SAN FRANCISCOCurrent evidence suggests that colonoscopy screening can reduce mortality from colorectal cancerif these procedures are performed in a high-quality manner, David A. Lieberman, MD, said at the 2006 Gastrointestinal Cancers Symposium."The rates of colonoscopy in the United States have been skyrocketing over the last 5 years, to the extent that some have suggested that we have inadequate resources to be able to perform all the colonoscopy that needs to be done," observed Dr. Lieberman, head of the Division of Gastroenterology, Oregon Health & Science University, Portland. However, he said, evidence that this strategy reduces colorectal cancer mortality is indirect at this time and mainly derived from clinical trials.
The indirect support of a lifesaving benefit of colonoscopy screening comes from several sources, Dr. Lieberman said. Four trials have assessed fecal occult blood testing (FOBT) as primary screening for colorectal cancer, finding mortality was lower among screened participants overall (reduction, 15% to 33%) and more so among those who were adherent (reduction, 33% to 39%). One trial also found a reduced incidence of cancer (Mandel et al: N Engl J Med 343:1603-1607, 2000). "The identification of early-stage cancers was beneficial, but that was done with colonoscopy. The incidence reduction, which was attributed by the authors to detection and removal of adenomas, was also due to colonoscopy," he pointed out.
Sigmoidoscopy studies also lend indirect support to a lifesaving benefit of colonoscopy screening, he said. Results of case-control studies suggest that mortality from cancers in the part of the colon examined is reduced by 60%. Moreover, VA Cooperative Study 380 found that among men, the rate of detection of advanced adenomas was 70% if the colon was examined to its sigmoid-descending part, but 80% if the colon was examined to the splenic flexure (Lieberman et al: N Engl J Med 343:162-168, 2000). "The implication from these studies is that an endoscopic evaluation of the colon with detection and removal of polyps could reduce mortality," he said. "And if you examine more of the colon, theoretically, that could result in higher . . . mortality reduction."According to Dr. Lieberman, more indirect support comes from the National Polyp Study, in which patients with adenomas underwent colonoscopy with polypectomy and were followed for almost 6 years (Winawer et al: N Engl J Med 329:1977-1981, 1993). During follow-up, the actual number of interval cancers was only about one-tenth of the expected number. In addition, a case-control study found that patients with colorectal cancer were only about half as likely as their healthy counterparts to have undergone colonoscopy (Muller et al: Ann Intern Med 123:904-910, 1995).
Despite this evidence suggesting benefit, a variety of factors may make colonoscopy screening less effective than thought, according to Dr. Lieberman. "We have known for a long time that colonoscopy is not perfect," he commented. For example, he said, chemoprevention studies in patients with adenomas who underwent colonoscopy with complete polypectomy have found a substantial rate of interval cancers 3 years later (1.7 to 2.4 per 1,000 person-years). These interval cancers may arise in several ways, he said.
First, 5% to 7% of colonoscopies performed in the United States are not complete to the cecum, suggesting some lesions are missed because of incomplete exams (Bowles et al: Gut 53:277-283, 2004). Second, he noted, in 2% to 12% of patients with adenomas measuring 1 cm or greater in diameter detected by CT colonography, the adenomas are not detected by colonoscopy, suggesting that other lesions are missed because they are overlooked.