Two rare cases of FET::CREB epithelioid mesenchymal tumors, displaying atypical immunophenotype (myoepithelial-like), unraveled on next-generation sequencing.

The clinicopathological spectrum of FET-CREB sarcomas is evolving with several distinct tumors entities described across various body sites, apart from the soft tissues. Lately, there are tumors primarily identified in the abdominal and extra-abdominal sites, including the soft tissues with an epithelioid morphology and displaying FET-CREB fusions, which are different from the established tumors showing the similar underlying gene fusions. An 18-year-old female patient presented with pain in her upper back. She underwent An An 18-year-old female patient presented with pain in her upper back. She underwent magnetic resonance imaging (MRI), which revealed a large lobulated, intraspinal, extra-axial mass, measuring 4.1 cm in the largest dimension. Histopathological examination of the biopsy revealed a tumor composed of moderate to focally, markedly pleomorphic, round to polygonal to spindle-shaped cells, including vacuolated forms in a myxoid-rich stroma with intervening branching blood vessels. By immunohistochemistry, the tumor cells were patchily positive for epithelial membrane antigen (EMA), and focally for pan keratin (AE1/AE3), S100 and desmin. A 43-year-old lady presented with a gradual, left-sided hearing loss of one year duration. On clinical examination, there was a soft palatal bulge. Computed tomogram (CT) scan showed a 4.9 cm-sized mass in the left parapharyngeal space. Microscopic examination of the excised mass revealed an infiltrating tumor composed of round to oval cells arranged in solid sheets, cords and cribriform pattern in a densely hyalinized and myxoid stroma. Few mitotic figures were noted. By immunohistochemistry, the tumor cells were focally positive for EMA, ALK and diffusely positive for S100 and SOX10. Fluorescence in-situ hybridization (FISH) revealed EWSR1 rearrangement. Next-generation sequencing (NGS) revealed EWSR1::CREB1 fusion in the former and EWSR1::ATF1 fusion in the latter tumor. The former patient is alive with the disease and the latter patient in on maintenance chemotherapy. The two tumors, occurring at distinct sites, histopathologically exhibiting a relatively atypical immunophenotype, including "myoepithelial-like" differentiation seem to further expand the clincopathological spectrum of tumors with epithelioid morphology and underlying FET-CREB fusions.