Gamma Knife Surgery of Vestibular Schwannomas: Volumetric Dosimetry Correlations to Hearing Loss Suggest Stria Vascularis Devascularization as the Mechanism of Early Hearing Loss

Objective: Determine which variables are correlated with early hearing changes after gamma knife surgery of vestibular schwannomas (VSs). Study Design: Prospective clinical study of hearing outcomes, radiation dosimetry, conformity, and tumor size of all sporadic unilateral VS patients treated between June 2000 and July 2009. Setting: Tertiary referral center. Patients: Fifty-nine VS patients with at least 6 months of follow-up data were studied. Interventions: Audiometry and imaging were performed to determine auditory thresholds, speech discrimination, and tumor size. Radiation doses to 5 volumes were measured. Main Outcome Measures: Pretreatment and posttreatment comparisons were performed with regard to change in tumor size; radiation dose to specific volumes including the internal auditory canal, cochlea, basal turn of the cochlea, and modiolus; and conformity of the treatment. Results: The mean follow-up was 63.76 months (standard deviation, +/-29.02 mo; range, 9-109 mo). The median follow-up was 65.5 months. A statistically significant association between maximum radiation dose to the cochlea volume and 3-frequency pure-tone average in patients starting with 50 dB or lesser PTA3 was demonstrated using linear regression analysis. Conclusion: Longitudinal changes in hearing occur over time, with the largest changes seen in the first 12 months after treatment. With our study outcomes as basis, limiting the dose of radiation to the cochlea to no more than 4 Gy would likely reduce vascular injury to the stria vascularis and improve hearing outcomes. Shielding the cochlea during the treatment planning process would be one mechanism to accomplish this goal.