Sinonasal adamantinoma-like Ewing sarcoma: A case report.

We describe the case of a sinonasal adamantinoma-like Ewing sarcoma in a 41-year-old male. Histologically, the tumor exhibited distinctive areas of nested growth pattern with prominent stromal fibrosis and metaplastic bone formation. The tumor cells were small and uniform with minimal amount of pale eosinophilic to clear cytoplasm and round or oval nuclei with finely dispersed chromatin and small nucleoli. Approximately 20% of the tumor parenchyma comprised of small clusters of basaloid cells within an osteofibrous background resembling adamantinoma. The tumor showed strong expression of keratins, p63, CD99 and Fli-1, and EWSR1 rearrangement. The diagnosis of sinonasal Ewing family tumors is particularly problematic owing to the large number of potential mimics. For any poorly differentiated or undifferentiated head and neck tumor, cellular monotony and CD99 immunoreactivity should prompt consideration for molecular studies that include analysis of both EWSR1 and FLI1, even in the presence of strong cytokeratin expression or focal keratinization.