Primary intracranial squamous cell carcinoma in a mule.

Primary intracranial squamous cell carcinoma is a malignant neoplasm reported only in humans and usually originates from epidermoid or dermoid cysts. We describe the first case of primary intracranial squamous cell carcinoma in an animal, emphasizing its clinical, pathological and immunohistochemical findings. A 15-year-old male mule was euthanized after bilateral blindness. At necropsy, the optic nerve, optic chiasm, pituitary gland and trigeminal nerve were surrounded by a multilobulated, firm, whitish mass interspersed by yellowish, irregular, friable multifocal areas of cavitation. Histologically, the mass was formed of neoplastic pleomorphic squamous epithelial cells that formed nests and sometimes contained areas with keratin pearls. Neoplastic cells were immunopositive for cytokeratin and immunonegative for vimentin and alpha fetoprotein. We conclude that primary intracranial squamous cell carcinoma can occur in animals and result in neurological signs, and should be considered as a differential diagnosis for diseases of the central nervous system.