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In what locations do carcinoid tumors occur?
Tumors most commonly arise from the embryonic midgut, i.e., small intestine – ileum, jejunum, with the high-est incidence occurring in the appendix. Occasionally, these tumors arise from the foregut, i.e., the stomach, pan-creas, thyroid, bronchus and from ovarian or testicular ter-atomas. Bronchogenic carcinoid is indistinguishable from undifferentiated small cell carcinoma of the lung. Pathologically, gastric tumors tend to produce histamine rather than serotonin-derived products, which are more common in tumors of the small intestine. Small cell tumors and bronchogenic carcinoid of the lung are associ-ated with the syndrome of inappropriate antidiuretic hormone (SIADH). These patients commonly present with euvolemic hyponatremia. Treatment requires vasopressin, an antidiuretic hormone (ADH) inhibitor, secreted from the posterior pituitary gland. Chronic hyponatremia is common in these patients. Hyponatremia is particularly problematic for patients undergoing transurethral resec-tion of the prostate (TURP). During TURP, absorption of glycine through the prostatic vascular bed can exacerbate the hyponatremia. Severe or acute hyponatremia can present with confusion and seizures.
The small intestine, particularly the ileum, is by far the most common site for carcinoid tumors of any clinical significance. These tumors tend to be small and tend to metastasize after reaching 2 cm in size. They have a strong tendency toward multicentricity, invading the mesentery and producing intermittent bowel obstruction, which is a common clinical presentation. As metastatic disease progresses, mesenteric lymph nodes become involved, and finally the liver. Bone is the next most frequent metastatic site. The more unusual metastatic locations include the breast, orbit of the eye, and the myocardium. Extrahepatic metastases occur in the bone where they tend to be osteoblastic rather than osteolytic. Tumors can also spread to the adrenal glands, ovaries, spleen, pancreas, and the lung.
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