Expanding the spectrum of FUS::CREM-rearranged neoplasms: a case of mesenchymal malignant tumor with neuroendocrine differentiation

We report an additional case of FUS::CREM-rearranged neoplasm, with a distinctive clinical presentation and phenotype, in order to expand the spectrum of these tumors and to underline the major issues they raise in diagnosis and classification. A 52-year male patient, with a remote history of seminoma, presented with multiple lung, pancreatic, and renal tumor nodules, containing large nests of medium-sized epithelioid, monomorphic, tumor cells, which did not express any epithelial marker, but expressed all neuroendocrine markers. S100 protein labeled sustentacular-like cells. GATA3 and Phox2B were undetectable; CD99 was strongly positive. ALK was heterogeneously expressed. Ki-67 index ranged from 5 to 15% according to the location. A FUS (exon 7)::CREM (exon 6) fusion was detected in two tumors and confirmed by FISH. This paraganglioma-like malignant neoplasm may belong to the group of FET::CREB-rearranged mesenchymal neoplasms, currently in the course of delineation, and points to its phenotypic diversity.