Objective To report a rare case of atypical choroid plexus papilloma and explore its clinical presentations and histopathological features, at the same time to take a review of relevant literature. Methods and Results A 1 ? year ? old male patient presented with vomiting for 3 days after a sudden tumbling. Computed Tomography (CT) showed a space?occupying lesion located in the left lateral ventricle. The lesion was with slight hyper?intensity signal compared to the adjacent brain parenchyma. Left lateral ventricle was enlarged and hydrocephalus was visible. Besides, edema occured in adjacent cerebral tissues. The surgical operation was performed under general anesthesia. During the operation, the tumor tissue was in heterogeneous consistency and rich blood supply, showing grey?white color, and attached to the choroid plexus. Microscopically, most tumor cells grew around the fibrovascular axis forming papillary pattern and showed no obvious atypia. However, in some areas, solid and sheet?like patterns were identified. Within these areas, increased cell density, mild to moderate cellular atypia and focal necrosis were appreciated. Some cells with acidophilic cytoplasm, few giant tumor cells and accidental mitosis were also seen, and the counting of mitosis was 2/10 HPF. Immunohistochemistry showed positive expression of pancytokeratin (PCK), synaptophysin (Syn), cytokeratin (CK)8/18, vimentin (Vim) and podoplanin (D2?40) in tumor cells. Integrase interactor?1 (INI?1) were also positively and focally expressed. The Ki?67 labeling index of tumor cells was 8％ . S ? 100 protein, CK19, neuronal nuclei (Neu ? N), glial fibrillary acidic protein (GFAP), epithelial membrane antigen (EMA), chromogranin A (CgA), CK7 and CK20 were negatively stained. The patient was followed up for 6 months after operation and no recurrence was found. Conclusion Atypical choroid plexus papilloma, the biological behavior of which is between benignancy and malignancy, is one of the rare tumors of central nervous system that occurs in the ventricular system. The incidence is low. It often occurs in the early childhood with atypical histopathological features. Follow up is suggested although patients with complete resection usually have a good long?term prognosis. DOI：10.3969/j.issn.1672?6731.2012.06.017.
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