Anatomic Pathology: Bone and Joint Pathology

• Low-grade or juxtacortical osteosarcoma is a surface fibroosseous malignancy of bone accounting for 3% of osteosarcoma. It affects older individuals (20 to 30 years old) compared with conventional osteosarcoma. It has a predilection for the long bones of the lower extremities; 80% affect the posterior surface of the distal femoral metaphysis.

• The most common clinical feature is the presence of a conspicuous, palpable hard mass protruding in the popliteal space and causing pain and discomfort. The duration of symptoms is several months as opposed to several weeks with conventional osteosarcoma.

• The clinical radiographic images are diagnostic, revealing a mushroom-shaped, lobulated mass of bone protruding from the posterior aspect of the distal metaphysis of the femur and attached to the cortical surface by a broad base. The center is densely radiopaque, whereas the periphery is more lucent, which is an important differential diagnostic point with myositis ossificans. The mass reflects over the surface of the adjacent cortex from which it is separated by a radiolucent line. There is no Codman triangle. In contrast to osteochondroma, the mass does not communicate with the medullary cavity of the femur.

• The lesion comprises a mass of well-differentiated, seemingly benign trabeculae of woven bone lacking a border of active osteoblasts in a fibrous stroma and appears deceptively benign. The trabeculae often show a directionality, streaming in parallel arrangement and frequently admixed with round, dotlike profiles. It may contain foci of cartilage. It may be difficult to distinguish between this lesion and fibrous dysplasia because both lesions share trabeculae lacking a lining of osteoblasts as well as a fibrous stroma. The trabeculae are most often delicate and curvilinear (resembling letters of the alphabet) in fibrous dysplasia, a feature seen less often in parosteal osteosarcoma. This tumor has a cellular spindle cell stroma with fascicular patterns compared with the hypocellular, bland stroma of fibrous dysplasia. In this neoplasm, a subtle blending between the spindle cells and the trabecular margins is best seen in the fibrous periphery of the tumor. Although the degree of atypia is often subtle, hyperchromasia and mitoses are consistently present.

• This tumor is slow-growing, and the prognosis is better than conventional osteosarcoma. If treated early and appropriately, a cure can be achieved. The recommended treatment is en bloc resection with wide margins of uninvolved tissue. This lesion can dedifferentiate if treatment is inadequate.