The sections show a moderately cellular tumour comprising well-defined papillary structures with fibrovascular cores. No solid areas are seen. The papillae are lined by cuboidal epithelium which shows occasional pseudostratification. The tumour cells have round nuclei with even chromatin, inconspicuous nucleoli and moderate amounts of eosinophilic cytoplasm. No mitoses, microvascular proliferation or necrosis is seen. There are scattered spots of microcalcification in the background. The Ki-67 index is about 2%. The tumour cells are transthyretin, thyroglobulin, TTF-1 and BER-EP4 negative.
DIAGNOSIS: 4th ventricle lesion: Choroid plexus papilloma (WHO grade I).