Clinical and Histopathologic Features of Recurrent Vestibular Schwannoma (Acoustic Neuroma) after Stereotactic Radiosurgery

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Abstract
Stereotactic radiosurgery for vestibular schwannoma entails uncertain long-term risk of tumor recurrence and delayed cranial neuropathies. In addition, the underlying histopathologic changes to the tumor bed are not fully characterized. We seek to understand the clinical and histologic features of recurrent vestibular schwannoma after stereotactic radiation therapy. Retrospective review. Tertiary referral center. Four patients who underwent microsurgical resection of vestibular schwannoma after primary stereotactic radiation therapy. Patients were treated primarily with gamma knife radiosurgery or fractionated stereotactic radiotherapy followed by salvage microsurgery. Retrosigmoid craniotomy was used in all cases. Histopathologic review. Preoperative and postoperative facial nerve function was assessed with the House-Brackmann scale. We observed highly inconsistent radiation changes in the cerebellopontine angle and internal auditory canal. Fibrosis outside and within the tumor bed varied markedly, complicating microsurgical dissection. Light microscopy confirmed the presence of viable tumor in all cases. Histopathologic features were typical of vestibular schwannoma, and there was no significant scarring that could be attributed to radiation effect. The variable fibrosis in the cerebellopontine angle and lack of radiation changes seen histopathologically in irradiated vestibular schwannoma suggest that a uniform treatment effect was not achieved in these cases. Although all four patients with preoperative cranial neuropathies were found intraoperatively to have fibrosis in the cerebellopontine angle, excellent preservation of facial nerve anatomy and function was possible with salvage microsurgical resection. Additional analyses are needed to clarify the histopathologic and molecular characteristics associated with vestibular schwannoma growth after stereotactic radiation.