"Undifferentiated" neoplasms of the sinonasal region: differential diagnosis based on clinical, light microscopic, immunohistochemical, and ultrastructural features.

Sinonasal neoplasms and neoplasm-like proliferations composed of light microscopically poorly differentiated or undifferentiated, small- to medium-sized cells cause considerable diagnostic confusion. Lesions in this category include lymphoepithelioma (undifferentiated carcinoma), olfactory neuroblastoma, small-cell undifferentiated (oat cell) carcinoma, sinonasal undifferentiated carcinoma, malignant melanoma, pituitary adenoma, lymphoid hyperplasia, malignant lymphoma, plasmacytoma, lymphomatoid granulomatosis, rhabdomyosarcoma, mesenchymal chondrosarcoma, small cell osteosarcoma, Ewing's sarcoma, and synovial sarcoma. Many of these lesions can be definitively diagnosed based on light microscopic features alone, but, in some instances, additional techniques such as immunohistochemistry are of value. The authors review the pertinent clinicopathologic features of the above lesions, with emphasis on light microscopic, immunohistochemical, and ultrastructural features of particular utility in differential diagnosis.