Oral fibrosarcoma is the third most common malignant tumour of the mouth in dogs, arising from fibrous connective tissue cells (fibroblasts). A critical feature of this cancer in large breed dogs, particularly Golden Retrievers, is that it presents a diagnostic trap: under the microscope the tumour cells may appear relatively benign and well-differentiated (low-grade histology), yet the tumour actually behaves very aggressively with high rates of local invasion into bone and a significant risk of distant spread. This disconnect between microscopic appearance and clinical behaviour is well-recognised in veterinary oncology and means that pathology reports can be misleadingly reassuring. The tumour most commonly develops in the upper jaw (maxilla) and is treated with aggressive surgical excision — the adequacy of surgical margins is the most important factor in determining outcome.