Functioning oncocytic islet-cell carcinoma. Report of a case with electron-microscopic and immunohistochemical confirmation.

A case of a 58-year-old woman with an unusual variant of malignant islet-cell tumor showing oncocytic features is described. Using the light microscopy technique, the tumor appeared comprised of solid nests of uniform cells with abundant, eosinophilic cytoplasm and round nuclei with granular chromatin. Ultrastructurally, the cells contained numerous abnormal mitochondria, dilated rough endoplasmic reticulum, and scattered dense-core neurosecretory granules, often associated with cytoplasmic filaments. Tumor cells were focally immunoreactive for insulin, glucagon, and somatostatin and diffusely immunoreactive for alpha 1-antitrypsin as assayed by the avidin--biotin technique. The tumor was immunonegative for human chorionic gonadotropin, gastrin, adrenocorticotropic hormone, and serotonin. The patient exhibited some of the clinical features associated with glucagonoma syndrome, including diabetes mellitus and chronic diarrhea. The tumor behaved in a malignant fashion, with widespread lymphatic involvement and bony metastases at the time of presentation. This report of an oncocytic islet-cell carcinoma supports the concept of oncocytic differentiation in islet-cell tumors in a fashion analagous to oncocytic carcinoids.