Conventional osteosarcoma is the most common type, having its highest incidence in patients in their second decade and affecting males slightly more often than females. It has a predilection for the knee region (distal femur and proximal tibia), whereas the second most common site is the proximal humerus (Fig. 21.2). Patients usually present with bone pain, occasionally accompanied by a soft-tissue mass or swelling. At times, the first symptoms are related to pathologic fracture.

The distinctive radiologic features of conventional osteosarcoma, as demonstrated by radiography, are medullary and cortical bone destruction, an aggressive periosteal reaction, a soft-tissue mass, and tumor bone either within the destructive lesion or at its periphery as well as within the soft-tissue mass (Fig. 21.3). In some instances, the type of bone destruction may not be obvious on the conventional studies, but patchy densities representing tumor bone and an aggressive periosteal reaction are clues to the diagnosis (Fig. 21.4).

The degree of radiopacity in the tumor reflects a combination of the amount of tumor bone production, calcified matrix, and osteoid. Tumors may present as purely sclerotic lesions or purely osteolytic lesions, but mostly a combination of both (Fig. 21.5). The borders are usually indistinct, with a wide zone of transition. The type of bone destruction is either moth-eaten or permeative and only rarely geographic.

The most common types of periosteal response encountered with osteosarcoma are the “sunburst” type and a Codman triangle; the lamellated (onionskin) type of reaction is less frequently seen (Fig. 21.6). In the past, computed tomography (CT) was an indispensable technique for evaluating these tumors (Fig. 21.7). This was particularly important if a limb-salvage procedure was contemplated because extension of the tumor into the medullary cavity is crucial information for effective surgical planning (see Fig. 16.11). Currently, magnetic resonance imaging (MRI) has become a modality of choice for evaluating


these tumors, particularly for intraosseous tumor extension and soft-tissue involvement. On T1-weighted images, the solid nonmineralized parts of osteosarcoma generally present as areas of low-to-intermediate signal intensity. On T2-weighted images, the tumor demonstrates a high signal intensity (Figs. 21.8, 21.9, 21.10). Osteosclerotic tumors demonstrate low signal intensity on all imaging sequences (Fig. 21.11). MRI may also effectively demonstrate peritumoral edema. This feature displays an intermediate intensity signal on T1-weighted and a high intensity on T2 surrounding the tumor. CT (see Fig. 16.12) and MRI are also essential in monitoring the results of treatment.

Based on the dominant histologic features, conventional osteosarcoma can be subdivided into three histologic subtypes: osteoblastic, chondroblastic, and fibroblastic. The last may occasionally mimic malignant fibrous histiocytoma (MFH). At times, the tumor cells may be so undifferentiated that on a purely cytologic basis; it is difficult to tell whether they are sarcomatous or epithelial. This variant of conventional osteosarcoma is sometimes referred to as epithelioid osteosarcoma. The diagnosis usually becomes evident from the patient’s age, the production of obvious tumor matrix, and a radiographic appearance typical of osteosarcoma.

This rare form of osteosarcoma (1% of all osteosarcomas) usually occurs in patients older than those presenting with conventional osteosarcoma, although the sites of predilection are similar. Radiographically, it may be indistinguishable from conventional osteosarcoma, but it grows more slowly and has a better prognosis. At times, its radiographic presentation clearly
mimics fibrous dysplasia (Fig. 21.13) or other benign lesion (Fig. 21.14). Histologic examination typically reveals abundant osteoid and spindle cells with a paucity of cellular atypia and mitotic figures.

A very aggressive type of osteosarcoma, the telangiectatic variant, also called hemorrhagic osteosarcoma by Campanacci, is twice as common in males than in females and is seen predominantly in patients in their second and third decades of life. It is rare, comprising approximately 3% of all malignant bone tumors. It is characterized by a high degree of vascularity and large cystic spaces filled with blood, which account for its atypical imaging presentation. Most of these tumors arise in the femur and tibia. On radiography, telangiectatic osteosarcoma most commonly presents as an osteolytic destructive lesion with or without matrix mineralization, and with an almost complete absence of sclerotic changes; a soft-tissue mass may also be present (Figs. 21.15, 21.16, 21.17). An aggressive periosteal reaction (lamellar, sunburst, or Codman triangle) is present in most patients, reflecting the malignant nature of this tumor, and a pathologic fracture is not uncommon
in the case of large lesions. On MRI, telangiectatic osteosarcoma often exhibits areas of high signal intensity on T1-weighted sequences, owing to the presence of methemoglobin. On T2 weighting, signal intensity is commonly heterogeneous (Fig. 21.18). Fluid-fluid levels can occasionally be seen (Fig. 21.19), similar to ones seen in aneurysmal bone cyst.

