Cases of unusual osteosarcoma comprising parosteal osteosarcoma, intraosseous well differentiated osteosarcoma, postradiation osteosarcoma, epithelioid osteosarcoma, and MFH-like osteosarcoma were reported. They were apparently differed from conventional osteosarcoma in the point of biologic activity and prognosis. Osteosarcoma is, therefore, considered as a heterogeneic tumor. It is important to make an exact diagnosis and to make an appropriate treatment depending on the diagnosis.
