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INTRODUCTION

Primary 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, and lymphatic and mesenchymal cells—the small bowel may also be the site for metastases from other primary tumors. The variety and uncommon nature of these tumors make generalizations regarding their management difficult. In this chapter, we will review the epidemiology and clinical diagnostic and management strategies for benign and malignant neoplasms of the small bowel.

EPIDEMIOLOGY

Although the small bowel accounts for 75% of the length and 90% of the mucosal surface area of the gastrointestinal tract, less than 2% to 5% 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% to 0.3% of hospital deaths—a rate 15 times the operative incidence of small bowel resections for tumors.1,2 Over the past few decades, the overall incidence of small bowel tumors has increased, likely due to enhanced detection with new diagnostic modalities. Although the incidence of adenocarcinoma has remained stable, there has been an almost 4-fold increase in the incidence of small intestinal neuroendocrine tumors (SI-NETs), which are now the most common primary tumor of the small bowel.1,2

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. Interestingly, these reports differ regarding the type of small bowel tumor, the distribution of tumors, and until the advent of molecular diagnostic criteria for gastrointestinal stromal tumors (GISTs), the classification of tumors of stromal origin. Nonetheless, in most series, adenocarcinomas (30%-50%), GISTs (15%), SI-NETs (carcinoid tumors; 20%-30%), and lymphomas (15%) compose the most common malignant tumors.2,3 Small bowel tumors are more prevalent in older patients, with over 65% of patients with small bowel adenocarcinoma being 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 these typically asymptomatic benign lesions.

There are no satisfactory explanations for the observed variation in prevalence of small bowel tumors around the world. Neuroendocrine tumors (NETs) are uncommon in Asian series, whereas GISTs compose a higher proportion of reported series in the East.4,5 Men are slightly more likely to develop small bowel neoplasms than women, with a male preponderance reported for both benign and malignant tumors.

PATHOGENESIS

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 other than the known polyposis syndromes. Only ...

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