Answer: Choroid plexus papilloma

Histology: The lesion is quite large and on tissue measures greater than 1 cm. The lesion consists of small papillary fronds lined by cells with abundant eosinophilic cytoplasm. Nuclei are uniform, lacking malignant features and lacking appreciable mitotic figures. Scattered calcifications are noted.

Discussion: Microscopically, choroid plexus papillomas can virtually be indistinguishable from normal choroid plexus. Consequently, the diagnosis of choroid plexus papilloma must be made in the presence of a mass lesion. Similarly, under the microscope, there should be a greater quantity of choroid plexus epithelium than expected in normal choroid plexus relative to the age of the patient. In some cases, the epithelial lining of choroid plexus papilloma is more crowded and piled up than in normal choroid plexus, although in the current case, cytology is virtually the same as normal choroid plexus. There is no such lesion such as hyperplastic choroid plexus. Choroid plexus carcinoma is overtly malignant with anaplasia and often with necrosis where in areas papillary architecture may not be readily appreciated. The major differential diagnosis in choroid plexus carcinoma is between metastatic carcinoma to the brain as opposed to between choroid plexus papilloma. However, in some cases one may see transitional areas between papilloma and carcinoma. Choroid plexus papillomas are entirely benign and can occur in any area of choroid plexus. They are more frequently seen in the lateral ventricles of children.