Two commonly recommended screening tests for colon cancer, flexible sigmoidoscopy and the fecal occult blood test (FOBT), when used together, will miss about a quarter of patients with precancerous colon polyps or invasive cancer when performed only once, according to a report in the August 23 New England Journal of Medicine (Vol. 385, No. 8: 555-560).

A colonoscopy is the most sensitive screening test for colorectal cancer, but it also is the most expensive and invasive, So some experts have questioned whether most people really need this test or whether simpler tests would suffice.

The FOBT detects blood in the stool, which can indicate the presence of precancerous growths or actual cancer, and a flexible sigmoidoscopy involves examining the lower third of the colon with a flexible lighted tube. A colonoscopy, however, examines the entire length of the colon.

In their new study, David A. Lieberman, MD, with the Oregon Health Sciences University in Portland, Oregon, and his colleagues with the Veterans Affairs Cooperative Study Group 380, performed colonoscopy on 2,885 patients at 13 different Veterans Affairs Medical Centers to determine whether or not the patients had invasive cancer, large colon polyps, or colon polyps that had visible or microscopic signs suggesting they were precancerous. These findings were called “advanced neoplasia” if they were found on the colonoscopic examination.

The participants also underwent FOBT before the colonoscopy. The researchers then performed colonoscopy, and made careful observations of their findings in the sigmoid colon and rectum (the regions of the colon that would usually be seen through the sigmoidoscope), and their findings throughout the remainder of the colon (which would usually be seen only with the colonoscope). The researchers then determined what percentage of cases detected by colonoscopy were also detected by the FOBT tests and were seen in the area usually examined with the sigmoidoscope, the two less extensive screening techniques.

Among patients who had advanced neoplasias detected by their colonoscopy, an FOBT detected only 24% of the patients with advanced neoplasias; sigmoidoscopy would have detected 70%; and combined testing would have detected 76%, meaning that about one fourth of all the patients with advanced neoplasias detected by colonoscopy would have been missed if a colonoscopy was not performed.

“This study provides accurate information of how many patients with serious, precancerous lesions in the colon are missed with one-time testing,” Lieberman says. “Removing such lesions can prevent colon cancers, so it is important that screening tests be able to detect the most serious pre-cancer lesions,” he notes.

But Lieberman says he was not surprised by the findings. “Prior studies suggested that there were limitations to FOBT and sigmoidoscopy,” he adds.

“Many health care providers currently believe that colonoscopy is the most effective test,” Lieberman says. “Therefore, I do not think it will be difficult to persuade doctors to perform the test,” he adds.

The American Cancer Society (ACS) recommends that people without risk factors, such as a family history of the disease or inflammatory bowel disease, begin screening at age 50. ACS has available colon cancer screening guidelines. The ACS has three preferred options for screening of people at average risk:

• a colonoscopy every 10 years
• a yearly FOBT combined with flexible sigmoidoscopy every five years
• a double contrast barium enema every 5 years.

The other two options, either an annual FOBT or a flexible sigmoidoscopy every five years, are considered less accurate than the combination of both tests.

Robert Smith, PhD, director of cancer screening and cancer control with the ACS, says that “patients have a range of options when it comes to colorectal cancer screening,” and advises that they should understand the differences among them in terms of accuracy, cost, discomfort, and complications.

“Colonoscopy is regarded as the most accurate and thorough screening test, but it may not be as readily available and may be outside the range of affordability for most people,” Smith says, pointing out that a colonoscopy ranges in cost from $700 or $800 to well over a $1,000 and is not always covered by insurance.

Despite the drawbacks, according to Smith, the best way to prevent getting cancer is to undergo a test that involves some endoscopy (i.e. sigmoidoscopy or colonoscopy), “because then we can identify polyps early and remove them,” he explains.

“Reducing intake of red meat, getting regular physical activity, eating a healthy diet that is rich in fresh fruits and vegetables may all reduce the risk of colorectal cancer,” he adds.

So, should you ask your physician for a colonoscopy? According to Smith, at this point, most physicians would probably regard colonoscopy as a more complicated test than the average person should get. “They regard it as more of a diagnostic tool rather than one that is used for screening.” But he says, you can bring it up with your physician.

The situation is different, however, for people at increased colon cancer risk, such as those with inflammatory bowel disease, those previously found to have polyps or those already treated for a colorectal cancer, and people with a strong family history of colorectal cancer. ACS guidelines specifically recommend colonoscopy for these individuals, at intervals that depend on which risk factors apply to them.

The authors also point out in their study that screening programs which use both FOBT and sigmoidoscopy may be more effective if the studies are repeated at the recommended intervals, rather than on a single occasion as was the case in this study. Although this study compared one flexible sigmoidoscopy plus one FOBT to one colonoscopy, the comparison that is more relevant to actual practice is two flexible sigmoidoscopies plus 10 FOBTs versus one colonoscopy. For this reason, the study results most likely overestimate the advantage of colonoscopy.

“If a physician does not recommend a colonoscopy,” Smith says, “this should not represent a barrier to screening. The ACS recommends a full spectrum of screening tests as options, because screening rates in the population are very low.”