Sequential volume mapping for confirmation of negative growth in vestibular schwannomas treated by gamma knife radiosurgery

Object. The purpose of this study was to confirm, by using a sequential volume mapping (SVM) technique, that gamma knife radiosurgery (GKS) induces negative growth in vestibular schwannomas (VS). Methods. Over a period of 5 years, 126 small- to medium-sized (< 15 cm(3)) VSs were treated using microradiosurgical techniques within a standard protocol. All patient data were collected prospectively. Sequential magnetic resonance imaging was performed every 6 months to assess the volume of the tumor, based on specially developed GammaPlan software. The mean follow-up duration was 22 months. At least three SVM measurements were obtained in 91 patients and at least four were obtained in 62 patients. The mean number of SVM measurements for each patient was 2.54. After GKS, the following patterns of volume change were seen: 1) 57 VSs showed transient increase in volume with a peak at 6 months, followed by shrinkage. Four VSs exhibited prolonged swelling beyond 24 months. Transient swelling and eventual shrinkage were independent of the initial VS volume; 2) 29 VSs showed direct volume shrinkage without swelling; and 3) five VSs showed persistent volume increase. All volume changes were greater than 10%. The overall mean volume reduction was 46.8% at 30 months. Conclusions. Sequential volume mapping appears to be superior to conventional two-dimensional measurements in monitoring volume changes in VS after GKS. It confirms that transient swelling is common. Ninety-two percent of tumors responded by showing significant volume shrinkage (mean 46.8%). It would seem that GKS can induce volume reduction in VS.