The tumor merges with the cortex by a stalk early in the disease or has a broad base later in the progression. The bone marrow of the bone does not extend into the base of the tumor. There is occasional medullary involvement by the tumor. CT scan is very useful in demonstrating a radiolucent zone of periosteum and fibrous tissue that becomes trapped between the encircling tumor and the cortex. MRI scan helps define the extent of the tumor extension.
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.