An osteoma is a mass of abnormally dense but otherwise normal bone, which generally arises from the periosteum of bones formed by intramembranous ossification.5 An uncommon tumor found in various species,5 osteomas are particularly rare or underreported in ferrets.1-4 The 2-year-old neutered male ferret (Mustela putorius furo) described in this case report was presented with an asymmetrical firm mass on the right dorsolateral skull. Radiographs and computed tomography revealed a well-marginated mineral-attenuating mass with a granular core that involved the dorsal right calvarium and was growing concentrically into the cranium. Surgical biopsy was pursued, and the cytological diagnosis was a sarcoma with reactive osteoblasts and osteoclasts, while the histopathological diagnosis was an osteoma. It was suspected that only the outer bony rim of the mass was collected for histopathological evaluation, thereby adversely affecting the diagnostic quality of the sample. Consequently, based on interpretation of the combined imaging, cytological, and histopathological findings, a multilobular tumor of bone was highly suspected. Due to clinical decline, the animal was euthanized 4 months after presentation, and postmortem examination yielded a definitive diagnosis of osteoma. This is the first report of an osteoma with cranial vault invasion in a ferret, and one of very few reports of this tumor in this species. The lack of neurological signs in the patient, marked and extensive growth of the mass over several months, and the similar clinical appearance to other bony tumors, leading to diagnostic ambiguity, contribute to the distinctiveness of this case.
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4. Ryland LM. What is your diagnosis? Focal, ovoid, smooth-bordered, osseus proliferation compatible with an osteoma of the parietal bone. J Am Vet Med Assoc. 1990;197(8):1065–1066.
5. Thompson KG, Pool RR. Tumors of bones. In: Meuten DJ, ed. Tumors of Domestic Animals. 4th ed. Ames, IA: Iowa State Press; 2002:248–252.