AIMS: Ossifying fibromyxoid tumour is a rare mesenchymal neoplasm predominantly affecting adults characterised by a multinodular growth pattern and the presence of a fibrous pseudocapsule with areas of ossification. Prompted by the recognition of a non-ossifying ossifying fibromyxoid tumour with lipomatous differentiation which caused diagnostic difficulty, we sought to further explore cases of ossifying fibromyxoid tumour with non-osseous heterologous elements. METHODS AND RESULTS: A search of our institutional and consultation archives revealed three additional cases that demonstrated lipomatous components and two cases with cartilaginous differentiation. RNA-sequencing revealed fusions involving PHF1 (n = 4) or EPC1 (n = 1) in all (five of five) cases tested, including EPC1::PHC1 and JAZF1::PHF1 fusions, which have not been reported before in ossifying fibromyxoid tumour. CONCLUSION: These six cases expand the histomorphological spectrum of ossifying fibromyxoid tumour, introducing lipomatous differentiation as a hitherto undocumented feature. Awareness of these rare variants will ensure appropriate diagnosis and clinical management.

Bioptical Laboratory Pilsen Czech Republic

Department of Anatomic Pathology Mayo Clinic Rochester Minnesota USA

Department of Pathology and Laboratory Medicine Cleveland Clinic Cleveland Ohio USA

Department of Pathology Charles University Faculty of Medicine in Pilsen Pilsen Czech Republic

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