[Clinicopathologic and molecular genetic characterizations of biphenotypic sinonasal sarcoma]

Objective: To investigate the clinicopathologic characteristics, immunophenotypes, molecular genetics, and diagnostic and differential diagnostic features of biphenotypic sinonasal sarcoma (BSNS). Methods: Three cases of BSNS were retrieved, the histomorphology, immunophenotype and molecular genetics were analyzed with review of literature. Results: There were 2 male and 1 female patient aged 45, 29 and 40 years, respectively.Computed tomography and magnetic resonance imaging examinations showed a large polypoid mass occupying the sinonasal cavity in all 3 patients. Microscopically, these tumors were un-circumscribed and composed of cellular spindle-shaped cells arranged in long and interlaced fascicles. A hemangiopericytoma-like growth pattern was frequently identified. The overlying hyperplastic respiratory epithelium invaginated down into the tumor forming a cystic (2 cases), glandular (1 case) structures and inverted in a papilloma-like (1 case)pattern, and foci of eosinophilic metaplasia were also noted in 2 of the three cases. The tumor nuclei were bland-appearing, mitoses were scarce and necrosis was absent. Immunohistochemically, the tumor cells showed co-expression of neural and myogenic markers in all the 3 cases, including that 3/3 showed diffuse and strong positivity of S-100 protein, 3/3 positivity of smooth muscle actin (1 diffuse and 2 focal), 1/2 diffuse positivity of calponin, 1/3 focal positivity of desmin, and 1/1 focal positivity of MyoD1.In addition, 1 detected for β-catenin showed focal nuclear positivity. None of the 3 showed positivity to cytokeratin, CD34 or SOX10 in the tumor cells.Ki-67 showed an index <5%, 10% and <2%, respectively. Fluorescence in situ hybridization analysis showed rearrangements of PAX3 gene in all 3 cases. In case 3, reverse transcription polymerase chain reaction, followed by Sanger sequencing, demonstrated an in-frame fusion between PAX3 and FOXO1.Follow-up information (range 3-15 months)showed no evidence of local recurrence or distant metastasis in three cases. Conclusions: BSNS is a newly described entity which can be readily confused with a variety of benign and malignant spindle cell tumors encountered in the sinonasal cavity; immunohistochemistry co-expression of neural and myogenic markers and PAX3 gene rearrangement can help distinguish this tumor from its many mimickers.