Mandibular ossifying fibroma in 2 cattle in Brazil.

Ossifying fibroma (OF) is a slow-growing, expansive, and benign fibro-osseous neoplasm that is rare in cattle. It mainly affects the craniofacial bones, especially the mandible. Here, we report 2 cases of mandibular OF in Nelore and mixed-breed steers with enlarged masses in the rostral portion of the mandible. Radiographic analysis of case 1 revealed an oval, lobed mass with the radiopacity of bone tissue that displaced the incisors laterodorsally. Histologically, both masses were composed of a proliferation of spindloid-to-stellate cells, supported by a dense fibrovascular stroma, with bony trabeculae surrounded by stroma and covered by a single layer of osteoblasts. Cellular pleomorphism was low, anisocytosis and anisokaryosis were negligible, and mitotic figures were not observed. The clinical, radiologic, gross, and histologic changes are compatible with OF. The primary differential diagnoses of OF are fibrous dysplasia and osteoma; their differentiation can be difficult. Fibroma, low-grade osteosarcoma, and multilobular sarcoma of bone are also differential diagnoses, with striking features that facilitate their exclusion.