As with other endocrine tumors, prediction of malignant behavior can not be made on the histologic appearance. Carcinoids are slow growing low grade malignant tumors most of which do not metastasize and are asymptomatic at death. Appendiceal and rectal carcinoids virtually never metastasize, while the great majority of metastasizing carcinoids arise in the Ileum, usually distal Ileum. The most common symptom is non-specific abdominal pain, which has often been present for a number of years. Bleeding is rare, and obstruction can occur due to size of the tumor, desmoplastic thickening of the intestinal wall, intestinal infarction secondary to mesenteric vessel compression by tumor, or intussusception.
Metastases first occur in regional nodes and then the liver.
The most striking presentation of these tumors is the Carcinoid Syndrome. The presence of the Carcinoid Syndrome indicates release of polypetoide hormones or vasoactive amines into the general circulation and in most cases denotes bulky hepatic metastases. The two most common symptoms of the Carcinoid Syndrome are flushing and diarrhea, and these are almost always associated with an elevated 5-HIAA, although the amines rsponsible for the symptoms are not clearly identified. Robbins Pathologic Basis of Disease.