Soft tissue tumors encompass a wide variety of mesenchymal neoplasms exhibiting diverse clinical, pathologic, and molecular features. Among these, osteoid and/or chondroid matrix deposition in some soft tissue tumors represents a noticeable characteristic. Unlike matrices present in bone tumors where they likely reveal the respective cells of origin (i.e., osteoblastic or chondroblastic precursors), those existing in soft tissue tumors more often denote a metaplastic phenomenon and reflect the diversity of differentiation these tumors can display. While many soft tissue tumor types can occasionally harbor metaplastic bone or cartilage as an incidental component or heterologous differentiation, in some other tumor types, the production of these matrices is a frequent and distinctive, if not diagnostic, feature. This review focuses on the latter tumor types where emerging immunohistochemical and molecular evidence has significantly improved our understanding of their respective pathogenesis and histopathological spectra. These tumor types include ossifying fibromyxoid tumor, phosphaturic mesenchymal tumor, synovial chondromatosis, soft tissue chondroma, calcifying aponeurotic fibroma, giant cell tumor of soft tissue, myositis ossificans and related diseases, mesenchymal chondrosarcoma, and extraskeletal osteosarcoma.
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