A case report of a choroid plexus carcinoma spontaneously occurring in the right lateral ventricle of a 14-week-old, female Donryu rat.

We encountered a brain tumor arising in the right lateral ventricle of a 14-week-old, female Donryu rat and investigated its histological and immunohistochemical characteristics. Macroscopically, the tumor appeared as a grayish mass with a size of 10 mm in diameter, present in front of the right hemicerebrum and well circumscribed on the cut surface. Histological examination revealed the tumor to be a hypercellular mass occupying the front part of the right lateral ventricle and expanding into the area in front of the hemicerebrum, continuing to the ependymal area at its edge. The tumor was constituted by columnar- or pleomorphic-shaped, highly atypical cells of epithelial origin surrounding fibrovascular cores as single or multiple cell layers. Growth was papillary with high proliferating activity. Immunohistochemically, the tumor cells proved positive for cytokeratin but negative for vimentin, S100 protein or glial fibrillary acidic protein, a profile characteristic for the epithelial cells of the choroid plexus, whereas the ependymal cells were found to be positive for all 4 items. In conclusion, the present tumor was diagnosed as a rat choroid plexus carcinoma, only the third such case to be reported in the world literature, with particular features.