Primary Encephalic Plasma Cell Tumor in a Dog

A 5-year-old female spayed Spitz dog had a 5-week history of right head tilt, seizures, and progressive quadriplegia. Analysis of cerebrospinal fluid revealed 27,600 white blood cells per microliter with 63% mononuclear phagocytes, 27% lymphocytes, 6% neutrophils, 3% plasmacytoid cells, and 1% eosinophils, and over 2000 mg/dl protein. On contrast-enhanced magnetic resonance images, a focal 1-cm oval lesion was identified in the right ventral brainstem. There was also marked contrast enhancement of the meninges in the following areas: surrounding the brainstem, outlining cerebellar folia, along the ventral floor of the brain and extending to the falx cerebri, and partially outlining the left frontal lobe. At necropsy, the areas of contrast enhancement corresponded to the presence of compact cellular sheets of pleomorphic, anisocytotic, oval to polygonal neoplastic cells with plasmacytoid differentiation. The smaller of these plasmacytoid cells stained predominantly for cytoplasmic immunoglobulin A using immunoperoxidase methodology. Ultrastructurally, the neoplastic cells had morphologic features typical of plasma cells, with large amounts of predominantly rough endoplasmic reticulum with variably prominent Golgi formation. This is the first report of a canine primary intracranial malignant plasma cell tumor.