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Carcinoid tumors were first described in a 1907 paper titled "Karzinoide Tumoren des Dünndarms," or "Cancer-like Tumors of the Gut," by the German pathologist Siegfried Oberndorfer.1 He used the word karzinoide to illustrate the benign behavior of a tumor whose cells appeared malignant under the microscope.1 Later in 1914, Gosset and Masson characterized the endocrine-related properties of carcinoid tumors.2 Over the years, a considerable amount of confusion has developed over the definition of carcinoid tumors, largely because, historically, the name carcinoid was used in reference to all neuroendocrine tumors. In general, the term carcinoid refers to endocrine tumors of the gastrointestinal (GI) tract, bronchopulmonary epithelium, and rare other sites but not to pancreatic neuroendocrine tumors (PNETs), also called islet cell tumors. This chapter focuses on GI carcinoid tumors, which arise from enterochromaffin, enterochromaffin-like, or Kulchitsky cells that are part of the diffuse neuroendocrine cell types of the gut. These cells are distinguished by their ability to secrete bioactive peptides and amines, such as serotonin, somatostatin, gastrin, and histamine.

In the United States, the overall age-adjusted incidence of carcinoid tumors has increased over the past 5 decades from about 1.5 to 2.75 per 100,000 people.3 This trend has mirrored the increased incidence observed in all neuroendocrine tumors over the same time period from about 2.5 to 5.0 cases per 100,000.3 Carcinoid tumors make up almost 50% of all neuroendocrine tumors, with the majority of carcinoid tumors (68%) occurring in the GI tract and the remainder in the bronchopulmonary system (25%).3 Rarely, carcinoid tumors have been reported in other locations, including the esophagus, ovaries, testis, thymus, and other endocrine tissues of the body.3 Classically, the appendix was thought to be the most predominant site within the GI tract for these tumors. However, more recent studies reveal that the majority of GI carcinoid tumors occur in the small intestine (42%), rectum (27%), and stomach (9%), with only about 5% of these tumors being found in the appendix. In addition, approximately 1% of patients present with more than one primary site of disease.

The rates of carcinoid tumors have been examined in various subsets of the population. The age-adjusted incidence of carcinoid tumors is higher in African Americans compared with whites and is highest in African-American men.3 Tumors with the highest rates of metastases include cecal, pancreatic, and small intestinal carcinoid tumors (82%, 72%, and 58%, respectively).

The exact etiology of carcinoid tumors remains unknown. However, certain risk factors and conditions are associated with a predisposition for developing carcinoid tumors. Furthermore, genetic studies have revealed chromosomal abnormalities that occur more frequently in patients with these tumors. As mentioned, African-American men have the highest rate of developing GI carcinoid tumors.3 People with well-educated social backgrounds (relative risk [RR] = 2.8), living in major metropolitan areas (RR = 1.39), or who have a family history of a first-degree relative with a carcinoid tumor (RR ...

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