Primary epithelioid sarcoma of bone: report of a unique case, with immunohistochemical and fluorescent in situ hybridization confirmation of INI1 deletion.

We report the clinical and pathologic features of, what is to the best of our knowledge, the first case of epithelioid sarcoma of bone. A 31-year-old woman with an unremarkable past medical history presented with pelvic pain and was found by computed tomography scan to have a destructive 5 cm, partially calcified intraosseous lesion of the iliac bone. Histologically, the tumor consisted of relatively uniform but clearly malignant-appearing epithelioid cells, with scattered rhabdoid-appearing cells. A hyalinized to partially calcified matrix was present between the tumor cells, with a "chickenwire" pattern of calcification. By immunohistochemistry, the neoplastic cells expressed cytokeratins, vimentin, epithelial membrane antigen and CD34, and showed complete loss of INI1 protein expression. Fluorescence in situ hybridization showed homozygous deletion of the INI1 gene. An extensive clinical and radiographic workup did not show evidence of a soft tissue tumor, and the diagnosis of a primary epithelioid sarcoma of bone was made. After this, the patient underwent a complete resection of her tumor, and is currently disease free, 6 months after surgery. These extremely rare tumors must be rigorously distinguished from other more common tumors of bone, in particular, chondroblastoma and osteosarcoma. Awareness that epithelioid sarcoma may occur in bone, careful histologic evaluation and ancillary immunohistochemistry for epithelial markers, CD34 and INI1 protein should allow for recognition of such tumors. Study of additional cases of primary epithelioid sarcoma of bone will be necessary to better understand its clinical behavior.