Do I need a colonoscopy? New study on colon cancer screening effectiveness.

Despite the findings of a large European study suggesting that the benefits of colonoscopies may not be as strong as thought, doctors in the U.S. say screening does detect colorectal cancer and saves lives.

The 10-year study, published Sunday in the New England Journal of Medicine, found that colonoscopies cut the risk of colon cancer by about a fifth, far below estimates from earlier scientific studies. What’s more, the procedure — usually performed with a long, flexible tube that is inserted up the rectum — didn’t help significantly reduce the risk of colon cancer deaths.

Doctors in the U.S. are concerned that the study’s results — based on more than 80,000 men and women ages 55 to 64 — could cause doubt about the effectiveness of colonoscopies, which are recommended for people in the U.S. every 10 years, starting at age 45.

“I fear that this is going to cause a lot of people to question whether they should get colon cancer screening,” said Dr. Otis Brawley, an oncologist at Johns Hopkins University and the former chief medical officer of the American Cancer Society.

Decades of research have shown that screening is very good at detecting colon cancer, said Brawley.

What did the colonoscopy study find?

In the trial, which was based in Norway, Sweden and Poland, people were randomly invited to get colonoscopies or receive regular care that didn't include colon cancer screening. Of those who were invited to have colonoscopies — whether they got them or not — there was an 18% reduction in developing the disease and no statistically significant reduction in the likelihood of colon cancer death.

A major sticking point for experts who reviewed the study’s findings was that only 42% of the people in the new study who were invited to get colonoscopies actually went through with them.

That was a "serious shortcoming" of the study, said Dr. Richard Wender, chair of the Department of Family Medicine and Community Health in the Perelman School of Medicine at the University of Pennsylvania.

"This is a flaw of randomized trials of cancer screening tests," said Wender. "They inevitably become trials of invitations to screen, not the test itself."

In fact, the results were actually quite good for the people in the study who got the colonoscopies — a 31% reduction in colon cancer risk and a 50% reduction in risk of dying from it, experts said.

“In order for a colonoscopy to be effective, you have to have it done,” said Dr. Robin Mendelsohn, a co-director of the Center for Young Onset Colorectal and Gastrointestinal Cancers at the Memorial Sloan Kettering Cancer Center.

Ng agreed, saying, “Colonoscopy only works if you actually go through with it.”

The study’s findings were based on a 10-year follow-up. Researchers in the study said they plan to conduct an additional analysis at 15 years.

With longer monitoring, the results could show a larger reduction of colon cancer risk, said Brawley of Johns Hopkins.

There wasn't enough follow-up time in the study, said Wender, noting that some of the screened patients were followed for only five or six years.

"Much of the effectiveness of colonoscopy comes from detecting and removing polyps," said Wender. "It can take over a decade to see mortality benefits. "

How well do colonoscopies detect cancer?

Cancer experts reached Monday said their recommendation for colorectal cancer screening remains the same: Regular colonoscopies are key to preventing colon cancer and finding it early.

The European study focused on colonoscopy; other colorectal screenings include an at-home, noninvasive stool blood test, or fecal immunochemical test, and a flexible sigmoidoscopy, which is a narrow tube with a camera that examines the rectum and the lower colon.

“It is still very important to undergo cancer screenings according to recommended guidelines,” said Dr. Kimmie Ng, a physician at Dana–Farber Cancer Institute in Boston. “These cancer screenings can still potentially save lives.”

Previous studies have shown that colonoscopies reduce the risk of colorectal cancer by 40% to 69% and reduce the risk of death from the disease by 29% to 88%, according to the study.

In the U.S., about 1 in 23 men are affected by the disease at similar rates as women, about 1 in 25. Black people in the U.S. have an estimated 40% higher chance of dying from the disease than white people.

According to the Centers for Disease Control and Prevention, about two-thirds of U.S. adults ages 50 to 75 are up to date on their colon cancer screenings.

How might the results differ in the U.S.?

The study has limitations, most notably quality issues, with some doctors not performing the procedure up to standards, according to the European scientists.

In addition, Europe doesn’t reflect the U.S., which is more socioeconomically, racially and ethnically diverse, said Dr. Arif Kamal, the chief patient officer of the American Cancer Society. Numerous studies have shown significant differences in health and health care quality among racial and ethnic minority groups in the U.S,, meaning some groups may detect colon cancer from screenings at higher rates than others.

High-quality screening is critical for detecting cancer. Another limitation was the time period the data was collected; researchers began performing the screenings in 2009. Kamal said the technology used to do colonoscopies has improved “by leaps and bounds since that time.”

Screenings work if you get them.

Doctors don't recommend changing the U.S. colonoscopy guidelines, but they said the study may provide more realistic expectations about the procedure.

In the U.S., “we have exaggerated the benefits of colon cancer screening,” Brawley said. “Now, that being said, that doesn’t mean that there is no benefit.”

He called the study’s potential impact an “unfortunate” consequence of scientists’ conducting “science out in public.”

Mendelsohn said the most important thing is for doctors to be honest and to tell patients that sometimes “we don’t know the answer."

“We have good studies that show that colon cancer screening is effective. Maybe it’s not the 80% that we thought, but the 30% is still a huge difference in somebody’s life,” she said.

Kamal said the study shows among "those who need screening and complete screening, their outcomes will be better."

Experts said that if you have symptoms that suggest colorectal cancer, you should get screened.

Colon cancer symptoms

According to the American Cancer Society, symptoms for colorectal cancer include:

• A change in bowel habits, including constipation or diarrhea that lasts for more than a few days.

• Rectal bleeding.

• Blood in the stool.

• Abdominal pain or cramping.

• Weakness and fatigue.

• Weight loss without trying.

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This article was originally published on NBCNews.com