On gross pathologic examination, the tumor resembles a “bag” of blood and is characterized by blood-filled spaces, necrosis, and hemorrhage. Histologically, it is composed of loculated blood-filled spaces, partially lined by malignant cells producing sparse osteoid tissue. It resembles an aneurysmal bone cyst, both radiologically and pathologically.

This is a rare variant of osteosarcoma, which histologically appears as an undifferentiated sarcoma with an overabundance of giant cells (osteoclasts) and a paucity of tumor osteoid and tumor bone. It comprises approximately 3% of all osteosarcomas and histologically is related to telangiectatic osteosarcoma and MFH-like osteosarcoma. Many of the typical imaging features of conventional osteosarcoma are not present, periosteal reaction is either scant or absent, and the soft-tissue mass is small. This presentation may sometimes create the difficulties in differentiation of giant cell-rich osteosarcoma even from benign lesions. In most cases, however, on the radiographs, the lytic lesion exhibits poorly defined borders, and MRI characteristics confirm malignancy (Fig. 21.20). The common location of this tumor is the metaphysis or diaphysis of a long bone, usually the femur and tibia. Histologically, because of abundance of giant cells, and because tumor osteoid is usually scant and difficult to identify, giant cell-rich osteosarcoma bears striking resemblance to giant cell tumor.

Described by Sim and associates, small cell osteosarcoma with preferential sites of distal femur, proximal humerus, and proximal tibia, usually occurs as a radiolucent lesion with permeative borders and a large soft-tissue mass. Its radiographic appearance thus mimics that of a round cell bone sarcoma. These lesions usually exhibit small round cells in many histologic

fields, much like Ewing sarcoma. The presence, however, of spindled tumor cells, as well as the focal production of osteoid or bone, helps to make a histologic diagnosis of osteosarcoma.

Fibrohistiocytic osteosarcoma, which resembles MFH, has recently been described in the literature. It can sometimes be confused with true MFH of bone because both of these tumors tend to arise at a greater age than conventional osteosarcoma, usually after the third decade. Both tend to involve the articular ends of long bones, and less periosteal reaction is typically present than in conventional osteosarcoma. Although on radiography both of these lesions tend to be radiolucent and thus do resemble giant cell tumor and fibrosarcoma, the MFH-like osteosarcoma usually exhibits areas of bone formation resembling cotton balls or cumulus clouds, whereas


MFH does not. When such areas are identified on imaging studies, a diligent search should be made for tumor bone in the resected specimen. Histologically, MFH-like osteosarcoma is characterized by pleomorphic spindle cells and giant cells, many of which have bizarre nuclei. This lesion therefore resembles giant cell-rich osteosarcoma. An inflammatory background is not unusual, and the storiform or spiral nebular arrangement, characteristic of MFH, although sometimes a dominant feature, may be less prominent or may be replaced by areas of large pleomorphic cells arranged in diffuse sheets. As in all other subtypes of osteosarcoma, the distinction from other sarcomas depends on the demonstration of osteoid or bone formation by malignant cells in the very typical patterns seen in osteosarcomas.

Intracortical osteosarcoma is one of the rarest forms of osteosarcoma. Very few of these tumors have been reported, with an age range of 9 to 43 years (average 24 years) and a male predominance. The presenting symptom is pain, often associated with activity. In some patients, a history of previous trauma has been elicited. The tumor involves the cortex, without extension into the medullary portion of the bone or the soft tissues. The radiographic presentation is that of a radiolucent lesion with surrounding cortical sclerosis. The size of the lesion varies from 1.0 to 4.2 cm. In some instances, the lesion mimics osteoid osteoma or intracortical osteoblastoma.

Gnathic osteosarcoma is osteosarcoma arising in the maxilla or mandible. Unlike osteosarcoma arising elsewhere in the skeleton, this tumor occurs in older patients (fourth to sixth decades, with a mean age of 35 years). It is usually a well-differentiated tumor with a low mitotic rate, possessing a predominantly cartilaginous component in a high percentage of cases and with less malignant potential and a better prognosis than for other forms of osteosarcoma.

The simultaneous development of foci of osteosarcoma in multiple bones is a rare occurrence (Figs. 21.21 and 21.22). Whether this entity is truly separate or represents multiple bone metastases from a primary conventional osteosarcoma remains a controversy. This type of osteosarcoma is currently recognized as having two variants: synchronous and metachronous. Multifocal osteosarcoma must be differentiated from osteosarcoma metastasized to other bones.

The term juxtacortical is a general designation for a group of osteosarcomas that arise on the bone surface (Fig. 21.23). Usually, these lesions are much rarer and occur a decade later than their intraosseous counterparts. The
majority of juxtacortical osteosarcomas are low-grade tumors, although there are moderately and even highly malignant variants.