NUT midline carcinoma of the parotid gland with mesenchymal differentiation.

Nuclear protein in testis midline carcinomas (NMC) are highly aggressive carcinomas typically arising in midline structures in young individuals. These carcinomas are characterized by the presence of a chromosomal rearrangement of nuclear protein in testis the (NUT) gene on chromosome 15 (15q14), resulting from a chromosomal translocation most commonly involving the BRD4 gene on chromosome 19p13. Rarely, in about 1/3 of cases, other translocation partners are involved (termed NUT-variants). Most cases have involved midline structures and with few exceptions were located in the upper aerodigestive tract and the mediastinum. Except for a single case, all reported NMC have been fatal, proving resistant to multimodality treatment. We report an exceptional case of a NMC presenting outside of midline structures in the parotid gland and showing mesenchymal chondroid differentiation in a 15-year-old male. The presence of the t(15;19) chromosomal translocation in the chondroid component was confirmed by fluorescence in situ hybridization analysis and immunohistochemical staining, indicating mesenchymal transdifferentation of the tumor. The findings demonstrate the first case of NMC arising within salivary gland, and the first example of mesenchymal differentiation in this group of tumors.