A 69-year-old man was referred to us with a 6-month history of a projecting lesion on his back, which had been suspected to be a malignant fibrous histiocytoma based on the results of a needle biopsy. Histopathologically, at the time of a wide resection, the tumor had a clear boundary between the tumoral component and the surrounding adipocytic component, and he was diagnosed as having dedifferentiated liposarcoma. However, reexamination of the case revealed no atypical cells suggesting liposarcoma in the adipose component. Immunohistochemically, the tumor cells were positive for cyclin dependent kinase-4, but negative for murine double minute-2 (MDM2). In addition, fluorescence in situ hybridization failed to reveal the amplification of the MDM2 gene, leading to the final diagnosis of undifferentiated pleomorphic sarcoma.
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