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Tumors of the small intestine, both benign and malignant, are rare. With the potential to arise from virtually every cell type within the small intestine—the epithelium, neural tissues, lymphatic and mesenchymal cells—the small bowel may also be the site of metastases from other primary tumors. The variety and uncommon nature of these tumors makes generalizations regarding their management difficult. In this chapter we review the epidemiology and clinical diagnostic and management strategies for benign and malignant neoplasms of the small bowel.

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While the small bowel accounts for 75% of the length and 90% of the mucosal surface area of the gastrointestinal tract, fewer than 3% of gastrointestinal malignancies arise in this organ. Most of these tumors are clinically silent. Autopsy series have identified incidental small bowel tumors in 0.2–0.3% of hospital deaths—a rate 15 times the operative incidence of small bowel resections for tumors.1

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Given the rare nature of these tumors, most published reports are collections of relatively small series of tumors accrued over a period of many years. Hence, interestingly, these reports differ regarding the type of small bowel tumors, the distribution of tumors, and until the advent of molecular diagnostic criteria for GIST (gastrointestinal stromal tumors), in the classification of tumors of stromal origin. Nonetheless, in most series adenocarcinoma, GIST, carcinoid tumor, and lymphoma comprise the most common malignant tumors and are approximately equally represented.2,3 Small bowel tumors are more prevalent in older than in younger patients, and a recent analysis identified that over 65% of patients with small bowel adenocarcinoma were age 60 or older.3 The proportion of small bowel tumors that are benign varies from 14 to 52% in different series, a disparity explained by the failure to detect the typically asymptomatic benign lesions.

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There are no satisfactory explanations for the observed variation in prevalence of small bowel tumors around the world. Carcinoids are uncommon tumors in Asian series, while GIST comprise a higher proportion of reported series in the East.4,5 Men are more likely to develop small bowel neoplasms than women, with a male preponderance reported for both benign and malignant tumors.

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Given the length of the small bowel and its large mucosal surface, it is intriguing that it is such an uncommon site for malignancy. Unlike the adenoma-carcinoma sequence seen in the colon, a clear molecular progression sequence has not been defined in the small bowel outside the known polyposis syndromes. Only periampullary adenomas are known to be premalignant lesions with the potential to progress to adenocarcinomas. Adenomatous polyps arising elsewhere in the small bowel presumably have similar potential for malignant transformation, although the molecular traits of this transformation remain unknown. Such progression has not been definitively documented at other sites in the small bowel.

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Based on theories of luminal injury defined in the colonic mucosa, several hypotheses are proposed regarding the pathogenesis of epithelial-derived small bowel tumors. Unlike the ...

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