An Unusual Mucinous Osteoma with Features of an Ossifying Fibroma in the Nasal Cavity of a Horse

SummaryA 4-month-old Warm-blooded colt presented with facial distorsion and an abnormal respiratory noise of 2 weeks’ duration. A proliferative mass was found endoscopically and by computer tomography. Clinical differential diagnosis included cyst formation of the concha, malformation and neoplasia. Euthanasia was followed by post-mortem examination, which revealed a well-circumscribed, semi-solid, jelly like structure, measuring 14.5×9.0×12.0 cm, in the left posterior nasal cavity extending into the left frontal and maxillary sinus. Histologically, the mass consisted of trabeculae of woven bone, focal lamellar bone and fibroblast proliferation within a moderate fibro-vascular stroma. Abundant mucin production was noted within areas of fibroblastic differentiation. Some areas contained irregularly shaped spicules of osteoid rimmed by osteoblasts, randomly distributed within a moderately vascularized, fibro-osseous stroma. On the basis of the histopathological features, a nasal mucinous osteoma with focal ossifying fibroma-like structures was diagnosed.