important differences in cancer prevalence, the availability and performance characteristics of screening tests, the values placed on benefits and harms, and the structures and resources of health care systems.

The evidence reviewed in this chapter was compiled from multiple sources, beginning with studies previously evaluated or currently under review by the U.S. Preventive Services Task Force (USPSTF) and other groups that have developed evidence-based cancer screening guidelines. It was supplemented by a manual search of recent literature on cancer screening and a computerized search of the National Library of Medicine’s bibliographical MEDLINE database, conducted in February 2001, of relevant studies published since 1995, the closing year for the review of evidence for the second edition of the Guide to Clinical Preventive Services, the report of USPSTF (1996). Evidence published after that date is not included in the report.

Colorectal Cancer

The colorectal screening tests considered in the review in this part of the chapter are the fecal occult blood test (FOBT), flexible sigmoidoscopy, double-contrast barium enema, and colonoscopy. The review does not consider a variety of investigational technologies, such as computerized colography (virtual colonoscopy), and testing of feces for mutations in DNA (Traverso et al., 2002), which are less invasive than current screening options but which have not been sufficiently validated for routine use in the clinical setting.

Recent studies have shown an association between screening for colorectal cancer and a decreased incidence of the disease (Mandel et al., 2000), lending support to the notion that the removal of polyps, precipitated by screening, prevents colorectal cancer. For many years, the most compelling evidence was from the National Polyp Study, which demonstrated that the detection and removal of adenomatous polyps in patients with a prior history of such lesions could reduce the subsequent incidence of colorectal cancer by 76 to 90 percent (Winawer et al., 1993b). Such evidence lends support to the existence of an adenoma-carcinoma sequence: the hypothesis that colorectal cancer arises largely from adenomatous polyps. That said, an unknown proportion of colorectal cancers may arise de novo or from hyperplastic polyps (Bedenne et al., 1992). Concerns remain over flat lesions that are not discernible on colonoscopy and that may progress to cancer (Rembacken et al., 2000).

Certain patients are at increased risk for colorectal cancer, accounting for 30 to 35 percent of colorectal cancer cases (Winawer et al., 1997). Risk factors include a personal or family history of polyps or prior colorectal cancer and inflammatory bowel disease.

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