The incidence of benign tumors is difficult to define since, if they are small or produce little effect on the lumen, they are often asymptomatic and escape attention. They are relatively common among clinically evident tumors, but the absolute incidence is probably much higher and probably exceeds that of malignancy .
Epithelial hyperplasias and adenomas may form sessile or pedunculated nodules. Adenomas are neoplastic, usually polypoid, excrescences due to proliferation of dysplas-tic epithelial cells. Excluding polyposis syndromes, they are usually solitary and are uncommon in the duodenum.
Villous tumors imply more severe dysplasia, with more aggressive growth. The lesions tend to be more sessile, larger, and have more irregular surface texture.
Barium trapped in the interstices of the villous surface gives a striated, reticulated, or 'soap bubbly' appearance (Fig. 10). In the duodenum they are usually found near the papilla and frequently harbor foci of carcinoma, especially if large [7, 8]. It is tempting to speculate that the dysplasia results from the irritating effect of bile or pancreatic secretions.
Mesenchymal tumors arise in the deeper layers. Thus, initially, their overlying epithelial layer of the mucosa is intact. The smooth luminal surface provides a convenient means of predicting the nature of such tumors (Fig. 11). Other aspects of appearance depend upon the consistency of the tumor tissue and its site of origin within the wall. Virtually any type of mesenchymal cell in the bowel wall may occasionally give rise to neoplastic or hamar-tomatous tumors, but only a very few occur frequently enough to warrant serious consideration.
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