Stereotactic radiosurgery (SRS) has fundamentally changed the management of vestibular schwannoma (VS), providing durable tumor control while avoiding the morbidity associated
with open surgery in appropriately selected patients. Contemporary series consistently demonstrate excellent radiographic outcomes. However, despite advances in treatment planning,
image guidance, and dose optimization, long-term hearing preservation remains a significant challenge.
The updated Congress of Neurological Surgeons (CNS) evidence-based guidelines report that fewer than half of patients with serviceable hearing before radiosurgery retain useful hearing at ten years. Although these estimates derive from heterogeneous studies, they highlight the progressive deterioration of auditory function despite successful tumor control.
This discrepancy raises a fundamental question for radiation oncologists: Are we adequately measuring hearing preservation, or are we relying on endpoints that fail to capture the true longitudinal impact of treatment?
Recent randomized trials, contemporary dosimetric studies, and renewed interest in auditory trajectories suggest that the next advance in vestibular schwannoma management may require a shift from conventional binary hearing endpoints toward individualized, time- dependent assessments of auditory function.
The Timing of Radiosurgery: What Have Randomized Trials Taught Us?
The optimal timing of radiosurgery in patients with small vestibular schwannomas and preserved hearing remains controversial. While early treatment may prevent further tumor growth, it also exposes patients to radiation-associated auditory risks that might otherwise be deferred or avoided.
The landmark V-REX randomized clinical trial compared upfront radiosurgery with a wait-and-scan strategy in 100 patients with small- to medium-sized vestibular schwannomas. At
four years, upfront radiosurgery achieved significantly greater tumor-volume reduction, with a mean follow-up-to-baseline volume ratio of 0.87 compared with 1.51 in the observation group.
However, most secondary clinical outcomes did not demonstrate statistically significant differences. More recently, Bartek et al. reported the results of a prospective randomized study comparing upfront Gamma Knife radiosurgery with initial conservative management in 52 patients with newly diagnosed vestibular schwannomas measuring no more than 20 mm. At five years, tumor control was achieved in all 24 patients assigned to upfront radiosurgery, whereas 14 of 28 patients initially managed conservatively ultimately required active treatment.
Serviceable hearing was maintained in 65% of the upfront radiosurgery group compared with 50% of the conservative management group. Despite this numerical difference, the hearing-preservation comparison was not statistically significant (P = .388).
These findings reinforce an important distinction: superior radiographic tumor control does not necessarily establish superior long-term hearing preservation.
However, the absence of a statistically significant difference should not be interpreted as proof that the strategies are equivalent. Limited sample sizes, treatment crossover, and the complex natural history of vestibular schwannoma constrain the conclusions that can be drawn.
For radiation oncologists, treatment timing should therefore remain individualized, incorporating tumor growth, baseline hearing, age, symptoms, and patient preferences rather than relying exclusively on radiographic progression.
Does Fractionation Offer an Auditory Advantage?
The potential role of fractionation in reducing radiation-associated cochlear injury hasbeen debated for years. From a radiobiological perspective, delivering radiation over multiple fractions might improve normal-tissue tolerance while maintaining tumor control.
The ACOUNEU randomized clinical trial directly investigated this hypothesis. The study randomized 108 patients with sporadic vestibular schwannoma and preserved hearing to hypofractionated radiosurgery (18 Gy in three consecutive fractions) or single-session radiosurgery (most commonly 12 Gy in one fraction).
Among 100 evaluable patients, the investigators found no statistically significant difference in hearing preservation between treatment approaches (hazard ratio 1.083; 95% CI, 0.603–1.946; P = .789). Baseline hearing and cochlear dose have been associated with hearing decline after radiosurgery more broadly. At a median follow-up of 62 months, overall local control was approximately 92%.
These findings challenge the assumption that fractionation inherently improves hearing preservation. Nevertheless, the trial does not exclude clinically meaningful differences in
selected patient subgroups, particularly those with unfavorable cochlear anatomy or high anticipated cochlear exposure. Fractionation should therefore be regarded as one component of treatment optimization rather than a universally superior hearing-preservation strategy.
Cochlear Dose: A Modifiable Risk Factor, but Not the Entire Story
Among the factors associated with post-radiosurgery hearing deterioration, cochlear radiation dose remains particularly relevant because it can potentially be modified during treatment planning. The International Stereotactic Radiosurgery Society (ISRS) practice guidelines identify cochlear dose of 4 Gy or less and favorable pretreatment hearing as predictors associated with better hearing preservation following single-fraction SRS, while a marginal dose of 13 Gy or less is associated with better facial nerve preservation.
