Tumor control
The tumor control rate after SRS was 98.9% at 3 years, 95.3% at 5 years, and 92.2% at 10 years. In univariate analysis, there was no significant difference in tumor control rates between HM and HD patients (p =0.535). Four patients had tumor progression during follow-up. Three patients were observed without additional treatment because the tumors had initial enlargement but then stabilized. These patients were one HD patient and two HM patients, but all three patients had hearing deterioration from 12 to 54 months after SRS. A single patient underwent additional SRS (margin dose 11.0 Gy) at 4.5 years after initial SRS. This patient was HM patient, maintained useful hearing, and had no complications at 23 months after repeat SRS.
Serviceable hearing preservation
At the last follow up, 42 of 67 HM patients maintained serviceable hearing, whereas 8 of 33 HD patients had preserved serviceable hearing. The detailed comparison of GR hearing at diagnosis, SRS, and last follow-up are shown in Table 2. The overall serviceable hearing preservation rate was 65.9% at 3 years, 52.5% at 5 years and 40.5% at 10 years (Fig. 2a). The serviceable hearing preservation rates of HM patients were 79.9% at 3 years, 63.4% at 5 years, and 51.2% at 10 years. In contrast the serviceable hearing preservation rates of HD patients were 40.0% at 3 years, 32.7% at 5 years, and 19.6% at 10 years (Fig. 2b). In univariate analysis, younger age (<55 years, p=0.006), better PTA at diagnosis (<20 dB, p=0.024), and HM status (p <0.001) were associated with improved long-term serviceable hearing preservation rates. The following factors were not significantly associated with serviceable hearing preservation rates: sex (p=0.119), tinnitus (p=0.643), vertigo symptoms (p=0.118), Koos grade (grade I & II vs. III & IV, p=0.882), SDS at diagnosis (<94% vs. >94%, p=0.603), target tumor volume (<0.6 cm3 vs. >0.6 cm3, p=0.510), margin dose (<13 Gy vs. >13 Gy, p=0.729), and observation interval (<2 years vs. >2 years, p=0.706). In multivariate analysis, younger age (<55 years, p=0.012, HR 2.21, 95% CI 1.19-4.11) and HM group (HM status, p <0.001, HR 2.85, 95% CI 1.68-5.41) were significantly associated with improved serviceable hearing preservation rates (Table 3).
Does deterioration of the PTA or SDS during observation affect hearing preservation after SRS?
During the observation interval (median interval 17.4 months) before SRS, the median PTA increased from 16.9 dB to 21.25 dB and the median SDS decreased from 96% to 92%. The median increase in PTA was 6.85 dB and the median decrease in SDS was 4%. In univariate analysis, patients with an increase in the PTA ≥15 dB during observation had reduced serviceable hearing preservation rates (p=0.024) (Fig. 2c). Similarly, patients whose SDS decreased ≥10% had reduced serviceable hearing preservation rates (p=0.024) (Fig. 2d). GR grade I HM patients whose PTA worsened by ≥6 dB had significantly reduced GR grade I maintenance (p=0.005) (Fig. 2e). Patients whose SDS decreased by ≥5% also had a reduced probability of maintaining GR grade I hearing (p=0.028) (Fig. 2f). Patients with a PTA at SRS of <20 dB (p=0.010) had a higher probability of maintaining GR grade I.
Facial and trigeminal neuropathy
In this series, 99% maintained House-Brackman[16] (HB) grade I facial nerve function after SRS. One patient developed mild facial neuropathy (HB grade III) at 20.7 months after SRS. The Margin dose of this patient was 12.5 Gy and the tumor volume remained stable. Three patients developed transient facial spasms 8 to 38 months after SRS. No patient developed trigeminal neuropathy (defined as decrease in sensation, new pain or new paresthesia within ipsilateral trigeminal nerve distribution after SRS) in this series.