A rare, usually malignant neuroendocrine tumor arizing from delta cells. This neoplasm produces large amounts of somatostatin, which may result in a syndrome characterized by diarrhea, steatorrhea, weight loss, and gastric hyposecretion. Sixty percent are found in the pancreas and 40% in the duodenum or jejunum. The peak incidence occurs between 40 and 60 years of age; women are affected more than men by 2:1.