Epidemiology of colorectal adenomatous polyps.

Colorectal cancer is currently the second leading cause of cancer death in the United States (1); nearly 7% of Americans are expected to develop this disease within their lifetime (2). Efforts to identify causes and effective preventive measures for this disease have led to the hypothesis that adenomatous polyps (adenomas), benign tumors that arise in the large bowel, are precursor lesions for the vast majority of colorectal cancers (3). The evidence implicating adenomas is of necessity indirect; the clinical practice has been to resect these lesions when discovered rather than to observe their natural history. A few carcinomas appear to have arisen out of flat mucosa and in the absence of adenomatous tissue (4-6). However, the fact that cancer patients are on average 5-7 years older than patients diagnosed with adenomas (7) suggests that adenomas precede malignant lesions. Pathologists have long recognized the presence of carcinoma in situ and frankly invasive carcinoma within adenomas (8, 9). Consequently, most experts currently accept the hypothesis of an adenoma-carcinoma sequence (10). The time is now ripe for a critical review of what has been learned about the epidemiology of adenomas.