Extraventricular Choroid Plexus Papilloma: A Case Report.

The patient is a 25-year-old female transferred from an outside facility with week-long complaints of a headache, nausea, and vomiting. Computed tomography (CT) and magnetic resonance imaging (MRI) revealed obstructive hydrocephalus and a large cystic mass of the posterior fossa. Subsequent external ventricular drain placement and surgical resection were performed. The patient was discharged six days postoperatively with resolution of symptoms. Pathological examination of the fleshy component within the cystic structure after surgical resection was relatively inconclusive, with findings suggestive of a choroid plexus tumor. However, the lesion demonstrates the immunohistochemical characteristics of a choroid plexus papilloma (CPP) but with a cystic structure surrounding it. CPPs are rare and typically benign tumors that originate from the choroid plexus, which is responsible for cerebrospinal fluid (CSF) production. These growths can occur throughout the ventricular system of the brain, more commonly in the lateral ventricles and less commonly in the third and fourth ventricles. Differential diagnosis based on imaging is broad due to the shared visual characteristics of various brain tumors. Consequently, immunohistochemical markers are essential for the identification and diagnosis of CPPs. In the case of our patient, the small enhancing nodule is unusual in that it is surrounded by a large cystic lesion. We present the unusual case of a cystic tumor of the posterior fossa with a small nodular component located within, suggestive of a CPP by immunohistochemical and morphological characteristics. A multidisciplinary diagnostic approach is necessary to exact a proper classification of lesions such as this one, including identification by neuroradiology, resection by neurosurgery, and immunohistochemical and morphological analysis by pathology.