We report herein on a juxtacortical Ewing sarcoma of the humerus in an 8-year-old girl. Radiological findings revealed a periosteal mass with periosteal reaction, suggestive of a periosteal osteosarcoma, chondrosarcoma or chondroma. Imprint cytology on site, however, showed uniform small round cells with round nuclei containing fine chromatin and scanty cytoplasm. Subsequent genetic analysis using the fresh specimen showed EWSR1-FLI1 fusion gene transcripts. The pathological findings revealed that the tumor was composed of small round cells that were immunoreactive with CD99 and NKX2.2. In this case, imprint cytology played a pivotal role in guiding the genetic analysis and confirming the diagnosis of a juxtacortical Ewing sarcoma.
