Neoplasms of the appendix are rare. They are found in about 1 percent of appendectomy specimens  and account for only about one-half of 1 percent of intestinal neoplasms . Carcinoid tumors are the most common, comprising over 50 percent of appendiceal neoplasms in most series (table 1) .
However, the distribution of appendiceal neoplasms might be changing over time. In a large series of appendiceal tumors derived from the Surveillance, Epidemiology and End Results (SEER) database of the National Cancer Institute between 1973 to 2003, the most frequent histology was a mucinous adenocarcinoma followed by intestinal-type adenocarcinoma; carcinoid tumors comprised only 11 percent (table 2) . The difference in natural history of these different histologies is illustrated by their differing five-year disease-specific survival rates.
The term “carcinoid” is generally applied to neuroendocrine tumors originating in the digestive tract, lungs, or rare primary sites such as kidneys or ovaries. Use of the term carcinoid usually implies well-differentiated histology, but the term can be applied to the rare high-grade or poorly differentiated neuroendocrine tumors. (See "Pathology, classification, and grading of neuroendocrine tumors arising in the digestive system", section on 'Classification'.)
As with other intestinal carcinoid tumors, appendiceal carcinoids can secrete serotonin and other vasoactive substances (table 3). These substances are responsible for the carcinoid syndrome, which is characterized by episodic flushing, wheezing, diarrhea, and right-sided valvular heart disease. Midgut carcinoids (ie, appendix, small bowel) are more commonly associated with the classic carcinoid syndrome than are those that arise in the foregut or hindgut (table 4). More than 90 percent of patients with the carcinoid syndrome have metastatic disease, typically to the liver. (See "Clinical characteristics of carcinoid tumors" and "Clinical features of the carcinoid syndrome".)
Appendiceal carcinoids are detected most commonly in patients in their forties, which is much younger than the average age for other primary malignant appendiceal neoplasms [4-6]. Epidemiologic studies show a slight but consistently higher incidence in women. (See "Clinical characteristics of carcinoid tumors", section on 'Appendix'.)