The prevalence of small (< 1 cm) simple tubular adenomas in adults over the age of 50 years exceeds 30%. These common small adenomas, however, have a very low malignant potential. Studies indicate that most remain static or actually regress with time, whereas only a few develop the additional acquired genetic changes that make them grow, develop the advanced histologic changes of villous architecture or high grade dysplasia, and turn eventually to cancer. Advanced adenomas as defined by the National Polyp Study are those that are > 1 cm in size or contain villous tissue or high grade dysplasia. These advanced polyps are much less common, but much more likely to progress to cancer if not detected by screening. A large body of recent scientific data indicates that we clinicians should shift our focus away from simply detecting and removing large numbers of small tubular adenomas, toward strategies that reliably detect most advanced adenomas. Long term postpolypectomy studies nicely demonstrate the validity of this important concept (Bond, 2000). Follow-up studies from the Mayo Clinic and from St. Mark's Hospital in London show that patients with resection of only one or two small tubular adenomas have no measurable increased risk of developing subsequent colorectal cancer compared with the average population. In contrast, patients with large (> 1 cm) or multiple (> 3) adenomas, or adenomas with villous changes or high grade dysplasia have a risk of metachronous cancer that is increased 3- to 6-fold. Thus, the objectives of colorectal cancer screening are to (1) detect cancers that have developed while they are still confined to the bowel and surgical cure is very likely or (2) to detect and resect advanced adenomas thereby preventing cancer. The choice of a screening option should be guided by how well it accomplishes these two objectives.
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