Predictors of Hearing Loss After Gamma Knife Radiosurgery for Vestibular Schwannomas: Age, Cochlear Dose, and Tumor Coverage

BACKGROUND: Deterioration in hearing after Gamma Knife radiosurgery of vestibular schwannomas is a well-documented risk. Recent studies suggest a correlation between cochlear radiation dose and hearing preservation. OBJECTIVE: This study identifies additional variables that predict hearing loss after radiosurgery. METHODS: Retrospective analysis of 53 patients with audiogram follow-up. Median marginal tumor dose was 12.5 Gy. Mean tumor volume was 1.11 cm(3). Statistical analysis included multivariate stepwise backward linear regression and multivariate logistic regression. Variables included age, prescription dose, tumor volume, intracanalicular length, and maximum and mean cochlear dose. Dose volume histograms were generated. The percentage of the cochlear volume that received 3.6 Gy or greater, 4.7 Gy or greater, and 5.3 Gy or greater was calculated. Plan conformality indicators were calculated. RESULTS: Forty-two patients had a less than 20-dB change in their pure tone average, with a hearing preservation rate of 79%. Two statistically significant predictors of hearing loss were identified using multivariate analysis: tumor coverage (odds ratio: 1.38 x 10(18)) and age (odds ratio: 1.1 per year). Multivariate linear regression was used to predict change in pure tone average. Age and percentage of the cochlear volume receiving 5.3 Gy or greater were found to be statistically significant predictor variables. CONCLUSION: Older patients are more vulnerable to detrimental effects of Gamma Knife radiosurgery on hearing. We propose that cochlear dose volume histograms be created and used to reduce the percentage of the cochlear volume exposed to radiation doses greater than 5.3 Gy. This is the first report to suggest that the conformity index tumor coverage may be an important predictor of hearing outcomes.