Gamma Knife Radiosurgery For Vestibular Schwannomas: A Multivariate Analysis Of Measures Of Cochlear Radiation Dose And Predictors Of Hearing Preservation Rates

Melandee Brown1, Christopher Chen2, Bradley Lega3, Michael Ruckenstein4, John Y.K. Lee1

1Department of Neurosurgery 2 University of Pennsylvania School of Medicine 3Philadelphia, United States 4 Department of Otolaryngology

Keywords: vestibular schwannoma, hearing function, gamma knife, hearing preservation, outcome

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     Vestibular schwannomas are benign tumors that arise from the vestibulocochlear nerve sheath. Although sensorineural hearing loss is the most common presenting symptom most patients do have useful hearing at the time of diagnosis. Further deterioration in hearing following radiation treatment is a well documented risk. Recent studies have suggested that there is a correlation between hearing preservation and radiation dose to the cochlea.
     This study supports these findings and suggests a novel measure of cochlear radiation dose that predicts hearing loss in treated patients.
     Retrospective study involving vestibular schwannoma patients treated with Gamma Knife Radiosurgery from January 2006 until March 2009.
     Mean marginal tumor dose was 12.5 Gy. Mean tumor volume was 1.11 cm3. For each patient the prescription dose, tumor volume, intracanicular tumor length, maximum cochlear dose, and mean cochlear dose were recorded. Additionally, dose volume histograms were generated and the percentage of the cochlear volume that received = 3.6 Gy, =4.7 Gy, and =5.3 Gy was calculated for each patient. Pure tone and speech discrimination audiometry was conducted. Speech discrimination and pure tone average (PTA) thresholds were taken as a measure of hearing.
     Pre and post-treatment audiograms were available for 32 patients. Pre-treatment audiograms were preformed within 68 days of GK; Post-treatment audiograms were performed on average 359 days following treatment. Ten patients whose pre-treatment audiometry demonstrated nonservicable hearing were excluded. Multivariate linear regression analysis was performed for 22 patients. The difference between the pre and post-GK PTA was the dependant variable. Tumor volume and the percentage of the cochlear volume receiving greater than 5.3 Gy (V20) were the two independent variables. V20 was a significant predictor of hearing loss (P=0.011). The predictive value of tumor volume on hearing loss approached significance (P=0.104).
     This is a retrospective study.
     The percentage of the volume of cochlea that received greater than 5.3Gy of radiation was predictive of hearing loss. In this study maximal radiation dose to the cochlea was not predictive.
     The use of cochlear dose volume histograms as an adjunct to conformal planning may be useful in limiting radiation exposure to the cochlea and preserving hearing in vestibular schwannoma patients.


Project Roles:

M. Brown (), C. Chen (), B. Lega (), M. Ruckenstein (), J. Lee ()