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Fluctuating response of a cystic vestibular schwannoma to radiosurgery: case report.

OBJECTIVE
A vestibular schwannoma (VS) is a benign tumor of the VIIIth cranial nerve that can often be treated by microsurgery or radiosurgery and demonstrates high tumor control rates. Radiosurgery is typically performed as gamma knife surgery (GKS), although other modalities are being applied with increasing frequency. A differentiating feature in responsiveness to microsurgery or GKS is whether the VS is cystic or solid. A cystic VS is less responsive to GKS than a solid VS, representing a challenging clinical problem. GKS treatment of a cystic VS usually results in sustained expansion, sustained regression, or transient expansion followed by sustained regression. In this article, we report an atypical fluctuating course of a cystic VS after GKS, ultimately requiring surgical intervention.


CLINICAL PRESENTATION
A 66-year-old woman presented with asymmetric hearing loss and tinnitus. Magnetic resonance imaging revealed a 2.0-cm unilateral cystic VS within the cerebellopontine angle.


INTERVENTION
After GKS with a 12-Gy dose to the 50% isodose line, the tumor expanded transiently to 3.2 cm and then regressed to 1.0 cm over the next 2 years. She presented several months later with new-onset dizziness, ataxia, and facial numbness. Magnetic resonance imaging revealed a 3.2-cm multicystic VS that compressed the brainstem. After microsurgical tumor excision, the patient's symptoms abated.


CONCLUSION
Our case report is a novel demonstration that a cystic VS that has regressed after GKS is still at risk for expansion. The mechanisms responsible for radiation-induced cystic tumor expansion have not been thoroughly elucidated. The risk of unpredictable tumor enlargement should be discussed with patients when considering GKS for cystic tumors.