Malignant transformation of acoustic neuroma/vestibular schwannoma 10 years after gamma knife stereotactic radiosurgery.

Only a handful of cases of de-novo malignancies of the vestibulocochlear nerve have been reported. Even rarer is the malignant transformation of a previously histologically diagnosed benign vestibular schwannoma. We present the case of a young adult who had combined operative/Gamma knife treatment for a benign vestibular schwannoma, followed by further surgery 2 years later. He represented 10 years after original diagnosis with facial numbness and ataxia, MRI showing gross tumor recurrence. After radical resection, histology showed malignant transformation to a malignant peripheral nerve sheath tumor. Within 3 months there was rapid, aggressive recurrence with brainstem compression, requiring further surgery for brainstem decompression. Histology confirmed further de-differentiation to an anaplastic sarcoma. While awaiting radiotherapy the tumor recurred again, the patient succumbing. The patient had no features of neurofibromatosis type 2. In the literature there are 13 other cases of malignant vestibular schwannomata. Only six had radiotherapy and of these only two had histological confirmation of a benign lesion preradiotherapy. Neither of these had neurofibromatosis. Three other cases had histological proof of malignancy postradiosurgery, but with no preradiotherapy histology; of these, two were positive for neurofibromatosis. The tumor biology of vestibular schwannomata as well as the radiobiology in the context of malignant transformation is discussed.