Hearing preservation in vestibular schwannoma stereotactic radiosurgery: what really matters?

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Stereotactic radiosurgery (SRS) for vestibular schwannomas has evolved and improved over time. Although early short-term follow-up reports suggest that fractionation yields hearing preservation rates equivalent to modern single-dose SRS techniques, significant questions remain regarding long-term tumor control after the use of fractionation in a late responding tumor with a low proliferative index and α/β ratio. With single-dose SRS, critical hearing preservation variables include: 1) strict attention to prescription dose 3D conformality so that the ventral cochlear nucleus (VCN) receives ≤ 9 Gy; 2) careful delineation of the 3D tumor margin to exclude the cochlear nerve when visualizable with contrast-enhanced T2-weighted MR volumetric imaging techniques and exclusion the dura mater of the anterior border of the internal auditory canal; 3) a tumor margin dose prescription ≤ 12 Gy; 4) optimization of the tumor treatment gradient index without sacrificing coverage and conformality; and 5) strict attention to prescription dose 3D conformality so that the modiolus and the basal turn of the cochlea receive the lowest possible dose (ideally < 4–5.33 Gy). Testable correlates for the relative importance of the VCN versus cochlear dose given the tonotopic organization of each structure suggests that VCN toxicity should lead to preferential loss of low hearing frequencies, while cochlear toxicity should lead to preferential loss of high hearing frequencies. The potential after SRS for hearing toxicity from altered endolymph and/or perilymph fluid dynamics either via impaired fluid production and/or absorption has yet to be explored. Serous otitis media, ossicular or temporal bone osteonecrosis, and chondromalacia are not likely to be relevant factors or considerations for hearing preservation after SRS.

Abbreviations used in this paper: DCN = dorsal cochlear nucleus; GKS = Gamma Knife surgery; IAC = internal auditory canal; SRS = stereotactic radiosurgery; VCN = ventral cochlear nucleus; VS = vestibular schwannoma.

Article Information

Address correspondence to: Mark E. Linskey, M.D., Department of Neurological Surgery, University of California, Irvine Medical Center, 101 The City Drive South, Building 56, Suite 400, ZOT 5397, Orange, California 92868. email: mlinskey@uci.edu.

© AANS, except where prohibited by US copyright law.

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Figures

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    A: Nissl stained cross section 14× of the brainstem at the level of the lateral recess of the fourth ventricle at the pons-medulla junction demonstrating the root entry zone of the VII and VII cranial nerves as well as the VCN and the DCN. The cochlear nuclei are tonotopically organized with low frequencies (L) represented ventral and laterally, Mid frequencies represented in the center of each nucleus (M), and high frequencies (H) represented medially and dorsally. The cochlear nerve splits the VCN and the DCN and the medial low frequency portion of the DCN (*) is located at a slightly more caudal level. B: with the location of a vestibular schwannoma indicated within the cerebellopontine angle (Tumor), it becomes clear that the low frequency representations of the cochlear nuclei are closest to the tumor margin and differentially susceptible to radiosurgery toxicity.

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    Heavily T2-weighted volumetric, contrast-enhanced target image of a left VS. Inferior level (A) and superior level (B) revealing cranial nerve VIII relative to the tumor volume, and (C) GKS dose plan prescribing 12 Gy to the 50% isodose line, taking these relationships into account.

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    A: Cadaver histological transverse section through the temporal bone at the level of the modiolus of the cochlea (M) demonstrating the cochlear nerve (CN), the lamina spiralis ossea (LSO), the scala vestibule (SV), the scala tympani (ST) and the liagamentum spirale cochleae (LSC). B: T2-weighted MR image demonstrating the same structures. C: Computed tomography bone window scan at the level of the modiolus indicating the area for point dose sampling after SRS. D: Computed tomography bone window scan at the level of the inferior basal turn of the cochlea indicating the area for point dose sampling after SRS.

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    Schematic of the tonotopic organization of the hair cells within the cochlea. The highest hearing frequencies are located in the inferior basal turn of the cochlea and these gradually lower as the turns of the cochlea wind around the cochlear modiolus, with the lowest frequencies represented at the apical turn.

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