Ewing sarcoma with myxoid stroma: Case report of an unusual histological variant

We describe the case of a Ewing sarcoma with prominent myxoid stroma of the temporal bone in a 26-year-old female. Histologically, the tumor exhibited a fascicular growth pattern of round to spindled cells with a minimal amount of pale eosinophilic to clear cytoplasm and oval or spindled nuclei with finely dispersed chromatin and small nucleoli. Myxoid changes were prominent (>50%), with reticular or pseudoacinar growth of the loosely cohesive cells. The tumor showed strong expression of CD99, FLI1, and CD56 and was positive for the EWSR1-FLI1 fusion transcript. The diagnosis of Ewing sarcoma with myxoid stroma (myxoid variant) is particularly problematic owing to the large number of potential mimics. The tumor extends the morphologic spectrum of Ewing sarcoma beyond the previously described histological variants, and broadens the differential diagnosis. For any round/spindle cell sarcoma, prominent myxoid stroma and CD99 immunoreactivity should prompt consideration for molecular studies that include analysis of both EWSR1 and FLI1.