Murine double minute 2-negative dedifferentiated liposarcoma with osteosarcomatous differentiation: a diagnostically challenging presentation with an unusual molecular signature

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A man in his early 70s presented with a painful left thigh mass. MRI showed a 10 cm intramuscular lesion with necrosis and calcifications without bone invasion; a small adjacent lipomatous area was noted. Biopsy revealed a high-grade spindle and pleomorphic sarcoma lacking specific lineage markers (AE1/AE3, CD45, desmin, smooth muscle actin, myogenin, MyoD1, CD31, S100, SOX10). Metastatic workup showed only a 5 mm subpleural lung nodule. The tumour was excised with reconstruction using a right vastus lateralis free flap. Grossly, it was lobulated yellow-grey with necrosis and ossification. Histology confirmed a high-grade sarcoma (Fédération Nationale des Centres de Lutte Contre le Cancer (FNCLCC) grade 3) showing lipogenic, chondrosarcomatous and osteosarcomatous differentiation with angiolymphatic invasion. Fluorescence in situ hybridisation testing showed non-amplified murine double minute 2 (MDM2) gene with polysomy 12 in 50% of cells. The overall features supported a diagnosis of dedifferentiated liposarcoma with heterologous osteosarcomatous and chondrosarcomatous elements.