Peer-Review ReportCochlea Radiation Dose Correlates with Hearing Loss After Stereotactic Radiosurgery of Vestibular Schwannoma
Introduction
Vestibular schwannoma is a benign tumor arising from the vestibulocochlear nerve. Treatment options include observation, open microsurgical resection, and radiation (conventionally fractionated radiotherapy and stereotactic radiosurgery [SRS]). In previous studies, authors have shown that SRS produces high rates of tumor control with low risk of cranial nerve injury (4, 12). Although the specific mechanism of hearing loss after SRS has yet to be determined, proposed mechanisms include direct radiation injury (to the nerve, cochlea, and/or auditory canal), vascular damage, and tumor-related effects (3, 15). In several studies of single-fraction SRS, investigators have suggested an association between radiation dose, particularly to the cochlea, and the preservation of hearing after SRS (15, 19, 20). To determine the relationship between the dose and volume of the irradiated cochlea and hearing in patients receiving trisession SRS, we conducted a retrospective, dosimetric study of patients with vestibular schwannoma treated with SRS at Stanford University Hospital.
Section snippets
Patient Population
With approval from the Stanford University Institutional Review Board, patient clinical and dosimetric information was retrospectively obtained. Entry criteria included treatment of a vestibular schwannoma with trisession SRS (18 Gy over the course of 3days) between 1999 and 2005, Gardner–Robertson (GR) classification (Table 1) grade I or II hearing before SRS, complete audiogram, and magnetic resonance imaging (MRI) follow-up. A cochlear dose–volume histogram was generated for each patient's
Results
Ninety-four patients fulfilled our inclusion criteria: 71 had good-excellent and 23 had serviceable pretreatment GR hearing. In this series, of the 24 patients with intracanalicular tumors, serviceable hearing was lost by only four, a sample size too small for further subanalyses. Patients had a median age of 52 years (range, 20–79 years), with equal gender distribution (53% male, 47% female). Forty-nine percent of tumors were left sided (n = 46). One patient had neurofibromatosis type II. The
Discussion
Hearing preservation rates after SRS for vestibular schwannomas have been improved (from 26% to >70%), without compromising tumor control rates, by decreasing the single session treatment dose to 13 Gy (2, 5, 12, 16, 17). Likewise, multisession radiosurgery (18 Gy in three sessions) is safe and effective, with a 5-year tumor control rate of 96% and serviceable hearing preservation rate of 76% at a median follow-up of 2.4 years (8).
The underlying pathophysiology of hearing loss after
Conclusion
Excellent tumor control and acceptable rates of hearing preservation can be achieved by the use of trisession stereotactic radiosurgery for vestibular schwannomas. Larger cochlear volume is associated with lower risk of hearing loss. Controlling for this phenomenon, we find a statistically significant increase in risk for hearing loss of 1% to 6% per unit cochlear volume (mm3) receiving 10 to 14 Gy of radiation (delivered in three radiosurgery sessions). This study quantifies the associations
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Conflict of interest statement: Dr. John Adler is Vice President of Varian Medical, Inc.; Dr. Iris Gibbs is a member of the Clinical Advisory Board and the Speakers' Bureau of Accuray, Inc., manufacturer of the CyberKnife radiosurgical system. This work is in part supported by gifts from Robert C. and Jeannette Powell, Alan Wong and Sylvia Tang, and Paula and William Zappettini to Steven D. Chang, MD. M. G. Hayden Gephart is supported by a postdoctoral fellowship from the California Institute of Regenerative Medicine.