Dose Volume Histogram Analysis Of Hearing Preservation As A Function Of Mean And Partial Cochlear Dose In Treatment Of Vestibular Schwannoma With Gamma Knife Radiosurgery

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     To assess auditory acuity outcome with Gamma Knife Radiosurgery (GKRS) in treating Vestibular Schwannoma (VS), results of a Dose-Volume Histogram (DVH) analysis of cochlear dose are presented.
     From 2006-09, 28 patients were treated at our institution with GKRS for VS and had serial auditory testing (Gardner Robertson GR & AAO).
     For all patients, marginal dose was 12.5 Gray (Gy) prescribed to the 50% isodose volume. Plugging was used to minimize cochlear dose. Pre- and post-treatment auditory acuity tests GR/AAO were done at 6, 12, 18and 24 mo after GKRS. The acuity testing results were correlated with the cochlear mean, maximal dose and % cochlear volume receiving 3, 5, 8 and 10 Gy (V3, V5, V8, V10).
     Median age was 62.9 (26.0 - 88.4). Median follow-up (FU) was 16.5 mo with auditory tests to minimum 18 mo. The cochlear mean dose was 2.7 Gy. The V3, V5 and V8 ranged from 0-84%, 0-44%, and 0-15%, respectively. Linear regression analysis revealed a GR decrement > V8 and V10 (p=0.012, 0.029 respectively). Analysis showed an AAO decrement > V10 (p=0.039). These cochlear volumes receiving 3, 5, 8 and 10 Gy showed that the higher the % cochlear volume receiving > 8 Gy resulted in hearing decrement of GR/AAO. With V3 at > 72% and V5 at > 13% there is a trend in decrement of AAO (p=0.102, 0.206). Advancing age showed less incremental AAO change (p=0.02). Maximal cochlear dose had a trend toward significance for GR (p=0.056). Mean cochlear dose, maximal tumor volume and dimension had no significance for GR and AAO.
     This is a retrospective study.
     Hearing preservation with GKRS is a function of patient & treatment-related factors. In DVH analysis of maximal dose and % cochlear volume receiving > 8 Gy to resultant hearing outcome GR/AAO, a significant decrement in GR and AAO is found with % cochlear volumes receiving > 8 Gy and > 10 Gy, respectively. Increasing age shows less change in AAO.
     Continued FU and additional patients will be needed to determine long-term hearing preservation as a function of age and partial cochlear organ dose with GKRS for VS.


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