Low-grade central osteosarcoma (LG-COS) is an uncommon variant of osteosarcoma (OS) that sometimes progresses to high-grade OS post-recurrence. We herein present a case of dedifferentiated LG-COS with extensive cystic change arising in the right iliac bone of a 26-year-old man. The LG-COS was initially diagnosed and managed as a simple bone cyst. The lesion recurred thrice, and high-grade OS was diagnosed during the third recurrence. The first lesion appeared as a typical benign cystic mass on radiography. However, a huge malignant osteoblastic mass subsequently developed in the right pelvis at the third recurrence. Extended hemipelvectomy with ipsilateral hemisacral resection was performed. Histologic analysis showed tumor necrosis and irregular neoplastic tumor osteoid, while immunohistochemistry revealed that the tumor was diffusely positive for MDM2 and CDK4. The histologic diagnosis was high-grade OS dedifferentiated from a preceding cystic lesion. Our final diagnosis of the primary lesion was LG-COS with extensive cystic change.
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