Dedifferentiated parosteal osteosarcoma with well-differentiated metastases.

Metastases of dedifferentiated sarcoma usually contain a dedifferentiated component. We report a rare case of dedifferentiated parosteal osteosarcoma (dd-POS) with well-differentiated multiple metastases in a 65-year-old woman with a painful firm mass on her thigh. Radiological examination revealed that the mass arose from the surface of her femur without medullary involvement. Multiple intramuscular metastases were detected in her lower leg on MR imaging. Small subcutaneous palpable masses were identified on her left lower leg, buttock, chest wall and head. An open biopsy and above-the-knee amputation were performed, and the mass on her femur was diagnosed as a dd-POS. However, histological examination on the subcutaneous lesions in her lower leg, buttock and head showed low-grade conventional POS without dedifferentiated components. To the best of our knowledge, this is the first report of a dd-POS with multiple metastases that do not contain any dedifferentiated components.