SS18::POU5F1-fused sarcoma of the parotid with divergent ganglioneuromatous differentiation-a novel manifestation of a rare tumor.

SS18::POU5F1-fused sarcomas are rare tumors that show an undifferentiated round cell morphology. We report for the first time a case of SS18::POU5F1-fused sarcoma occurring in the parotid gland, with a highly unusual biphasic morphology comprising of epithelioid/undifferentiated round cell component and a ganglioneuromatous component consistent with divergent differentiation. We illustrate the cytology, histomorphological, immunohistochemical profile of the tumor, and show that both components of the tumor share a common origin. We discuss the differential diagnoses of this tumor, discuss its possible histogenesis, and perform a broad review of the literature covering this uncommon tumor.