Canine mesenchymal chondrosarcoma of the ribs.

The tumor of the thoracic cavity, which arose from the ribs, was diagnosed as mesenchymal chondrosarcoma. No distant metastasis was observed. Histologically, the tumor was characterized by the nests of well-defined cartilaginous tissue within a proliferation of primitive mesenchymal cells. Additionally, the deformed blood vessels compressed by the proliferating mesenchymal cells exhibited clear stag-horn appearance. Immunohistochemically, most neoplastic cells that formed multifocal cartilaginous islands were positive for S-100 protein, while the surrounding mesenchymal cells were negative. This is the first report of canine mesenchymal chondrosarcoma of the ribs.