Primary Epithelioid Sarcoma of the Zygomatic Bone.

Primary epithelioid sarcoma (ES) of bone is extremely rare with only 2 reported cases in the English literature. A previously healthy 18-year-old man presented with a 6-month history of right facial numbness and tingling and right eye diplopia. A computerized tomography scan revealed an ill-defined mass with dense osseous matrix centered in the right zygomatic bone. An outside biopsy was read as osteosarcoma. The resection specimen revealed large epithelioid and spindle cells embedded in a prominent hyalinized matrix with focal metaplastic bone formation. The tumor cells were strongly and diffusely positive for AE1/AE3 and epithelial membrane antigen, but a definitive diagnosis of ES was not immediately reached due to the presence of dense hyalinized matrix and weak expression of SAT2B by tumor cells. Deficient INI1 protein expression by immunohistochemistry and homozygous loss of the SMARCB1 gene by chromosomal microarray analysis ultimately justified this tumor's designation as ES.