Gamma knife radiosurgery for vestibular schwannomas: results of hearing preservation in relation to the cochlear radiation dose.

Abstract

OBJECTIVES/HYPOTHESIS: This study was designed to evaluate hearing preservation after gamma knife radiosurgery (GKRS) and to determine the relation between hearing preservation and cochlear radiation dose in patients with a sporadic vestibular schwannoma (VS). METHODS: Prospective study involving patients suffering from VS who received GKRS from June 2003 until November 2007. Pure tone and speech audiometry were conducted before and after GKRS. The thresholds at pure tone audiometry were taken as a measure of hearing. Pure tone average (PTA) was defined as the mean threshold at 0.5 kHz, 1.0 kHz, 2.0 kHz, and 4.0 kHz. Hearing was classified according to the 2003 consensus meeting in Tokyo. Stereotactic surgery was performed using a Leksell 4C Gamma Knife (Elekta, Stockholm, Sweden). RESULTS: A total of 69 patients were included in the study. Mean tumor size was 17 mm. Mean marginal dose at the tumor was 11.0 Gy (range, 9.3 Gy-12.3 Gy), mean maximal dose was 19.7 Gy (range, 16 Gy-25.5 Gy). Mean maximal dose at the cochlea was 10.27 Gy (range, 3.1 Gy-16.1 Gy), and mean minimal dose at the cochlea was 2.6 Gy (range, 0.9 Gy-7.4 Gy). Mean PTA before GKRS was 43 dB (standard deviation [SD] 20 dB), mean PTA after GKRS was 63 dB (SD 30 dB). Mean interval between pre-GKRS audiometry and GKRS was 8.0 months. Between GKRS and post-GKRS audiometry, mean interval was 14.2 months. Hearing was considered to be preserved (max +1 class, Tokyo classification) in 52 (75%) of 69 patients. However, only 32 patients had class A, B, or C (serviceable hearing) before GKRS. Within this group, only 13 patients (41%) had a hearing class A, B, or C after GKRS. A significant relation was found between the maximal cochlear dose and the difference in PTA before and after GKRS. CONCLUSIONS: Hearing preservation is correlated to the maximal radiation dose at the cochlea. The purpose of developing GKRS techniques was to avoid collateral damage in healthy tissues. This study emphasizes the need for exact radiation planning to reduce the cochlear radiation dose if the hearing is to be preserved. Laryngoscope, 2009.