Diffuse leptomeningeal malignant histiocytosis in the brain and spinal cord of a Tibetan terrier

An 8-year-old male Tibetan Terrier showed prolonged astasia, complete paral ysis, ticlike signs, and seizure and died 2 months after the onset of sympt oms. Histopathologically, there was moderate to severe infiltration of pleo morphic histiocytic mononuclear cells bilaterally in the basiarachnoidal an d ventricular areas of the brain. The spinal dura mater, arachnoidal space, and leptomeninges were also affected by infiltrative proliferation of thes e mononuclear cells. The infiltrating cells had the morphologic characteris tics of histiocytes but exhibited moderate pleomorphism and atypia, with ab undant mitotic figures. With immunohistochemistry and lectin histochemistry , most of the infiltrating cells were positive for lysozyme and lectin RCA- 1 and negative for glial fibrillary acid protein, suggesting that they were of monocytic/histiocytic-origin. Positive proliferating cell nuclear antig en immunostaining demonstrated that most nuclei of the histiocytic cells we re in the S phase of the cell cycle, consistent with a proliferating popula tion of cells. Based on these findings, the case was diagnosed as diffuse l eptomeningeal malignant histiocytosis.