Ossifying choroid plexus papilloma recurring with features of atypical papilloma

A case of ossifying choroid plexus papilloma (CPP) with an unusual clinical course is reported. The patient was a 46-year-old woman who underwent surgery for a 3-cm tumor occupying the fourth ventricle. The tumor showed typical histopathological features of CPP, and the formation of psammoma bodies and mature bone trabeculae was prominent in the stroma. The tumor recurred locally after a dormant period of 15 years, and the recurrent lesion showed invasion of the cerebellum as well as increased cellularity, cellular stratification, nuclear atypism, and mitotic activity, all of which were consistent with a diagnosis of "atypical" CPP. The recurrent tumor did not show ossification of the stroma, although many psammoma bodies were found. There have been very few reported cases of ossifying CPP, and all of the previous cases behaved in a benign fashion. The present case is the first report of ossifying CPP that showed postoperative recurrence with progression to atypical CPP after an unusually long dormant period.