YAP1::MAML2-Rearranged Poroid Squamous Cell Carcinoma Arising in a Non-sebaceous Lymphadenoma of the Parotid Gland.

Salivary gland lymphadenoma is an uncommon neoplasm and malignant transformation is even rarer. Reported tumor types include basal cell adenocarcinoma, sebaceous carcinoma, EBV-associated lymphoepithelial carcinoma, and undifferentiated carcinoma; a squamous phenotype has not been reported. Here we describe the case of a 62-year-old female who underwent total parotidectomy for a painful parotid gland mass of uncertain duration. On microscopic examination, a high grade lymphoepithelial carcinoma with squamous and poroid features arising in a non-sebaceous lymphadenoma was recognized. Both benign and malignant components expressed CK5/6 and ΔNp63 (p40), however SOX2 was overexpressed in the malignant component. S100, SOX10, CD117, PLAG1 and HMGA2 immunostains were negative. In-situ hybridization for high-risk HPV RNA and EBER were also negative. Fluorescence in situ hybridization revealed MAML2 rearrangement with RNASeq confirming a YAP1::MAML2 fusion product. This case highlights a novel salivary gland malignancy type arising from lymphadenoma, for which molecular testing was critical in establishing primary origin, and thus excluding the more common metastatic squamous cell carcinoma.