts of blood in the stool, and is typically done annually. Another option is a combination test — such as Cologuard -- which can detect both blood and DNA changes that may come from a cancerous or precancerous polyp in the stool. People who choose this option are typically advised to do the test every three years.

Patients at Kaiser Permanente opt for the FIT tests more commonly. Sometimes, the biggest hurdle is remembering to take the test, so Corley says at Kaiser they mail the stool-based tests to patients and then keep contacting patients, until tests are sent back.

Each screening approach "has its pluses and minuses," Corley says. If the stool sample test is positive, then a colonoscopy will likely be recommended to get a better look and remove any polyps. Colonoscopies do carry small risks, including the risk of a perforation or tear in colon, bleeding at the site of a biopsy (if the doctor removes a polyp or takes a biopsy), and potential side-effects from sedation during the procedure.

Typically, "If you are at higher risk for colon cancer, such as you've had close family members who've had colon cancer, especially if they're at a young age, then it's recommended that you have a colonoscopy," Corley says. If you're not at high risk, he says, you should pick the test that you'll be able to complete, whether that's a colonoscopy or one of the less invasive and more frequent stool-based tests.

Regardless of which method of colon cancer screening you choose, Corley says, the important thing to follow through and get screened.

You can find Allison Aubrey via Twitter @AubreyNPR.