Granulofilamentous meningioma.

A 55-year old female was referred to the Department of Neurosurgery, Kitasato Hospital, because of a hearing impairment. Neuroimaging revealed a typical meningioma attached to the falx in the right frontal region. During surgery, an encapsulated, circumscribed, reddish-gray, slightly hard tumor attached to the falx was completely removed by an interhemispheric approach. On light microscopy, many of the tumor cells contained eosinophilic inclusions with single or multiple vacuoles that displaced the cytoplasm. The nuclei of the tumor cells were eccentric. There were no signs of malignancy in the specimen. Electron microscopy revealed that most of the eosinophilic inclusions were composed of filaments measuring 12 nm in diameter. There have been several reports of benign meningiomas with eosinophilic inclusions composed of intermediate filaments. The microscopic differences between these types of tumor and rhabdoid meningiomas are very subtle, and it is important the two types of tumors are not confused. Benign meningiomas with eosinophilic inclusions comprising intermediate filaments, for example the tumor described in this report, have been diagnosed as granulofilamentous meningiomas, which is a subtype of benign meningioma.