Diagnostic challenges in a dog with synovial histiocytic sarcoma initially managed as degenerative osteoarthritis

Synovial histiocytic sarcoma may mimic chronic degenerative joint disease, potentially delaying diagnosis. A 12‐year‐old, spayed, female, mixed‐breed dog with a 1‐year history of medically managed degenerative joint disease of the right elbow, diagnosed radiographically at another veterinary clinic, was referred because of progressive lameness and periarticular swelling. Radiographs obtained at our institution suggested an aggressive periarticular lesion, and a surgical bone biopsy was initially interpreted as showing reactive and degenerative changes. Progressive clinical deterioration approximately 5 months later prompted forelimb amputation. Histopathology revealed a pleomorphic spindle‐cell sarcoma, and immunohistochemistry demonstrated strong Iba‐1 and vimentin immunoreactivity with negative Sox‐10 staining, supporting a diagnosis of synovial histiocytic sarcoma. Adjuvant chemotherapy was administered. At 11–12 months after amputation, three‐view thoracic radiographs showed no pulmonary metastasis, and the dog remained clinically stable. This case highlights the diagnostic challenge of distinguishing synovial histiocytic sarcoma from degenerative joint disease in dogs with persistent monoarticular disease.