More recently, a 2026 retrospective analysis of 286 patients undergoing Gamma Knife radiosurgery demonstrated excellent tumor control, with actuarial local control of 98.5% at five years and 96.5% at ten years. However, hearing loss increased progressively across mean cochlear dose categories, from 38.7% among patients receiving 2.9 Gy or less to 73.8% among those receiving more than 4.5 Gy.
Although these findings support the clinical importance of cochlear sparing, they must be interpreted cautiously. Retrospective dose-response relationships may be influenced by baseline
auditory status, tumor location, intracanalicular extension, and anatomical factors that simultaneously affect achievable cochlear dose and hearing prognosis. Furthermore, the cochlea is not a biologically homogeneous structure. Conventional dose-volume metrics may inadequately characterize radiation exposure to specific auditory structures, including the modiolus, cochlear nerve, and individual cochlear turns.
Future studies should investigate whether spatially resolved dosimetry and anatomically informed dose mapping improve hearing-outcome prediction beyond conventional mean cochlear dose. Importantly, cochlear sparing should not compromise adequate tumor coverage or established normal-tissue safety constraints.
Beyond Binary Endpoints: Hearing Loss Is a Longitudinal Process
Most vestibular schwannoma studies define hearing preservation according to Gardner–Robertson class I–II or equivalent American Academy of Otolaryngology–Head and Neck Surgery classifications. Although these classifications are clinically useful, they reduce a continuous and potentially nonlinear process to a categorical outcome. Two patients classified as
having serviceable hearing may have substantially different pure-tone averages, word- recognition scores, and functional auditory experiences. Likewise, patients who eventually lose
serviceable hearing may follow very different trajectories.
The importance of longitudinal analysis was recognized more than a decade ago. In a seminal study by Yomo et al., investigators examined serial hearing outcomes in 154 patients
who underwent observation before Gamma Knife radiosurgery. The investigators reported mean annual hearing deterioration rates of 5.39 dB before treatment and 3.77 dB afterward, without a statistically significant overall difference.
Kondziolka et al. also reported that, among patients with initially excellent hearing, deterioration rates differed significantly between the pre- and post-treatment periods. Other work describes an early phase of rapid hearing decline after radiosurgery, followed by a more gradual decline. These observations illustrate why hearing loss should not be interpreted solely as a binary treatment outcome. They also highlight the difficulty of distinguishing radiation-associated deterioration from tumor-related progression and age- related auditory decline.
A more informative framework would incorporate repeated pure-tone audiometry, word- recognition scores, time to clinically meaningful deterioration, and the rate of auditory change
over time. Rather than simply reporting the proportion of patients retaining serviceable hearing at five years, future studies should characterize whether patients experience early rapid
deterioration, gradual progressive decline, or prolonged auditory stability.
Toward Personalized Prediction of Hearing Outcomes
Recent evidence suggests that hearing preservation after SRS is influenced by multiple interacting factors, including baseline auditory function, age, tumor anatomy, cochlear dose, and
potentially electrophysiological and imaging characteristics. However, these variables are frequently evaluated independently, and existing predictive models have not been sufficiently
validated for routine clinical decision-making.
The next generation of research should integrate clinical, anatomical, and dosimetric variables into longitudinal prediction models capable of estimating individualized auditory outcomes. Potential approaches include mixed-effects models, time-to-event analyses, and carefully validated trajectory-based methods. Where adequate data are available, bilateral
audiometry may also help contextualize age-related changes, although the contralateral ear cannot independently establish the causal effects of treatment.
Advanced MRI analysis and radiomics may provide additional prognostic information, but their contribution should be demonstrated through incremental predictive performance,
external validation, and clinically meaningful improvements in decision-making. Ultimately, the objective should be to move beyond population-level hearing-preservation percentages toward
individualized estimates of long-term auditory function.
Redefining Success in Vestibular Schwannoma Radiosurgery
Recent randomized evidence has clarified important aspects of treatment timing, while contemporary dosimetric studies continue to emphasize the importance of cochlear radiation
exposure. Nevertheless, no single treatment strategy or dosimetric threshold has definitively solved the problem of durable hearing preservation. For radiation oncologists, the next major
advance may lie not only in refining treatment delivery but also in redefining how treatment success is measured.
The question should no longer be simply whether hearing was preserved after radiosurgery, but how much hearing was preserved, for how long, and whether our treatment decisions meaningfully altered its natural trajectory.
Written By Ahmed Galal, MD