ABSTRACT
Small cell osteosarcoma is a rare form of osteosarcoma and the histological differential diagnosis from other small round cell tumors (SRCTs) is difficult. The immunohistochemical stain for MIC2 has been considered an useful diagnostic marker for Ewing's sarcoma and primitive neuroectodermal tumors but recently, other SRCTs such as malignant lymphoma and embryonal rhabdomyosarcoma also showed positive reaction. Therefore, the usefulness of MIC2 must still be proven. We experienced a case of small cell osteosarcoma of the mandible in a 25-year-old man. Histologically, the tumor consisted of small round cells that resembled those of Ewing's sarcoma. Immunohistochemically, the tumor cells expressed diffuse strong positive reaction for MIC2 gene products. However, the scanty foci of lacy osteoid material between the tumor cells seemed to be diagnostic of osteosarcoma. The histologic and immunohistochemical findings of this case suggest close relationship between small cell osteosarcoma and Ewing's sarcoma.