A case of third eyelid hibernoma in a dog

Purpose: To describe an unusual tumor of the third eyelid. Methods: A 10‐year‐old, female, beagle was presented for an ophthalmological consult with a slowly progressive pro- trusion of the third eyelid OS. The dog received a complete ophthalmic examination. The only abnormal ocular finding was a very subtle lateral strabismus and slight third eyelid displacement. For diagnostic investigation, an MRI was performed, demonstrating an oval, well‐defined mass, 18 × 19 mm, at the base of the third eyelid. Treatment included surgical excision with orbital exenteration. All orbital tissues were submitted for histopathology. Results: Histologically, the nictitans gland was completely replaced by a large, multilobulated, partially encapsulated neoplasia composed of small dense lobules of polygonal or elongated cells, separated by subtle fibrovascular septa. Foci of mineralization were multifocally detectable. Neo- plastic cells were characterized by well‐defined cell bor- ders, abundant, finely vacuolized cytoplasm, and small round paracentral nuclei. Anisokaryosis and anisocytosis were mild, and mitotic index was 0.1. Immunohistochemi- cally, neoplastic cells stained negative for pancytokeratin and cytokeratin 14 (epithelium), PNL2 (melanocytes), Iba1 (histiocytes), chromogranin A, and NSE (neuroendocrine/ neuronal cells), occasionally positive for vimentin (mes- enchyme) and positive for UCP‐1 and alpha‐smooth muscle actin (smooth muscle). Based on histological and immuno- histochemical results, a hibernoma was diagnosed. The dog was recurrence‐free one year after surgery without adjunc- tive therapy. Conclusions: Hibernomas have been demonstrated to express both muscle antigens and UCP1. To the best of author's knowledge, hibernomas have been previously described in the canine orbit but not within the third eyelid.