Atypical (dedifferentiated) chondrosarcoma or osteosarcoma with preponderant chondroblastic differentiation?

A bone tumor was detected in the distal right femur of a 15-year-old girl. The age of the patient, the localization, and the X-ray picture were in favor of an osteosarcoma. However, histology was consistent with a "highly differentiated chondrosarcoma with dedifferentiation and focal osteosarcomatous metaplasia". The tumor was resected with preservation of the limb. One year later the tumor recurred, and the leg was exarticulated. Histology was again of the same type, as was the case in a single lung metastasis taken out another half year later. Two series of chemotherapy according to an osteosarcoma protocol had no recognizable effect. An unambiguous diagnosis cannot yet be made.