Biphenotypic sinonasal sarcoma: demographics, clinicopathological characteristics, molecular features, and prognosis of a recently described entity.

Biphenotypic sinonasal sarcoma (BSNS) is a recently recognized type of sarcoma arising exclusively in the sinonasal tract displaying unique clinical course, histopathology, and genetics. Due to its rarity, only case series and case reports are available. In order to provide an overview of the current understanding of this disease, we present a comprehensive review of the literature and present three previously unreported cases of BSNS. A total of 55 genetically characterized and 41 cases without molecular data were identified in the literature. Two-thirds of patients were female and the peak incidence was in the fifth decade. Fatal outcome was rare (two cases with intracranial extension) and local recurrence occurred in 31.6%, all occurring within 5 years after initial treatment. Histologically, BSNS is highly cellular in the majority of cases and composed of fascicles of spindle cells, with entrapped hyperplastic surface epithelium being a frequent finding. The immunohistochemical profile is characteristic due to the biphasic nature of this lesion, with shared features of both myogenic and neural origin. Rhabdomyoblastic differentiation is apparent in a subset of cases. The most common genetic event is the PAX3-MAML3 fusion (58.6%) but isolated PAX3 rearrangement (19.2%), absence of rearrangements (9.1%), PAX3-FOXO1 (8.1%), PAX3-NCOA1 (4%), and isolated MAML3 rearrangement (2%) have also been reported. In conclusion, the recognition of BSNS is crucial due to its relatively indolent clinical course. A selected immunohistochemical panel and/or molecular confirmation can be used to aid in appropriate diagnosis and consequently in prognostication and to avoid overtreatment with chemotherapy regimens used in its mimics.