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 b elong 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 ultrastructur al 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: esthesioneu roblastoma (EN), 12; undifferentiated carcinoma, 6; melanoma, 3; lymph oma, 3; melanotic neuroectodermal tumor, 1; rhabdomyosarcoma, 1; and p ituitary adenoma, 1. In the remaining six cases, the ultrastructural f eatures were those of poorly differentiated carcinomas. They usually e xhibited some epithelial characteristics as well as neuroendocrine fea tures by immunohistochemistry and electron microscopy. These tumors co uld be best described as poorly differentiated neuroendocrine carcinom as (malignant neuroepitheliomas). The most controversial diagnostic pr oblems existed between the tumors categorized as esthesioneuroblastoma s and neuroendocrine (NE) carcinomas. Esthesioneuroblastomas were char acterized by uniform round nucleated cells with variable amounts of de ndritic processes containing numerous dense core granules ranging from 150 to 350 nm in the perikarya and dendritic processes. Dendritic pro cesses contained longitudinally arranged neural tubules and revealed a n 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 rou nd cells with scanty cytoplasm that lacked any feature of neuroblastic cells. The tumor cells in this category often were epithelioid in app earance and exhibited a varying degree of cytokeratin positivity. Neur on-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 mic roscopic appearance is very similar. The study indicates that EM is us eful in the diagnostic categorization of sinonasal tumors of uncertain nature, particularly when it is used in conjunction with immunohistoc hemistry.