A Non-joint Tissue Biphasic Synovial Sarcoma in a Dog

A subcutaneous tumour was identified in the malar region of a 14-year-old neutered female mixed breed dog. The dog was humanely destroyed and necropsy examination was performed. The tumour did not invade neighbouring tissues and no metastasis was found. Microscopically, the tumour showed a range of features including the presence of multinucleated giant cells, chondrocyte differentiation and cystic or slit-like structures. All of these features are consistent with previously reported descriptions of synovial sarcomas in dogs. Mesenchymal cells accounted for the majority of the tumour, but cytokeratin-positive epithelioid components were also confirmed by immunohistochemistry. The tumour was diagnosed as a biphasic type of synovial sarcoma. Synovial sarcoma in man may develop in tissues unrelated to joints and this is the first report of a non-joint synovial sarcoma in a dog.