A rare insulin‐immunoreactive neuroendocrine tumor of the duodenum in a 54 year old male is reported. The incidentally identified tumor was located on the anterior free wall of the duodenal bulb and measured approximately 6 mm in diameter. Uncomplicated endoscopic resection of the tumor was carried out. The lesion exhibited classic histologic features of insulinoma of the β‐islet cell type with stromal amyloid deposition. In addition to positive reactivities of chromogranin A, neuron‐specific enolase, synaptophysin, Leu 7 (CD57), cystatin C, CA15–3 and cytokeratin, the non‐argyrophilic tumor cells were strongly immunoreactive for insulin and C‐peptide. The stromal amyloid was clearly labeled for amylin. A few cells were stained for somato‐statin, whereas other hormones were negative. Interestingly, a few isolated insulin‐positive cells were identiii in the non‐neoplastic duodenal mucosa in the proximity of the tumor. lmmunoelectron microscopy using paraffin sections disclosed insulin‐immunoreactive secretory granules in the cytoplasm. The patient exhibited no signs or symptoms of hypoglycemia. Serum insulin levels were not measured prior to resection. No tumors were demonstrated in the pancreas. Magnetic resonance imaging revealed a 1 cm asymptomatic pituitary mass, in association with moderately elevated serum prolactin levels. The patient is currently being followed up in t
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