Giant-cell rich extraskeletal osteosarcoma of the superficial soft tissues: A case report.

Summary: We present a case of giant-cell rich extraskeletal osteosarcoma of the superficial soft tissues of the thigh in an middle aged male. The patient presented with complaints of a slowly growing swelling in the left thigh. Imaging showed no relationship to the adjacent bone. An excision of the lesion was performed. Histopathological examination revealed a multinodular tumor with an osteoid formation, composed of undifferentiated spindle-shaped and epithelioid cells, with multiple osteoclast-like multinucleated giant cells. The tumor demonstrated over 10 mitotic figures per mm², including atypical mitoses. The initial differential diagnosis included extraskeleral osteosarcoma, malignant GCT with an osteoid formation, undifferentiated pleomorphic sarcoma of the soft tissue, and, less likely, dedifferentiated liposarcoma, dedifferentiated or metastatic melanoma, MPNST with heterogeneous differentiation, anaplastic angiosarcoma, and leiomyosarcoma with giant cells. Immunohistochemical studies showed weak cytoplasmic expression of CD68 in osteoclast-like multinucleated giant cells, with no expression of MCK, ERG, SMA, Desmin, MDM2, S100, SOX-10, MelanCoctail, PRAME. Given the conjunction of osteoid formation and undifferentiated spindle cells, the main hypothesis was the osteogenic nature of the tumor. Subsequent IHC study with SATB2 demonstrated nuclear expression in undifferentiated spindle cells, confirming the diagnosis of extraskeletal osteosarcoma of the soft tissues.