Laboratory Studies
No laboratory studies have been shown to be helpful for diagnosing low-grade central osteosarcoma, with the exception of histologic examination.
Imaging Studies
Standard radiography
Standard radiography may be helpful. Radiographs show a large amount of variation among low-grade osteosarcomas (see the image below).
Characteristic features include the following:
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Location within the bone - Metaphyseal, 67%; at the junction of the diaphysis and the metaphysis, 17%; diaphyseal, 16%
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Degree of margination - Poor, 50-68%; intermediate, 11-20%; sharp, 22-30%
Other features seen on standard radiographs include the following:
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Cortical destruction - 55-70%
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Expansive lesion - 40-51%
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Soft-tissue extension - 40-55%
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Periosteal new bone formation - 22-50%
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Crossing of previously fused physes - 60% (In one study, 0 of 11 cases crossed radiographically open physes.)
Radiographic findings may be suggestive of malignancy.
Enneking reported that the main radiologic feature distinguishing low-grade central osteosarcoma from fibrous dysplasia is increased radiopacity. [17]
At least two of the following three radiographic signs were present in all eight cases presented by Ellis et al [18] :
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Cortical discontinuity (the radiographic sign that was considered to be the most useful feature for distinguishing low-grade central osteosarcoma from benign entities)
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Poorly marginated soft-tissue extension
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Cloudlike tumor matrix pattern
Accordingly, these signs are thought to be suggestive of malignancy.
Standard anteroposterior and lateral chest radiographs are used to assess pulmonary metastases.
Computed tomography
Computed tomography (CT) is useful for evaluating the degree of cortical destruction. On CT, 85% of low-grade central osteosarcomas show cortical destruction (compared with 55% on plain radiography). CT scans show the trabecular pattern of the lesion and its relation to adjacent healthy bone more clearly than do other images. CT of the chest is also used to evaluate pulmonary metastases.
Magnetic resonance imaging
Compared with other techniques, magnetic resonance imaging (MRI) enables better assessment of potential soft-tissue and marrow extension (see the images below).
Magnetic resonance image (MRI) of low-grade central osteosarcoma of the distal femur.
Magnetic resonance image (MRI) of low-grade central osteosarcoma of the distal femur.
Magnetic resonance image (MRI) of low-grade central osteosarcoma of the distal femur.
Bone scanning
Bone scanning reveals intense radioisotope uptake by the lesion, demonstrates the approximate local extent of the lesion, and may depict distant metastatic lesions to bone (see the image below).
Angiography
Angiography is usually not necessary. Angiographic findings are normal until soft-tissue extension (which typically occurs late) is present.
Biopsy
Biopsy of the lesion may be necessary to obtain tissue for histologic diagnosis.
Core needle biopsy (CNB) may be performed to obtain tissue when sufficient soft-tissue extension is present. In most cases, a Craig needle or an equivalent biopsy needle is necessary to enter the bone to obtain tissue for diagnosis.
Formal open biopsy also may be performed. The authors recommend sealing the site of biopsy with bone cement to prevent local extension of the tumor. However, because of the risk of tumoral dissemination into the venous circulation and lungs, this technique is controversial.
Histologic Findings
General histologic characteristics of low-grade central osteosarcomas include the following (see the images below):
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Spindle cell tumor
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Irregular bone production - Chinese-characters pattern of fibrous dysplasia (14%); desmoid pattern with scanty osteoid production and many collagen fibers (33%); parosteal sarcoma pattern with heavy, irregular osteoid seams (40%)
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Low cellularity - Small foci of cartilage differentiation (18%); small clusters of benign-appearing giant cells (36%)
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Few mitotic figures - In 82% of low-grade central osteosarcomas, one or two mitotic figures are present per 10 high-power fields; in 18% of low-grade central osteosarcomas, three or four mitotic figures are present per 10 high-power fields
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Minimal cytologic atypia
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Other - Low-grade central osteosarcomas are often indistinguishable from parosteal osteosarcoma at histologic analysis [19, 20, 21, 22, 23, 24] ; differentiation of these two osteosarcomas is often based on whether a tumor is located within bone (low-grade central osteosarcoma) or has a juxtacortical presence (parosteal osteosarcoma; see also Juxtacortical Tumors)
Photomicrograph of a low-grade central osteosarcoma (original magnification, X40). Courtesy of Dr Ronald Burns, Palmetto Richland Department of Pathology.
Photomicrograph of a low-grade central osteosarcoma (original magnification, X100). Courtesy of Dr Ronald Burns, Palmetto Richland Department of Pathology.
Photomicrograph of a low-grade central osteosarcoma (original magnification, X400). Courtesy of Dr Ronald Burns, Palmetto Richland Department of Pathology.
Detection of GNAS mutations in primary bone tumors has been useful in clinical practice for diagnosing fibrous dysplasia; however, a study by Salinas-Souza et al found that GNAS mutations are highly specific for fibrous dysplasia and rarely, if ever, occur in low-grade osteosarcomas. [23]
Staging
Two staging systems are used to stage osteosarcomas and other cancers of bone: the American Joint Committee on Cancer (AJCC) TNMG (tumor-node-metastasis-grade) staging protocol and the Musculoskeletal Tumor Society (MSTS) staging system. [25, 26]
The AJCC staging system is based on tumor grade, size, and location (specifically, whether the tumor is within the cortex or extends beyond it), as well as on the presence of regional nodal or distant metastases. The MSTS system is based on tumor grade, site (ie, extracompartmental vs intracompartmental), and metastases. In both systems, low-grade central osteosarcoma is, by definition, a stage I tumor unless metastasis has occurred.
In the MSTS system, stage IA includes intracompartmental low-grade tumors without metastases, whereas stage IB includes extracompartmental low-grade tumors. Tumors that have metastasized are classified as stage III lesions. In the AJCC system, stage IA includes low-grade tumors that are confined within the cortex without metastases, whereas stage IB includes low-grade tumors that have extended beyond the cortex. Tumors that have metastasized to regional lymph nodes are classified as stage IVA, and those with distant metastasis are classified as stage IVB.
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Standard radiographs of a low-grade central osteosarcoma of the distal femur.
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Magnetic resonance image (MRI) of low-grade central osteosarcoma of the distal femur.
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Magnetic resonance image (MRI) of low-grade central osteosarcoma of the distal femur.
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Magnetic resonance image (MRI) of low-grade central osteosarcoma of the distal femur.
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Bone scan of a low-grade central osteosarcoma of the distal femur.
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Photomicrograph of a low-grade central osteosarcoma (original magnification, X40). Courtesy of Dr Ronald Burns, Palmetto Richland Department of Pathology.
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Photomicrograph of a low-grade central osteosarcoma (original magnification, X100). Courtesy of Dr Ronald Burns, Palmetto Richland Department of Pathology.
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Photomicrograph of a low-grade central osteosarcoma (original magnification, X400). Courtesy of Dr Ronald Burns, Palmetto Richland Department of Pathology.
