Dedifferentiated parosteal osteosarcoma with high-grade osteoclast-rich osteogenic sarcoma at presentation.

We report a case of a 32-year-old woman who presented with parosteal osteosarcoma of the distal femur with simultaneous dedifferentiation to a high-grade osteoclast-rich osteogenic sarcoma. This pattern of dedifferentiation is rare, particularly at the time of presentation. We are aware of three other somewhat comparable cases in the literature; however, none is quite similar to our case.