Musculoskeletal Imaging

Osteosarcoma

a malignant neoplasm of bone composed of proliferating tumour cells that in most instances produce osteoid or immature bone. Osteosarcomas can be classified according to location of the tumour within the bone (intramedullary or central, intracortical, surface, periosteal, or parosteal); degree of cellular differentiation (high grade or low grade); histologic composition (osteoblastic, chondroblastic, fibroblastic, fibrohistiocytic, telangiectactic, small cell, clear cell); number of foci of involvement (single or multicentric); and status of the underlying bone (normal or the site of disease, injury or another neoplasm).

Conventional osteosarcoma

most typically involves the long tubular bones, especially about the knee (Fig.1). A metaphyseal location predominates. Most typically bone involvement shows a mixed pattern consisting of both osteolysis and osteosclerosis. Conventional osteosarcomas in the tubular bones are usually evident as poorly defined intramedullary metaphyseal lesions that extend through the cortex and produce a sizeable soft tissue mass. A periosteal reaction (Codmans triangle or sunburst pattern) and sometimes a pathologic fracture are additional radiographic features. In other skeletal sites the radiographic features of osteosarcoma are similar.

Radionuclide scanning demonstrates an increased uptake of the bone-seeking radiopharmaceutical agent. Angiography allows identification of the extraosseous component of the tumour and defines the degree of displacement (or invasion) of vessels. MR imaging appears to be superior to CT scanning in defining the intraosseous and extraosseous extent of the tumour. On T1-weighted spin-echo MR images the tumour is typically of low signal intensity, whereas on T2-weighted spin-echo MR images it is usually of high signal intensity.

Gnathic osteosarcomas

of the mandible or maxilla are distinct from conventional osteosarcomas. They may be purely osteolytic, purely osteosclerotic, or mixed osteolysic and osteosclerotic.

Telangiectatic osteosarcomas

represent tumours of large cystic cavities filled with blood. They are primarily tumours of the long tubular bones. On radiographs a large expansile osteolytic lesion often lacking periosteal bone production may be encountered, as may pathologic fractures and soft tissue masses (Fig.2). On MR images areas of high signal intensity in T1-weighted spin-echo images and fluid-fluid levels similar to those seen in an aneurysmal bone cyst may be noted.

Small cell osteosarcomas

consist of cells resembling those of Ewings sarcoma. The femur, humerus, tibia and ilium are affected most frequently, and prognosis appears to be poorer than for conventional osteosarcoma. On radiographs the lesions are seen to be large and predominantly osteolytic, involving the medullary and cortical bone, and accompanied by periostitis, a soft tissue mass, or both. Permeative bone destruction is characteristic.

Intraosseous low grade osteosarcomas

are uncommon and possibly represent the intramedullary counterpart of the parosteal variety of tumour. The characteristics of these tumours may simulate those of benign neoplasms. A relatively large metaphyseal lesion, which may be purely osteosclerotic or both osteolytic and osteosclerotic, may be seen on radiographs. Prognosis is better than for conventional osteosarcoma.

Intracortical osteosarcoma

, the rarest form of osteosarcoma, arises within the cortex as an osteolytic lesion with surrounding cortical sclerosis and without radiating bone spicules. This tumour usually occurs as a diaphyseal lesion of the tibia or femur.

Surfac sessile fashion to the external cortex (Fig.3). The cortex itself may be thickened, and a thin radiolucent line, or cleavage plane, may separate the remaining part of the tumour from the underlying bone. CT scanning and MR imaging may aid in the evaluation of a parosteal osteosarcoma and may help document the presence of medullary involvement. In some cases radiolucent regions are noted within the otherwise dense tumour mass; these can represent fibrous or cartilaginous tissue, normal fat, entrapped benign soft tissues or dedifferentiated, high-grade areas of parosteal osteosarcoma.

Multicentric osteosarcomas

may result from multicentric lesions that arise simultaneously (osteosarcomatosis), multicentric lesions that arise metachronously, or a unicentric lesion in which one or more skeletal metastases occur. These lesions are difficult to differentiate. Multiple metachronous osteosarcomas may develop after the initial treatment of a primary osteosarcoma and represent late metastases from the original neoplasm or new primary tumours. Multicentric osteosarcomas may also occur in persons with Pagets disease or after irradiation.

Soft tissue osteosarcomas are also encountered, generally in the deeper tissues of the extremities, thighs or shoulder region.

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Fig.1

AP (a) and lateral (b) radiographs of the femur demonstrate a large soft tissue mass with an aggressive periosteal reaction creating the sunburst appearance. Patchy osteosclerosis and osteolysis in the distal femur secondary to conventional osteosarcoma also observed.
Osteosarcoma, Fig.1 (a)
Osteosarcoma, Fig.1 (b)
Osteosarcoma, Fig.2 (a)
Osteosarcoma, Fig.2 (b)
Osteosarcoma, Fig.3