Poorly differentiated sinonasal tract malignancies: A review focusing on recently described entities

Sinonasal tract malignancies are uncommon, representing no more than 5% of all head and neck neoplasms. However, in contrast to other head and neck sites, a significant proportion of sinonasal neoplasms tend to display a poorly/ undifferentiated significantly overlapping morphology and a highly aggressive clinical course, despite being of diverse histogenetic and molecular pathogenesis. The wide spectrum of poorly differentiated sinonasal epithelial neoplasms with small "basaloid" blue cell morphology includes basaloid squamous cell carcinoma (both HPV+ and HPV-unrelated), nasopharyngeal-type lymphoepithelial carcinoma (EBV+), small/large cell neuroendocrine carcinoma, esthesioneuroblastoma, poorly differentiated carcinoma of salivary type (myoepithelial carcinoma and solid adenoid cystic carcinoma), NUT midline carcinoma, the recently described SMARCB1-deficient sinonasal carcinoma, sinonasal teratocarcinosarcoma and, as a diagnosis of exclusion, sinonasal undifferentiated carcinoma (SNUC). On the other hand, a variety of sarcomas, melanoma and haematolymphoid malignancies have a predilection for the sinonasal cavities, and they occasionally display aberrant cytokeratin expression and show small round cell morphology thus closely mimicking poorly differentiated carcinomas. This review summarizes the clinicopathological features of the most recently described entities and discuss their differential diagnosis with emphasis on those aspects that represent pitfalls.