Abdominal small round cell tumor with osteoid and EWS/FLI1.

In this report, we described a case of multiple intraperitoneal tumors. Histologically, the tumors were composed of small round cells with malignant phenotype, necrotic areas, and islands of osteoid matrix in the stroma. In immunohistochemical and molecular analyses, the tumors expressed CD99 and EWS-Fli1 fusion gene. Production of osteoid by small round tumor cells was consistent with the histologic criteria of small-cell osteosarcoma, whereas expression of EWS-Fli1 was a characteristic genetic feature of Ewing's sarcoma family of tumor. Such tumors have been limited to a case in which histologically proven small-cell osteosarcoma of the scapula showed a chromosomal translocation, t(11;22)(q24;q12).