MICROSCOPIC DESCRIPTION: The sections show a moderately cellular papillary tumour. It has well-developed papillae with central fibrovascular cores. The lining cells have mildly enlarged and slightly clefted nuclei, generally small nucleoli and moderate amounts of clear foamy to pale eosinophilic cytoplasm. True ependymal rosettes or perivascular pseudorosettes are absent. Mitoses are inconspicuous. There are increased numbers of blood vessels at the tumour periphery but no microvascular proliferation is seen. No necrosis is identified. The tumour cells are diffusely CAM5.2 and focally GFAP positive. The topoisomerase index is 2%. They are transthyretin, synaptophysin, inhibin, EMA, CD10, Napsin-A, thyroglobulin and TTF-1 negative. The features are those of choroid plexus papilloma.
DIAGNOSIS: Posterior fossa tumour: Choroid plexus papilloma (WHO Grade I).