Primary paranasal sinus clear cell carcinoma with EWSR1-ATF1 fusion: report of 2 molecularly confirmed cases exhibiting unique histopathology.

Hyalinizing clear cell carcinoma (HCCC) is a rare low-grade tumor of the salivary glands made up of clear cells that form cords and nests in hyalinized stroma. To date, primary HCCCs of the paranasal sinus have not been described. This article presents 2 cases of HCCC of the maxillary sinus with unusual glandular formation and lymphoplasmacytic stroma in case 1 and a characteristic solid nest pattern and fibrocellular and hyalinized stroma in case 2. Immunohistochemical studies excluded myoepithelial origin and sinonasal renal cell-like adenocarcinomas. Negativity for p63 and p40 in case 1 ruled out a squamous cell origin. Both cases showed a rearranged EWSR1 gene. Reverse-transcription polymerase chain reaction detected EWSR1-ATF1 fusion gene transcripts, and Sanger sequencing confirmed an EWSR1 exon 11 fused in-frame to ATF exon 3.