Origin and general involvement and presentation
Carcinoid tumors are derived from primitive stem cells in the gut wall but can be seen in other organs (Broaddus, 2003), including the lungs (Moraes, 2003), mediastinum, thymus (Soga, 1999), liver, pancreas, bronchus, and ovaries (Piura, 1995). In children, most tumors occur in the appendix and are benign and asymptomatic. Although rare, aggressive and metastatic disease (eg, to the brain) does occur; even tumors in the appendix can metastasize (Hlatky, 2004; Volpe, 2000). Depending on size and location, carcinoid tumors can cause various symptoms, including carcinoid syndrome. Carcinoid tumors of the ileum and jejunum, especially those larger than 1 cm, are most prone to produce this syndrome, at least in adults.
Classification
Carcinoid tumors generally are classified based on the location in the primitive gut (ie, foregut, midgut, hindgut) that gives rise to the tumor. Foregut carcinoid tumors are divided into sporadic primary tumors and tumors secondary to achlorhydria. The term sporadic primary foregut tumor encompasses carcinoids of the bronchus, stomach, proximal duodenum, and pancreas. Midgut tumors are derived from the second portion of the duodenum, the jejunum, the ileum, and the right colon. These account for 60-80% of all carcinoid tumors (especially those of the appendix and distal ileum) in adults and are also seen in children (Schmittenbecher, 2001). Appendicular carcinoid tumors are most common (Bethel, 1997; Pelizzo, 2001). In children, more than 70% of these tumors occur at the tip of the appendix and are often an incidental finding in appendectomy specimens. In one study, carcinoid tumors were found in 0.169% of 4747 appendectomies (Doede, 2000). Bulky tumors are relatively rare and require somewhat extensive cecectomy or, when tumor infiltration is beyond the cecum, ileocecal resection (Soreide, 2000; D'Aleo, 2001; Pelizzo, 2001).
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