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The majority of carcinoid tumors are slow-growing tumors that can be treated and often cured, especially in early stages. The occurrence of metastasis from carcinoid tumors relates directly to the size of the primary tumor (lesions 1 cm or less rarely metastasize; lesions greater than 2 cm frequently metastasize). They are classified as neuroendocrine or amine precursor uptake and decarboxylation tumors. Rarely, they may be a part of the multiple endocrine neoplasia syndrome type 1. Carcinoid tumors may arise from various sites, most commonly the gastrointestinal tract and the lung. The appendix, small bowel, and rectum account for over 90% of surgical cases occurring in the gastrointestinal tract. Small bowel carcinoids may occur in multiple sites in the same patient. Symptoms may be chronic, suggesting partial obstruction or intussusception. Carcinoid tumors, except those originating in the rectum, produce a variety of endocrine substances, the most frequent of which are serotonin (5-hydroxytryptamine) and kallikrein (an activator of bradykinin release). The diagnosis of carcinoid syndrome (a syndrome associated with flushing, diarrhea, bronchoconstriction, cardiac valvular lesions, and telangiectasia) is aided by demonstrating elevated 24-hour urinary 5-hydroxyindoleacetic acid levels. This test is not useful in the diagnosis of carcinoids at a curable stage, except in some rare cases in which the tumor arises from a site outside of the gastrointestinal tract, such as the lung.[2-7] Blood chromogranin A assay may also be a useful, though nonspecific, confirmatory test for carcinoid or neuroendocrine tumors. Primary carcinoids of the extrapelvic colon are uncommon, typically present with metastatic disease, and have a poor prognosis.[9,10] Patients with carcinoid tumor are at increased risk for synchronous or metachronous second malignancies. The most common site for a second primary malignancy is the gastrointestinal tract.
The relatively rare carcinoid syndrome relates to the release of bioactive substances, but precise pharmacologic mechanisms are still unclear. Because of efficient hepatic metabolism of vasoactive amines, the carcinoid syndrome rarely occurs in the absence of liver metastases. Exceptions are circumstances where venous blood draining from tumors enters directly into the systemic circulation (for example, pulmonary and ovarian primaries, and pelvic or retroperitoneal involvement by metastatic or locally invasive small bowel carcinoids or extensive bone metastases).
Surgical resection is the standard curative modality. If the primary tumor is localized and resectable, 5-year survival rates are excellent (70%-90%). Even in patients with distant metastasis, the disease is usually very indolent, with median survivals of 2 years or more. When necessary, excellent palliation may be achieved by bypass surgery or resection of large hepatic metastases that may produce the carcinoid syndrome. Radiation therapy has a minor role in patients with regionally unresectable disease and may palliate the pain of bone metastasis. Patients with carcinoid syndrome can usually be effectively palliated by injections of somatostatin analogue 2 to 3 times a day. A long-acting somatostatin analogue that can be given as an injection once a month, with equivalent efficacy, is now available.
Patients with symptomatic metastatic carcinoid disease are appropriate candidates for clinical trials examining combination chemotherapy, since single-agent standard chemotherapy provides minimal palliation. However, chemotherapeutic drug combinations occasionally do offer long-lasting (in excess of 1 year) palliation. In patients with the carcinoid syndrome, palliation is sometimes obtained with pharmacologic agents that suppress production or block the action of vasoactive amines; of particular interest is a somatostatin analogue. Some patients benefit from the use of interferon alfa. Toxic effects associated with interferon treatment that frequently outweigh therapeutic gains can occur in some patients, but these effects are reversible once treatment has been discontinued and usually do not occur with smaller doses. Anecdotal reports of biologic activity indicate that some patients may respond to combined octreotide and interferon alfa treatment. Patients with asymptomatic metastases that cannot be resected for cure will often remain symptom-free for long periods of time.
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