Flat adenoma in colon: two decades of debate

The existence of flat adenomas in the colon is well recognized. Whether they represent a distinct disease with a pathogenetic pathway different from that of the classical adenoma-carcinoma sequence in colorectal tumorigenesis and have higher malignant potential remains a matter of debate. To review the epidemiology, clinical features, detection and management of flat and depressed (non-polypoid) colonic neoplasm, we performed a thorough literature review on studies focusing on the prevalence, histological features, genetics, detection and treatment of flat and depressed (non-polypoid) colonic neoplasm. A high percentage of severe dysplasia in flat colonic adenomas has not been consistently demonstrated. Their malignant potential appears to be size-dependent. Flat adenomas are found to have a lower incidence of major genetic abnormalities involved in the classical adenoma-carcinoma sequence and that has raised suspicions that they may have a different pathogenesis. The depressed type of colorectal carcinoma is uncommon but shows more aggressive behavior. More advanced colonoscopic techniques, such as chromoendoscopy, may enhance the detection of small and inconspicuous colonic neoplastic lesions that lack a protruding configuration. It is essential for endoscopists to appreciate the existence and clinical significance of flat and depressed colonic lesions as an important variant of colonic neoplasms so that the goal of reducing colorectal carcinoma incidence by polypectomy can be better achieved.