Follow-up after gamma knife radiosurgery for vestibular schwannomas: volumetric and axial control rates.

OBJECTIVES/HYPOTHESIS: A prospective long-term follow-up study was conducted to evaluate the results of gamma knife radiosurgery (GKRS) for vestibular schwannoma (VS) patients. Both axial and volumetric measurements are used to determine tumor size during follow-up. STUDY
DESIGN: Individual prospective cohort study.
METHODS: A total of 110 VS patients were referred for radiosurgery between 2002 and 2007. All patients were treated with a Leksell 4C gamma knife. There were 12.5 to 13 Gy prescribed to the isodose covering 90% of the tumor volume. The resulting marginal dose was on average 11.0 Gy (range, 9.3-12.5 Gy). Tumor size and tumor volume were determined before and after gamma knife treatment at regular intervals. The minimal follow-up period was 2 years.
RESULTS: There were 100 patients included in the study. Eight patients needed additional treatment after a mean follow-up period of 38 months. One patient experienced a temporary facial nerve deficit. No growth pattern could be recognized for tumor growth after GKRS. Based on the measurements of the largest extrameatal diameter, the tumor size would have decreased or remained stable in 94%. Based on volumetric measurement, the tumor size was decreased or remained stable in 79%.
CONCLUSIONS: High tumor control and low complication rates make GKRS a good therapy for VS. If tumor growth occurs after GKRS, a conservative management can be considered because continued tumor growth is uncertain. The extrameatal diameter on axial magnetic resonance imaging seems to be a reliable parameter of the size of a VS. Volumetry is the preferred method to assess the dimensions of a VS, although the consequences of strong volumetric increase, especially in small tumors, can be different depending on individual differences in tumor size.