Extraskeletal osteosarcoma histologically mimicking parosteal osteosarcoma.

An extraskeletal osteosarcoma histologically mimicking parosteal osteosarcoma arising in a 63-year-old female is described. The intramuscular lesion in the left lower thigh was a 7 x 4 cm, well-defined mass with marked central calcification and ossification. The tumor was composed predominately of a centrally located osteocartilaginous component and a peripheral, non-osseous fibrous component. The former was characterized by a complex interanastomosing pattern of woven-bone trabeculae with small osteocytes and a minor cartilaginous element. The intratrabecular stroma consisted of benign-appearing fibroblasts. The peripheral fibrous part was characterized by fascicular and interlacing proliferations of spindle cells with mild atypia, rare mitosis and low to moderate cellularity. There were a few scattered foci of proliferating pleomorphic cells, constituting 5% of the tumor, indicating high-grade transformation. There was no evidence of zoning phenomena, cortical erosion, periosteal reaction or intramedullary involvement. The patient is well without recurrence or metastasis at 12 months follow up. This soft tissue neoplasm, mimicking parosteal osteosarcoma, should be distinguished from other bone-forming tumors or tumor-like lesions with which they may be confused. Pathologists and physicians should know the existence of this type of extraskeletal osteosarcoma and treat it as a high-grade extraskeletal osteosarcoma.