Compelling evidence for this polyp-cancer sequence includes their similar prevalence in different world populations, their common etiology, and their similar site distribution in the colon. Furthermore, colorectal cancers often contain residual benign adenomatous tissue, whereas small cancers without such residuals are extremely rare. As adenomas slowly increase in size, they develop some of the abnormal chromosomal patterns and histologic characteristics found in colorectal cancers. Non neoplastic polyps, which include hyperplastic polyps, lymphoid aggregates, hamartomas, and inflammatory lesions, have no recognized premalignant potential. Clinical studies have now conclusively shown that most patients with colorectal adenomas have a greater risk of developing subsequent cancer and that resection of these adenomas substantially decreases this risk.
Colorectal cancer is the second most common visceral malignancy in the United States.
The latter approach is referred to as secondary prevention and is the topic of this Clinical Update. Only a small minority of adenomas undergo this entire malignant sequence. Cancer risk increases with polyp size, multiplicity, villous histology, and the presence of a family history of colorectal adenomas or cancer. Some persons are at higher risk for the disease because of a personal or family history of colorectal neoplasia or as the result of long-standing inflammatory bowel disease. Clearly colonoscopy is the most accurate method of detecting smaller polyps, and it has an additional obvious advantage over barium enema x-ray examinations in that it provides an immediate means of resecting most polyps and of obtaining biopsy specimens of other suspicious lesions.