Dedifferentiated chondrosarcoma of the rib with a malignant mesenchymomatous component: an autopsy case report.

A rare variant of dedifferentiated chondrosarcoma with malignant mesenchymomatous component in a 57-year-old male is reported. The patient presented with a posterior mediastinal mass arising from the left eighth and ninth ribs showing well differentiated, low-grade chondrosarcoma. Five years later, local recurrence occurred and an excised specimen also showed the same histological features as the primary tumor. Another 6 years later, the tumor recurred and metastasized to the multiple organs, the patient dying 4 months later. Autopsy revealed that the recurrent and metastatic tumors showed malignant mesenchymomatous 'dedifferentiation' of chondrosarcoma composed of rhabdomyosarcoma, angiosarcoma, chondrosarcoma, osteosarcoma, and leiomyosarcoma, in addition to fibrosarcomatous areas. Although the less differentiated component of dedifferentiated chondrosarcoma usually shows the histological features of malignant fibrous histiocytoma and fibrosarcoma, multilineage differentiation can occur in that component. The phenomenon of 'dedifferentiation' in chondrosarcoma and the relationship to and distinction from malignant mesenchymoma of soft tissue and bone are discussed.