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
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.
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