Biphenotypic Sinonasal Sarcoma With High-Grade Features: A Diagnostic Challenge.

Biphenotypic sinonasal sarcoma (BSNS) is a rare neoplasm of the sinonasal tract. These tumors show neural and myogenic differentiation and are characterized by PAX3 translocations. The immunophenotypic features reflect their dual differentiation. They are low-grade sarcomas that show monomorphic spindle cells in sheets, fascicles, and herringbone patterns and are positive for S100 and smooth muscle actin. These tumors are common in elderly female patients and have a locally aggressive course. High-grade presentation or transformation was not documented until recently. Total 3 BSNSs have now been documented in the literature and we report a fourth tumor with high-grade transformation 8 years after the initial presentation. We identify the morphologic and immunohistochemical features of the high-grade areas and we highlight the stark differences with the low-grade areas based on literature and our specimen. We also discuss the diagnostic challenges that may come up with such a presentation.