Abstract
The sinonasal cavities host a wide variety of undifferentiated malignancies with round cell morphology, including neoplasms of epithelial, mesenchymal, neuroectodermal, and hematolymphoid lineage. The differential diagnosis may be difficult, especially in small biopsy material, due to overlapping morphology, but their correct classification is clinically relevant. The aim of this review is to provide practical guidelines for the differential diagnosis of these malignancies, with emphasis on recently described entities and special reference to the role of ancillary techniques.