Osteosarcoma is a malignant tumor that is characterized by the direct formation of bone or osteoid tissue by the tumor cells. The World Health Organization’s histologic classification  of bone tumors separates the osteosarcomas into central (medullary) and surface (peripheral) [2,3] tumors and recognizes a number of subtypes within each group.
Central (Medullary) Tumors
Surface (Peripheral) Tumors
The most common pathologic subtype is conventional central osteosarcoma, which is characterized by areas of necrosis, atypical mitoses, and malignant osteoid tissue and/or cartilage. The other subtypes are much less common, each occurring at a frequency of less than 5%. Telangiectatic osteosarcoma may be confused radiographically with an aneurysmal bone cyst or giant cell tumor. This variant should be approached as a conventional osteosarcoma.[4,5]
Malignant fibrous histiocytoma (MFH) of bone is treated according to osteosarcoma treatment protocols. MFH should be distinguished from angiomatoid fibrous histiocytoma, a low-grade tumor that is usually noninvasive, small, and associated with an excellent outcome with surgery alone. One study suggests similar event-free survival rates for MFH and osteosarcoma.
Extraosseous osteosarcoma is a malignant mesenchymal neoplasm without direct attachment to the skeletal system. Previously, treatment for extraosseous osteosarcoma followed soft tissue sarcoma guidelines, although a retrospective analysis of the German Cooperative Osteosarcoma Study identified a favorable outcome for extraosseous osteosarcoma treated with surgery and conventional osteosarcoma therapy.
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