Adrenocortical oncocytoma: case report and review of the literature.

Only 9 oncocytic neoplasms of true adrenal origin have been described to date. It therefore seemed of interest to study the histochemical and ultrastructural features of a non-functioning monolateral adrenocortical oncocytoma which was incidentally detected by ultrasonography and magnetic resonance imaging in a 28-year-old woman. The tumor was round, well encapsulated and weighed 73 g. It consisted of islets of eosinophilic cells, and did not display any sign of necrosis. The proliferation rate (as evaluated by mitotic index and percentage of MIB-1 Ki67 positive cells) was low, and atypic mitoses were absent; some rare cells with nuclear atypias were observed and the capsule was focally invaded by oncocytes. Immunocytochemistry did not show expression of vimentin or cytokeratin. The oncocytes had an abundant cytoplasm packed with mitochondria containing plate-like cristae. Smooth endoplasmic reticulum was virtually absent, while rough endoplasmic reticulum cisternae and free ribosomes were abundant. Although the classic histological approach clearly indicates the benign nature of the tumor, the immunocytochemical and ultrastructural features of oncocytes may suggest their potential for malignant behavior.