Calcitonin-producing insulinoma. An immunohistochemical and electron microscopic study.

A pancreatic endocrine tumor measuring 1.5 X 1.0 X 1.0 cm in a 67-year-old man with recurrent hypoglycemic attacks was examined by immunohistochemistry and electron microscopy. The tumor cells were relatively uniform, and almost all of them were immunoreactive to insulin. A few calcitonin-positive cells could be identified scattering in tumor cell strands, and the content of calcitonin in the tumor was 85 ng/g wet tissue. Coexistence of insulin and calcitonin in tumor cells was demonstrated by using adjacent semithin sections of the Epon-embedded material. Electron microscopically, the tumor consisted of a single type of cells with secretory granules identical to B-cell granules in the normal counterpart.