LAUSR

Simple Summary Choroid plexus tumors are commonly described as intraventricular mass lesions and account for 7–10% of intracranial, primary tumors in dogs. This paper reports about an unusual case of a choroid plexus carcinoma in a 3-year-old Shetland sheepdog that presented for slowly progressive lethargy, vision impairment and cognitive deficits. On magnetic resonance imaging, a subdural fluid accumulation overlying and compressing the left parietotemporal lobe as well as multifocal changes consisting of cyst-like lesions, supposed intra-axial brain lesions and generalized meningeal changes were identified. Cerebrospinal fluid analysis showed a mononuclear pleocytosis with negative results for infectious agents. The dog was treated with prednisolone followed by burr hole craniotomy with puncture of the subdural fluid accumulation and subsequent chemotherapy that resulted in temporary improvement. The dog deteriorated again and was therefore euthanized. Post-mortem examination revealed a diffuse, highly invasive choroid plexus carcinoma that involved the entire central nervous system. In conclusion, a choroid plexus carcinoma should be considered as a possible cause of a subdural fluid accumulation even in the absence of an intraventricular mass lesion. Abstract Choroid plexus tumors are commonly described as intraventricular mass lesions and account for 7–10% of intracranial, primary tumors in dogs. A 3-year-old Shetland sheepdog was presented with a history of slowly progressive lethargy, vision impairment and cognitive deficits. On magnetic resonance imaging, a subdural fluid accumulation (SFA) overlying and compressing the left parietotemporal lobe as well as multifocal changes consisting of cyst-like lesions, supposed intra-axial brain lesions and mild, multifocal meningeal thickening and generalized contrast enhancement were identified. Cerebrospinal fluid (CSF) analysis showed a mononuclear pleocytosis with negative results for infectious agents. The dog was treated with prednisolone followed by burr hole craniotomy with puncture of the SFA, which macroscopically appeared to be CSF-like fluid. After initial improvement, the dog deteriorated despite continuation of prednisolone and cytarabine therapy and was euthanized four weeks after surgery. Histopathology was consistent with a disseminated, neuroinvasive choroid plexus carcinoma (CPC) that involved the entire neuroaxis including the meninges of the brain and spinal cord. Immunohistochemical examination showed a strong Kir7.1 and a heterogenous cytokeratin-immunoreactivity in neoplastic cells. In conclusion, a CPC should be considered as a possible cause of a SFA even in the absence of an intraventricular mass lesion.