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Cancer of the Small Bowel


Summary of Key Points

  • Incidence

  • Small bowel tumors are rare, with an incidence of less than 10% of new gastrointestinal (GI) cancer diagnoses and less than 1% of all newly diagnosed cancers each year.

  • Genes involved include APC, KIT, and mismatch repair genes.

  • Biologic Characteristics

  • Malignant primary small bowel tumors consist of carcinoid (37%–39%), adenocarcinoma (31%–37%), lymphomas (17%–18%), and gastrointestinal stromal tumors (GISTs) (8%–10%).

  • Staging Evaluation

  • Staging is based on the American Joint Committee on Cancer (AJCC) tumor-node-metastasis (TNM) staging system. GI lymphoma is staged with the Ann Arbor system based on lymphatic and extralymphatic involvement above and below the diaphragm.

  • Primary Therapy

  • Primary therapy for most small bowel neoplasms is surgical resection. Appropriate lymphadenectomy must be performed for adenocarcinoma and carcinoid tumors. Some forms of lymphoma are primarily treated with chemotherapy.

  • Adjuvant Therapy

  • Chemotherapy is often used for adenocarcinoma; however, there is no clear benefit for prognosis. GISTs are treated with tyrosine kinase inhibitors.

  • Locally Advanced Disease

  • Locally advanced disease typically requires surgical resection.

  • Palliation

  • GI bleeding, mechanical obstruction, pain, and perforation may require palliative surgery in situations when curative resection is not possible. The use of palliative chemotherapy is also being explored.

  • Treatment of Metastatic Disease

  • Surgery may be used to control complications of local symptoms such as bowel obstruction. Metastatic carcinoid tumors are treated with somatostatin analogues.

Small bowel neoplasms encompass a rare grouping of diseases. Although the majority of small bowel tumors are secondary malignancies, this chapter focuses on benign and malignant primary small bowel neoplasms. The most common presentations of malignant tumors are pain, obstruction, and intraluminal bleeding. Most benign small bowel tumors remain small and asymptomatic, but when large they can manifest similarly. The chapter describes the overall epidemiology, pathogenesis, and clinical presentation of small bowel tumors before discussing the most common subtypes of benign and malignant tumors, which include adenocarcinoma, neuroendocrine tumors (NETs), lymphomas, and gastrointestinal stromal tumors (GISTs).

Epidemiology

Small bowel cancer is rare, comprising less than 1% of new cancer diagnoses each year. The estimated incidence in the United States of new cases of small bowel malignancies in 2016 was 10,090, with 1330 deaths. This is in contrast to the estimated 134,490 new cases of colorectal cancer and 49,190 deaths. Unlike colorectal cancer, the incidence of small bowel cancers has risen in recent years.

Much of this increase appears to be due to a rising incidence in small bowel NETs, also referred to as carcinoid tumors. Historically, the most common type of small bowel malignancy was adenocarcinoma. However, based on the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) database, the incidence of small bowel carcinoid tumors increased more than fourfold in the United States between 1973 and 2004. The most common histologic types of primary small bowel malignancies now reported are carcinoid (37% to 39%), adenocarcinoma (31% to 37%), lymphomas (17% to 18%), and stromal tumors (8% to 10%). Mean age at diagnosis is approximately 65 years. There is a slight male predominance, and a higher incidence in blacks than whites.

Distribution throughout the gastrointestinal (GI) tract tends to vary based on type of malignancy. Most adenocarcinomas are found in the duodenum, whereas carcinoid tumors are most commonly located in the ileum. Lymphomas and stromal tumors are more evenly distributed.

Etiology and Biologic Characteristics

The rarity of small bowel cancers has made the study of pathogenesis difficult. Although the small bowel contains 90% of the mucosa of the GI tract, only 1% to 2% of GI malignancies occur there. Various theories have been proposed as to why cancer of the small bowel mucosa is so rare compared with colon cancer, including lower bacterial load, more rapid transit time of ingested carcinogens, and relative alkaline environment. As in other cancers, environmental and genetic factors play clear roles in pathogenesis. Other diseases involving the immune system, such as Crohn disease, celiac sprue, and immunosuppressive states, also increase the risk of particular types of small bowel malignancy.

Environmental Factors

Multiple dietary factors have been implicated with increased risk of development of small bowel adenocarcinoma, including red meat, smoked or salt-cured foods, saturated fat, and sugar. Cigarette smoking and ethanol have also been found to have significant associations with adenocarcinomas and carcinoids in some studies.

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