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PART A: SMALL BOWEL CANCER
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Small bowel cancer is a rare malignancy representing approximately 3% of gastrointestinal neoplasms (1). In 2014, it was estimated that 9,160 new cases of small bowel cancer and 1,210 small bowel cancer–related deaths would occur (1). The two most common histologies seen in cancers of the small intestine are carcinoids and adenocarcinomas (2). Because of the nonspecific clinical presentation of small bowel adenocarcinoma and the difficulty in imaging the small bowel, most patients with small bowel adenocarcinoma present with lymph node involvement or distant metastases. Even in patients with localized disease who undergo resection with curative intent, the prognosis is poor, and no studies have yet demonstrated a clear benefit from adjuvant therapy. However, there have been some recent advances in the use of chemotherapy as palliative treatment. In this chapter, the epidemiology, diagnosis, and treatment of small bowel cancers, in particular small bowel adenocarcinoma, are reviewed.
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Based on an analysis of the Surveillance, Epidemiology, and End Results (SEER) database, the age-adjusted incidence rate for small bowel cancers has slowly increased from 0.9 per 100,000 persons in the years 1973 to 1982 to 1.8 per 100,000 persons in the years 2000 to 2004 (3,4). The majority of this increase has been attributed to an increase in the incidence of carcinoid tumors. A recent analyses of 67,643 patients with small bowel malignancies between 1973 and 2004 using the National Cancer Data Base and SEER showed carcinoids to be dominant with 37.4% cases compared to 36.9% cases of adenocarcinomas (5). The incidence of histologic subtypes varies in the different sections of the small intestine, with adenocarcinomas representing 80% of duodenal cancers and carcinoids representing 60% of ileal cancers.
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The incidence of small bowel adenocarcinoma peaks in the seventh and eighth decades of life, with a mean age at diagnosis of 65 years. A slightly increased incidence is seen in men and blacks (6).
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One of the more interesting aspects of small bowel adenocarcinoma is its rarity in comparison to large intestine adenocarcinoma. Even though the small intestine represents approximately 70% to 80% of the length and over 90% of the surface area of the alimentary tract, the incidence of small bowel adenocarcinoma is 30-fold less than the incidence of colon adenocarcinoma. Numerous theories have been proposed to explain the small intestine’s relative protection from the development of carcinoma. Proposed protective factors have centered on two concepts. First, the rapid turnover time of small intestinal cells results in epithelial cell shedding prior to the necessary acquisition of multiple genetic defects. Second, the small bowel’s exposure to the carcinogenic components of our diet are limited due to rapid small bowel transit time, the lack of bacterial degradation activity that occurs in the small bowel, and the relatively dilute, alkaline environment of the small bowel. ...