Outcomes of Cochlear Radiation Dose on Hearing Preservation Following Stereotactic Radiosurgery and Fractionated Radiotherapy in Vestibular Schwannoma

Introduction: Radiation therapy has emerged as an effective treatment for vestibular schwannomas (VS) to achieve tumor control and hearing preservation. The purpose of this study was to evaluate the amount of radiation delivered to the cochlea during stereotactic radiosurgery (SRS) and fractionated stereotactic radiotherapy (FSRT), and determine its effect on hearing outcome.

Methods: Patients receiving SRS or FSRT treatment for VS at a single institution from 2009–2014 were retrospectively identified. The dosimetric data to the cochlea volume was calculated using tractography software (BrainLAB, iPlan RT Dose 4.5.3) and associated with hearing outcomes. Patients treated with SRS received a marginal dose of 12 Gy and patients treated with FSRT received a marginal dose of 50.4 Gy delivered through 28 fractions. Statistical analysis included univariate Mann-Whitney-U test and multivariate stepwise binary logistic regression to compare hearing outcomes between subgroups. Variables with p-values < 0.2 under univariate analysis were included in the logistic regression model. All statistical analyses were 2-tailed with a significance level of 0.05.

Results: 56 patients (25 SRS and 31 FSRT) underwent radiation therapy with a mean follow-up of 30 months. The mean age was 61.1 years and the average tumor volume was 1.87 cc for SRS and 3.18 cc for FSRT. The mean cochlea volume was 77.9 mm3. Patient characteristics are summarized in Table 1.

When comparing SRS patients with stable\improved hearing and decreased hearing, the mean cochlea volume (p = 0.008), mean tumor volume (p = 0.031) and minimum cochlear radiation dose (p = 0.031) were statistically different. Multivariate analysis kept 2 variables in the final model (p = 0.007) with minimum cochlea radiation dose (Odds ratio: 2.163) as a predictor of decreased hearing. Patient parameters and hearing outcomes after SRS are summarized in Table 2.

When comparing FSRT patients with stable/improved hearing and decreased hearing, none of the 9 variables evaluated were statistically different. Multivariate analysis kept 1 variable in the final model (p = 0.036) with cochlea volume (Odds ratio: 1.037) as a predictor of decreased hearing. Patient parameters and hearing outcomes after FSRT are summarized in Table 3.

Conclusions: Our data suggests that increased cochlear radiation dose is significantly associated with decreased rates of hearing preservation after SRS treatment, but not with FSRT treatment. Irradiation of cochlear structures may be a critically important aspect of radiosurgery treatment that warrants careful evaluation in the radiation treatment of vestibular schwannomas. However, other factors such as cochlea size may play a role in hearing preservation, especially in FSRT were tumor volumes tended to be larger. Future prospective studies may further validate these findings.