Imprint cytology of biphenotypic sinonasal sarcoma of the paranasal sinus: A case report.

Biphenotypic sinonasal sarcoma (BSNS) is a rare low-grade spindle cell sarcoma that predominantly affects middle-aged women with multiple tumors in the sinonasal tract. BSNS shows biphenotypic expression of neural and myogenic markers on immunohistochemistry (IHC) with a specific chimeric PAX3-MAML3 fusion. The cytological features of BSNS have so far not been reported. Here, we describe a case of BSNS including findings of imprint cytology, histology, IHC, and genetic analysis. A 30-year-old woman presented with a nodular tumor that completely occupied the ethmoid sinus. The tumor was resected and submitted for imprint cytology, which revealed relatively bland spindle tumor cells that had mildly enlarged oval to spindle-shaped nuclei with fine nuclear chromatin and a thin nuclear rim in a clear background. Nucleoli were inconspicuous and there was no significant nuclear atypia and pleomorphism. These cytological findings were consistent with the histology of low-grade spindle cell sarcoma in BSNS. On IHC, the tumor cells were focally positive for S-100 protein and α-smooth muscle actin; nuclear β-catenin expression was also seen. PAX3 split signals were detected in 52% of tumor cells by fluorescence in situ hybridization. Reverse transcriptase-polymerase chain reaction also identified a chimeric PAX3-MAML3 fusion gene. Based on these findings, we diagnosed the tumor as BSNS. Our findings revealed that a relatively bland spindle cell cytology with a clear background is a characteristic feature of BSNS. BSNS should therefore be differentiated from benign and bland-appearing malignant spindle cell tumors and the combination of cytology, histology, IHC, and genetic analysis facilitates the diagnosis of BSNS.