Skull-base chordoma in a dog.

Autopsy of a 16-y-old, castrated male Pomeranian dog revealed an incidental, friable, tan mass attached to the base of the skull. The mass stretched from the sella turcica to the foramen magnum, anatomically corresponding to the clivus. The mass neither firmly adhered to the cranium nor the medulla oblongata. No associated neurologic signs had been observed. Histologically, the mass had nests of round cells and islands of immature chondroid matrix. At the periphery of the chondroid matrix, physaliphorous cells with hyperchromatic nuclei were identified. Histochemical (von Kossa, Watanabe silver, Verhoeff-Van Gieson, and Masson trichrome stains; periodic acid-Schiff reaction) and immunohistochemistry (IHC; MUM1, CD3, CD20, IBA1, Ki67, S100, cytokeratin AE1/AE3, E-cadherin, vimentin, brachyury) procedures were performed. Immunohistochemically, neoplastic cells were positive for cytokeratin AE1/AE3, E-cadherin, vimentin, S100, and brachyury, and negative for MUM1, CD3, CD20, IBA1, and Ki67, supporting a diagnosis of chordoma. Our case highlights the potential for diverse morphology in chordomas and emphasizes the utility of an IHC panel, including anti-brachyury antibody, to aid in diagnosis. Our case also demonstrates the importance of evaluating the clivus during postmortem CNS examination.