Low-grade central osteosarcoma (LGCOS), which arises from the intramedullary cavity of the metaphysis of long bones, occasionally exhibits extraosseous spread. Approximately 10–30% of patients with LGCOS exhibit dedifferentiation, but it is rare to experience a primary tumor with a dedifferentiated component. A 38-year-old female patient presented with right knee pain for two months. Imaging studies revealed a bone mass with extraosseous involvement. Wide resection was performed, and pathologic examination led to the diagnosis of LGCOS with a dedifferentiated extraosseous lesion. A single defect in the bone cortex constituted the boundary between the low- and high-grade components. The extraosseous high-grade component included more tumor cells with p53 overexpression and more murine double minute 2 (MDM2) copies compared with the low-grade component. These genetic mutations and copy number alterations can be associated with malignant transformation of LGCOS.
