Parosteal Osteosarcoma

     Parosteal osteosarcoma is the most common form of surface osteosarcoma. It accounts for 5% of osteosarcoma and is found in the metaphysis of long bones, especially the posterior femur above the knee. The lesion arises from the surface of the bone and has a tendency to encircle the bone. This slow growing mass presents with mild symptoms such as a vague ache or limited range of motion due to interference with a joint. The median age of patients is 27 years old.
     The radiologic picture of parosteal osteosarcoma is very characteristic. 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. Approximately 80% of tumors are grade 1 and 20% are grade 2.2 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.

References
Bloem, JL and HM Kroon, Osseous Lesions, Radiologic Clinics of North America, 31(2):261-277, March 1993. 2

Inwards, CY and KK Unni, Classification and Grading of Bone Sarcomas, Hematology/Oncology Clinics of North America, 9(3):545-569, June, 1995.

Bullough, Peter, Orthopaedic Pathologv (third edition), Times Mirror International Publishers Limited, London, 1997.

Huvos, Andrew, Bone Tumors: Diagnosis. Treatment and Prognosis, W.B. Saunders, Co., 1991.
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