Tumor shrinkage of vestibular schwannomas after Gamma Knife surgery: results after more than 5 years of follow-up.

OBJECT: The authors prospectively analyzed volume changes in vestibular schwannomas (VSs) after Gamma Knife surgery (GKS). METHODS: Among 104 VSs treated by GKS at the Chiba Cardiovascular Center between 1998 and 2004, 87 consecutively treated unilateral VSs observed on follow-up MR imaging for at least 5 years were analyzed. These lesions were harbored by 31 men and 56 women, with a mean age of 58.6 years (range 29­80 years). The Gd-enhanced volume of each lesion was measured serially every 3 months during the 1st year and every 6 months thereafter using Gamma Plan or SurgiPlan. The frequency and degree of volume shrinkage were documented and possible prognostic factors were analyzed. RESULTS: The mean tumor volume at GKS was 2.5 cm3 (range 0.1­13.2 cm3). The lesions were irradiated by directing a mean dose of 12.0 Gy (range 10.5­13.0 Gy) to the tumor margin, which was located at the mean 52.2% isodose line (range 50%­67% isodose line). The mean follow-up period was 7.5 years (range 5.0­11.1 years). Peak tumor volume expansion was most frequently observed at 8.6 months after GKS and averaged 58% (range 0%­613%). Five years after GKS, the mean reduction in tumor volume was 31%, and 9 tumors still remained larger than their initial volumes. Tumors that homogeneously enhanced on MR images displayed less shrinkage than other tumors. CONCLUSIONS: Most VSs exhibit shrinkage 5 years after GKS. The mean volume reduction in this series was 31%. These results indicate that careful serial follow-up is necessary for patients who harbor tumors that display homogeneous enhancement on MR images and patients whose tumors continue to expand in size after GKS. (DOI: 10.3171/2010.8.GKS10960)