Usefulness of electron microscopy in the diagnosis of "small" round cell tumors of the sinonasal region.

The sinonasal region is known to harbor several types of tumors that belong to the general category of "small" round cell tumors and offer considerable diagnostic challenges. This study evaluated 33 cases of such tumors by electron microscopy to characterize their ultrastructural features in conjunction with immunohistochemistry, in an attempt to define diagnostic criteria of various types. Electron microscopy was useful in the proper classification of tumors in 27 cases: esthesioneuroblastoma (EN), 12; undifferentiated carcinoma, 6; melanoma, 3; lymphoma, 3; melanotic neuroectodermal tumor, 1; rhabdomyosarcoma, 1; and pituitary adenoma, 1. In the remaining six cases, the ultrastructural features were those of poorly differentiated carcinomas. They usually exhibited some epithelial characteristics as well as neuroendocrine features by immunohistochemistry and electron microscopy. These tumors could be best described as poorly differentiated neuro-endocrine carcinomas (malignant neuroepitheliomas). The most controversial diagnostic problems existed between the tumors categorized as esthesioneuroblastomas and neuroendocrine (NE) carcinomas. Esthesioneuroblastomas were characterized by uniform round nucleated cells with variable amounts of dendritic processes containing numerous dense core granules ranging from 150 to 350 nm in the perikarya and dendritic processes. Dendritic processes contained longitudinally arranged neural tubules and revealed an occasional synaptic junction. In three of the 12 cases of EN, cells with the appearance of sustentacular cells were recognized by electron microscopy. The NE carcinomas usually consisted of closely packed round cells with scanty cytoplasm that lacked any feature of neuroblastic cells. The tumor cells in this category often were epithelioid in appearance and exhibited a varying degree of cytokeratin positivity. Neuron-specific enolase was also positive in all cases, further suggesting their neuroepithelial nature. The greatest difference between EN and NE carcinomas was the absence of sustentacular cells in NE carcinomas. Immunohistochemical and electron microscopic studies are essential in the differential diagnosis of EN and NE carcinomas, because their microscopic appearance is very similar. The study indicates that EM is useful in the diagnostic categorization of sinonasal tumors of uncertain nature, particularly when it is used in conjunction with immunohistochemistry.