ABSTRACT
Neuroendocrine tumors (NETs), which include those that arise in the pancreas (also known as pancreatic neuroendocrine tumors or pancreatic NETs), the gastrointestinal tract and from neuroendocrine cells scattered in other tissues (carcinoids) are rare. These tumors were initially believed to arise from a putative common precursor, the amine precursor uptake and decarboxylation (APUD) cell, although this is now known not to be the case. These NETs are histologically closely related to melanoma, pheochromocytoma, and medullary carcinoma of the thyroid, which are described elsewhere (see chaps. 16, 30, and 48). All NETs have the potential to synthesize and secrete hormones. Functioning tumors are those in which hormone secretion by the tumor results in a clinical syndrome. Non-functioning tumors are those in which either there is no hormone secretion or hormone secretion does not result in a recognizable clinical syndrome. Functioning tumors usually present relatively early, due to the clinical syndrome, and may be a challenge for the radiologist to localize as they are often small. In contrast, non-functioning tumors generally go unrecognized for many years and present much later with mass effects.
