Diagnostic dilemma: Papillary third ventricular neoplasm with concurrent choroid plexus and ependymal features in a cat.

A 6-year-old, castrated male, domestic shorthair cat with progressive neurologic signs underwent magnetic resonance imaging, revealing a suprasellar mass, which resulted in euthanasia. Grossly, a tan-red tumor expanded the ventral third ventricle, compressed adjacent brain structures, and emerged ventrally at the midline. Histologically, numerous arborizing papillary formations protruded into a network of anastomosing luminal canals. Neoplastic cells immunolabeled for pan-cytokeratin, vimentin, and E-cadherin, and lacked immunolabeling for oligodendrocyte transcription factor 2 (OLIG2), glial fibrillary acidic protein (GFAP), and adrenocorticotropic hormone (ACTH). Transmission electron microscopy revealed apical microvilli, apical and lateral tight junctions, and a basal membrane. In this cat, the neuroanatomic location with ventral brain invasion was more suggestive of ependymal origin; however, there were overlapping histologic and immunohistochemical features, and ultrastructural features were more consistent with choroid plexus epithelium. Dual ependymal and choroid plexus differentiation could not be excluded. This case highlights species differences in both the occurrence and neurolocalization of intraventricular tumors in domestic animals and comparable features between papillary ependymoma and choroid plexus papilloma.