Purpose: To report the clinicopathologic features of a 17-year-old patient with primary Ewing sarcoma of the orbit. Methods: The patient was evaluated clinically before surgery with computed tomography scans of the orbit. After surgery, the patient was staged with computed tomography scans and bone scan and was treated with systemic chemotherapy and radiation therapy to the orbit. The orbital biopsy was evaluated with conventional light microscopy and immunohistochemistry. Results: Clinical evaluation revealed proptosis and limited upgaze. Computed tomography scans disclosed a mass involving the superior orbit, anterior cranial fossa, and temporal fossa. Microscopic examination revealed small, poorly differentiated cells with medium-sized nuclei containing finely granular chromatin and small nucleoli. The cytoplasmic borders of the cells were indistinct. A PAS stain revealed modest glycogen in many of the tumor cells. The tumor stained positive for O-13 and vimentin and was negative for neural, skeletal, and lymphoid cell markers. Computed tomography scan, bone scan, and blood chemistries revealed no other site of involvement. After treatment, the clinical symptoms and signs resolved, and there has been no evidence of residual orbital tumor or metastasis. Conclusions: Primary Ewing sarcoma of the orbit should be considered in the differential diagnosis of children or young adults with proptosis, diplopia, or periorbital swelling. Immunohistochemistry is essential to distinguish Ewing sarcoma from other small round cell tumors.
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