Synovial cell sarcoma is an uncommon cancer arising from the soft tissues around joints — including the joint capsule, tendons, and bursae. Despite its name, modern understanding suggests it does not arise directly from synovial cells but rather from undifferentiated mesenchymal cells near joints. The tumour most frequently affects the stifle (knee), elbow, or shoulder, presenting as progressive lameness with firm swelling around the affected joint. It is locally aggressive, destroying surrounding bone and soft tissue, and high-grade tumours have meaningful metastatic potential, particularly to the lungs. Treatment usually requires amputation of the affected limb above the involved joint, as the intimate relationship with the joint makes limb-sparing surgery very difficult. Medium to large breed dogs are typically affected.