Impact of Cochlear Dose on Hearing Preservation Rates following Gamma Knife Radiosurgery for Nonvestibular Schwannomas of the Lateral Skull Base

Background Radiation dose to the cochlea has been proposed as a key prognostic factor in hearing preservation following stereotactic radiosurgery (SRS) for vestibular schwannoma (VS). However, data regarding the impact of cochlear dose on hearing outcomes following SRS for treatment of other lateral skull base neoplasms are scarce.

Methods A prospectively maintained database was retrospectively reviewed for patients with posterior fossa meningioma or jugular paragangliomas, serviceable hearing (AAO-HNSF Grades A−B) prior to SRS, and SRS dose >5.3 Gy to the cochlear volume. A parallel cohort of patients with sporadic VS who received >5.3 Gy to the cochlear volume during the treatment period 2007−2013 were simultaneously reviewed to provide a control group. Student's t and chi-square tests were used for analysis of continuous and categorical variables, respectively.

Results Fifteen non-VS tumors and 43 VS met inclusion criteria. At last audiometric follow-up (median: 38 years, range: 12−17, and 24 years, range: 6−24, for non-VS and VS tumors, respectively), serviceable hearing was preserved in 12 non-VS patients (80%; Table 1), versus 23 in the VS control group (Table 1; 80 vs. 56%, p = 0.09; [Fig. 1]). Non-VS tumors with serviceable hearing had received a median cochlear volume dose of 6.9 Gy (SD ± 4.2, range: 5.7−19.2), as compared with 7.4 Gy (SD ± 1.2, range: 5.4−7.6) in patients who subsequently developed non-serviceable hearing. Sporadic VS subjects received a median cochlear modiolar point dose of 6.8 Gy (SD ± 1.5; range: 5.4−11.7). Non-VS tumors hearing loss remained stable after 8 Gy, while VS patients' hearing continued to decline.

Conclusion Our data suggest that patients with non-VS lateral skull base neoplasms may respond differently to comparable cochlear doses after SRS; correspondingly, conclusions or predictions regarding hearing preservation outcomes extrapolated from studies conducted on VS patients cannot be freely applied to non-VS tumors. A spectrum of important but challenging-to-assess factors that go beyond cochlea irradiation likely contribute to increased radiosensitivity and propensity toward hearing loss in VS patients who undergo SRS.

Fig. 1