Screening debate: Experts differ on whether colonoscopy is the best screening method for colon cancer
A recent study published in the New England Journal of Medicine concluded that 40% of all colorectal cancers could be prevented if people underwent regular colonoscopy screening, which some medical societies recommend occur every 10 years for people of average risk.
“The main finding of our study is that the 10-year screening interval is good for average-risk individuals but not for people with high-risk factors such as a family history of colon cancer or a smoking history,” says Shuji Ogino, MD, PhD, co-senior author of the study and associate professor in the department of epidemiology at the Harvard School of Public Health in Boston, Massachusetts. “We need more studies to determine the optimal screening interval for this group.”
Dr. Ogino was part of a team of researchers, including those from Dana-Farber Cancer Institute, Brigham and Women's Hospital, and the Harvard School of Public Health, who analyzed data from nearly 90,000 participants in 2 long-term studies: the Nurses' Health Study and the Health Professionals Follow-Up Study. They gathered their data from questionnaires participants filled out every 2 years between 1998 and 2008 on colonoscopy and sigmoidoscopy procedures.
The researchers found that both screening methods were associated with a decreased risk of either developing colorectal cancer or dying from the disease. Only colonoscopy decreased the risk for cancers originating in the proximal colon but not to the extent that it reduced distal colorectal cancers. Investigators estimated that if all participants had undergone colonoscopies, 40% of colorectal cancers, including 61% of distal cancers and 22% of proximal cancers, would have been prevented. In addition, they found that people who did not have polyps following a colonoscopy had a reduced colon cancer risk for up to 15 years after the procedure.
Several Screening Methods
Tim Byers, MD, MPH, associate dean of the Colorado School of Public Health, and a member of the board of directors of the American Cancer Society, says that the study “gives us more real-world observational evidence” that colonoscopy screening substantially reduces colon cancer.
“Our basis of knowledge about colorectal cancer screening comes from randomized, controlled studies of sigmoidoscopy,” he says. “We've always figured that colonoscopy is better, but we don't have completed trials yet. This is pretty solid evidence.”
Although colonoscopy has been widely publicized as the most effective colon cancer screening test available, not all medical experts are in agreement as to whether it is clearly the best method. “We don't have a best screening test,” says James Allison, MD, clinical professor of medicine emeritus, division of gastroenterology, University of California, San Francisco (UCSF). “It's not that colonoscopy is a bad test, but it's one of several good tests, and there's no proof that one is better than the other.”
Dr. Allison cites the American College of Physicians, the National Cancer Institute (NCI), the Centers for Disease Control and Prevention (CDC), and the US Preventive Services Task Force (USPSTF) as agreeing on this point. The CDC recommends 3 screening tests that are effective at saving lives: colonoscopy, stool tests (the sensitive guaiac fecal occult blood test [FOBT] or fecal immunochemical test [FIT]), and sigmoidoscopy, which is now seldom done. “The most important messages the public should have are to be screened and that several screening methods that have been denigrated in the past have lots of evidence in the literature for their effectiveness,” he adds.
Two studies are currently evaluating whether colonoscopy is indeed the best screening method for colorectal cancer. One is a study being conducted by the US Department of Veterans Affairs, which is a randomized, controlled trial that is comparing the use of FIT every year with a colonoscopy every 10 years in affecting colorectal cancer mortality. Additionally, a Spanish study is comparing the use of FIT every 2 years with colonoscopy every 10 years. Results of these studies will not be available for another 8 years, however.
“The studies will be able to tell us if colonoscopy is truly the best test,” Dr. Allison says, adding that, even if it is, society does not have enough money or endoscopists to provide a quality colonoscopy for all average-risk individuals.
Dr. Byers is less convinced that the results of these studies are necessary. “Given that we have solid evidence from 2 sigmoidoscopy trials that if we look at the colon and remove adenomas, it has a big effect on colon cancer incidence, I would say we don't really need these trials,” he adds.
The trials of flexible sigmoidoscopy included a British study, which showed that only 1 flexible sigmoidoscopy screening between the age 55 and 64 years could substantially reduce colorectal cancer incidence and mortality, and the US-conducted Prostate, Lung, Colorectal and Ovarian Cancer (PLCO) screening trial.[1, 2] The latter found that colorectal cancer screening with flexible sigmoidoscopy decreased both the incidence (21%) and death from the disease (26%) over an average of almost 12 years when compared with the usual care group.
The PLCO authors estimated that if they had used colonoscopy rather than sigmoidoscopy in their study, they would have identified 16% more cancers, two-thirds of which would have been proximal cancers. However, they were not able to determine what effect that may have had on proximal colorectal cancer mortality. (According to the NCI, some studies suggest colonoscopy is more effective against distal than proximal tumors.) In addition, sigmoidoscopy has never been directly compared with colonoscopy in a definitive clinical trial.
Dr. Byers notes that colonoscopy is a screening test that is not without side effects, including the need for preparation, the risk of colon perforation and bleeding, and the need for sedation. At the same time, FOBT/FIT—which are noninvasive methods—are becoming better and more sensitive. However, to detect colon cancers early through FOBT/FIT, the test needs to be performed annually. “The problem is that few health care systems can do the FOBT/FIT reliably year after year,” Dr. Byers says. “The value of colonoscopy is that it covers you for a long time.”
Screening Rates Still Low
Dr. Byers says his view of screening methods has shifted over time. Twenty years ago, he would have said that FOBT or sigmoidoscopy were the best methods, but he now feels that colonoscopy every 10 years is the best test for average-risk individuals. He says that colonoscopy is becoming easier to perform and that the cost is not prohibitive. Many US physicians seem to concur, because colonoscopy increasingly appears to be the screening method of choice in this country. However, Dr. Allison points out that many other countries, including the United Kingdom and Australia, disagree with that approach and have not embraced colonoscopy as the definitive test. The CDC, meanwhile, emphasizes that colonoscopy, FOBT/FIT, and sigmoidoscopy should all be offered as options to patients in an effort to increase overall colorectal cancer screening rates, which are still low among minority and underserved populations. They also add, in their November 2013 Vital Signs—a new program that offers data and calls to action on important public health issues—that about 23 million adults have never been screened for colorectal cancer.
CDC leaders further suggest in Vital Signs that physicians offer all recommended test options with advice about each and match patients with the test they are most likely to complete. They also urge public health professionals to create ways to make it easier for people to get FOBT/FIT kits in places other than a doctor's office, such as giving them out at flu shot clinics or mailing kits to people's homes.
The one thing experts can agree on is that while none of the tests is perfect, it is still important for average-risk individuals to be screened for colon cancer starting at age 50. They also can agree that more questions need to be answered through additional studies.
“All tumors are not the same, and each patient is different,” Dr. Ogino says. “Screening needs to be catered according to risk factors and different molecular subtypes—it won't be effective for everyone or equally in the same way,” he adds. “This is just the beginning of the new field of pathological epidemiology.”
We've always figured that colonoscopy is better, but we don't have completed trials yet. This is pretty solid evidence. —Tim Byers, MD, MPH
It's not that colonoscopy is a bad test, but it's one of several good tests, and there's no proof that one is better than the other. —James Allison, MD