Synovial sarcoma in a dog.

A 6.5-year-old, female Saint Bernard had a slowly growing tumor of the right tibiotarsal joint. Radiographically, bone surrounding the tumor was not involved. Subtotal surgical resection was undertaken. The tumor was classified histologically as a synovioma. Within 6 months, the tumor recurred locally and invaded the soft tissue proximal to the tibiotarsal joint. Eighteen months after initial examination, there was widespread right tibial invasion and pulmonary metastasis. The dog was euthanatized. The final histopathologic diagnosis was synovial sarcoma.