Internal Auditory Canal Drill-Out as a Prognostic Factor for Vestibular Schwannoma Recurrence Following Subtotal Resection

Background: Subtotal resection (STR) of vestibular schwannomas (VS) is often performed to preserve neurological function, often leaving residual tumor within the internal auditory canal (IAC). While postoperative tumor volume is a known predictor of progression, the independent contribution of IAC drill-out on local tumor control remains poorly defined. This study aimed to determine if surgical drill-out of the IAC is associated with reduced tumor recurrence rates in patients with sporadic VS undergoing STR. Methods: A retrospective cohort study was conducted at a single tertiary academic center, including 261 patients who underwent STR for sporadic VS between 2004 and 2024. The cohort was stratified by the surgical extent of IAC dissection (drill-out versus no drill-out). The primary endpoint was tumor recurrence, defined as volumetric growth requiring further intervention. Recurrence-free survival was analyzed using Kaplan-Meier methodology and compared with the log-rank test. A multivariate Cox proportional hazards regression model was used to identify independent predictors of recurrence, adjusting for potential confounders. Secondary outcomes included volumetric analysis of preoperative and residual tumor, 30-day surgical and medical complication rates, and functional outcomes, including House-Brackmann (HB) facial nerve scores. Results: The cohort included 129 patients in the IAC drill-out group and 132 in the no drill-out group. Baseline characteristics were similar, though the drill-out group had smaller mean preoperative tumor volumes (8.1 ± 9.5 cm 3 versus 11.6 ± 9.1 cm 3 ; p = 0.003). Critically, there was no significant difference in mean postoperative residual tumor volume (1.6 ± 3.4 cm 3 versus 2.2 ± 2.5 cm 3 ; p = 0.120) or percent residual tumor (16.2 ± 22.2% versus 20.2 ± 17.1%; p = 0.110). At a median follow-up of 30.6 months, the recurrence rate was significantly lower in the IAC drill-out group (8.5% versus 18.2%; log-rank p = 0.001). On multivariate analysis, IAC drill-out was an independent predictor of improved recurrence-free survival (Hazard Ratio 0.35, 95% CI 0.14−0.85; p = 0.021). In a subgroup analysis of patients managed without adjuvant radiation ( n = 128), the benefit of IAC drill-out was highly pronounced (3.4% versus 23.1% recurrence; log-rank p < 0.001). Conversely, no difference was observed among patients who received adjuvant radiation ( n = 133) (20.0% versus 16.1%; log-rank p = 0.829). The procedure was not associated with increased morbidity, with no differences in surgical complications (9.3% versus 9.8%; p = 1.000), medical complications (5.4% versus 6.1%; p = 1.000), or rates of good facial nerve function (HB I-II) at 6 months post-op (87.2% versus 85.4%; p = 0.751). Conclusion: Complete drill-out of the IAC during subtotal resection of vestibular schwannoma is associated with a 65% reduction in the hazard of tumor recurrence, independent of preoperative tumor volume. This benefit is most significant in patients who do not receive adjuvant radiation and is achieved without an associated increase in surgical morbidity. These findings suggest that the oncologic advantage of IAC dissection may extend beyond simple cytoreduction, potentially involving disruption of the tumor's microenvironment, and support its consideration as a key component of planned STR. Publication History Article published online: 27 February 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany