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

Departments of *Otolaryngology and Communication Sciences, ÞRadiation Oncology, and þRadiology,Medical College of Wisconsin, Milwaukee, Wisconsin, U.S.A.Objective: Determine which variables are correlated with earlyhearing changes after gamma knife surgery of vestibularschwannomas (VSs).Study Design: Prospective clinical study of hearing outcomes,radiation dosimetry, conformity, and tumor size of all sporadicunilateral VS patients treated between June 2000 and July 2009.Setting: Tertiary referral center.Patients: Fifty-nine VS patients with at least 6 months offollow-up data were studied.Interventions: Audiometry and imaging were performed to de-termine auditory thresholds, speech discrimination, and tumorsize. Radiation doses to 5 volumes were measured.Main Outcome Measures: Pretreatmentandposttreatmentcom-parisons were performed with regard to change in tumor size;radiation dose to specific volumes including the internal auditorycanal, cochlea, basal turn of the cochlea, and modiolus; and con-formity of the treatment.Results: The mean follow-up was 63.76 months (standarddeviation, T29.02 mo; range, 9Y109 mo). The median follow-upwas 65.5 months. A statistically significant association betweenmaximum radiation dose to the cochlea volume and 3-frequencypure-tone average in patients starting with 50 dB or lesser PTA3was demonstrated using linear regression analysis.Conclusion: Longitudinal changes in hearing occur over time,withthelargestchangesseeninthefirst12monthsaftertreatment.With our study outcomes as basis, limiting the dose of radiationto the cochlea to no more than 4 Gy would likely reduce vascu-lar injury to the stria vascularis and improve hearing outcomes.Shieldingthecochleaduringthetreatmentplanningprocesswouldbeonemechanismtoaccomplishthisgoal.KeyWords:AcousticneuromaVAudiometryVConformityVFacial nerveVGammaknifeVRadiosurgeryVTinnitusVVestibular Schwannoma.Otol Neurotol 31:1480Y1487, 2010.