A Rare Case of Primary Epithelioid Hemangioma of Bone with WWTR1::FOSB Fusion: A Benign Lesion with the Potential to Mimic Malignancy

Epithelioid hemangioma of bone is a rare benign, locally aggressive vascular tumor that can be particularly challenging to diagnose given its frequent multifocality, non-specific imaging findings, and wide range of morphologic appearances. Additionally, some epithelioid hemangiomas demonstrate atypical histologic features including increased cellularity, necrosis, and moderate cytologic atypia – characteristics that may raise concern for malignancy. Molecular studies can serve as a powerful, objective tool in the differential diagnosis of diagnostically challenging epithelioid vascular tumors. Importantly, FOS and FOSB gene rearrangements have been identified as the genetic hallmarks of osseous epithelioid hemangioma, present in greater than 70% of cases. FOSB-fusion-positive epithelioid hemangioma, in particular, may display atypical histologic features. While ZFP36 is the typical FOSB fusion partner in epithelioid hemangioma, we herein present a case of epithelioid hemangioma of bone with a rare WWTR1::FOSB fusion. This case demonstrates the diagnostic challenges associated with epithelioid hemangioma, especially in the setting of FOSB gene rearrangements, and the importance of genomic studies in the work up of these vascular tumors.