Parosteal osteosarcoma is a low-grade tumor microscopically. Accordingly, it has a well differentiated spindle cell stroma, minimal atypia and a low mitotic rate. The tumor originates from the outer fibrous layer of periosteum. The fibroblastic stroma may have foci of trabecular bone or cartilage. Parosteal osteosarcoma has a relatively good prognosis compared to conventional osteosarcoma.
Wide excision is the preferred treatment and there is normally no role for chemotherapy.
Unfortunately, parosteal osteosarcoma can dedifferentiate into a higher grade tumor. Treatment and Prognosis, W.B. Saunders, Co., 1991.