A malignant epithelioid tumor with EWSR1 fusion mimics primary pulmonary myxoid sarcoma (PPMS): a variant of PPMS or under-recognized entity?

Abstract Background Primary pulmonary myxoid sarcoma (PPMS) is an extremely rare soft tissue tumor that is characterized in most cases by the oncogenic fusion gene EWSR1-CREB1. Tumor cells with epithelioid differentiation have not been described in PPMS. We report a primary lung tumor with morphological, immunohistochemical and molecular features closely mimics PPMS, but with concomitant epithelioid differentiation. Is this a variant of PPMS or under-recognized entity? Case presentation: A 34-year-old female patient showed a CT scan showing nodular opacities around the upper lobe of the left lung with a diameter of 8 mm, which was mildly unevenly enhanced after enhancement. Histologically there were two tumor areas, one of which presented a typical PPMS morphology, and the tumor cells were stellate, polygonal, epithelial and arranged as reticular and cords in abundant myxoid stroma. The other tumor area presented an epithelioid cell morphology. The tumor cell immunophenotype was positive for vimentin, INI1 and negative for TTF-1, P40, S-100, SOX10, CK5/6, SMA, Desmin, PAX8, Brachyury and WT1. Interestingly, immunohistochemistry confirmed positive AE1/AE3 in epithelioid cell regions, while AE1/AE3 was negative in typical PPMS regions. Molecular analysis demonstrated the presence of EWSR1-CREB1 gene fusions in the tumor. Conclusion PPMS is a very rare lung malignant soft tissue tumor with distinct histological and genetic features. Our challenge case had morphological, immunohistochemical and molecular features similar to PPMS, but with epithelioid differentiation.We prefer to describe this tumour as a variant of PPMS instead of a new entity.