Characteristics of carcinoid tumors
Carcinoid tumors are neuroendocrine tumors of the lung, GI tract, or genitourinary system. They are classified based on their embryonic origin (i.e., foregut, midgut, and hindgut tumors). These tumors convert dietary tryptophan into serotonin. Serotonin is metabolized to 5-hydroxyindoleacetic acid (5-HIAA). Elevated levels of 5-HIAA are diagnostic. These tumors also secrete other vasoactive substances such as histamines, tachykinins, kallikrein, and prostaglandin. Clinical features associated with excess serotonin (i.e., diarrhea, valvular fibrosis) and histamine (i.e., flushing, bronchospasm) are referred to as carcinoid syndrome. The presence of these systemic features indicates the carcinoid tumor has extraintestinal tumor location or liver metastasis.