Primary osteosarcoma of the synovium in a dog

An eight-year-old, spayed female Labrador retriever was presented for evaluation of left thoracic limb lameness. Radiographs of the left elbow revealed mineralization of the soft tissues surrounding the joint, with no evidence of periarticular bony lysis. Biopsy of the synovial tissues of the left radiohumeral joint yielded a diagnosis of osteosarcoma (OSA). The dog was treated with palliative radiotherapy, piroxicam, and carboplatin chemotherapy, which resulted in excellent pain control for approximately eight months. Amputation of the affected limb was then performed. Six months later, the dog developed multiple subcutaneous OSA metastases, and the dog was euthanized 15 months after diagnosis due to refractory vomiting. This report documents the first known case of primary OSA of the synovium in any species.