Mixed cranial nerve tumors in neurofibromatosis type 2.

Five cerebellopontine angle tumors from four patients with neurofibromatosis (NF) are described. Three were surgical resection specimens from patients with NF2 and two were removed at autopsy from a fourth patient who appears to have had a variant of NF2. On microscopy the three biopsy specimens appeared to be histologically mixed tumors: although they were predominantly typical benign schwannomas, the tumors also contained islands of meningioma, the two tumor types being closely intermingled. In our fourth patient, consecutive decalcified sections of the tumors in the petrous temporal bones revealed florid arachnoidal proliferation around both schwannomas. This had resulted in the formation of multiple "micromeningiomas", some of which had become partially incorporated into the tumor. There are a few previously reported examples of mixed schwannoma-meningiomas in the literature. Possible mechanisms for such a mixed tumor are discussed: the most likely explanation for the appearances in cases of apparently mixed vestibular nerve tumors is that reactive meningeal changes adjacent to the tumor are responsible. Arachnoidal proliferation appears to be more exuberant in bilateral acoustic neurofibromatosis than in sporadic acoustic schwannomas, possibly the result of a disease-related growth factor.