Metastatic Sclerosing Epithelioid Fibrosarcoma in Bone Marrow

Sclerosing epithelioid fibrosarcoma (SEF) is an aggressive neoplasm thought to be related to low-grade fibromyxoid sarcoma, which typically occurs in middle-aged adults in the deep soft tissues of the lower extremities and trunk. It comprises nests and cords of relatively uniform epithelioid polygonal cells with clear or eosinophilic cytoplasm in densely sclerotic stroma, and it is typically associated with EWSR1 gene rearrangements, and most commonly EWSR1-CREB3L1 fusions. As primary SEF can arise in bone, and bone is also a common metastatic site for SEF, its recognition at this site is important. We illustrate bone marrow showing diffuse infiltration by SEF and highlight the potential for confusion with a range of neoplasms such as carcinoma, hematolymphoid neoplasms, and other sarcomas.