Vestibular dose predicts toxicity in stereotactic radiosurgery for vestibular schwannomas

Stereotactic radiosurgery (SRS) provides excellent tumor control in small and medium vestibular schwannomas (VS), but its impact on the vestibular system remains uncertain. This study examines the effects of SRS on subjective vestibular symptoms, vestibular function, and potential predictors of symptom worsening.

A retrospective analysis was conducted on adult VS patients treated with SRS over eight years. Vestibular symptoms were graded at baseline and six months post-SRS. The vestibular sensory organs (VSO) were defined as the combined volume of the saccule, utricle, and ampullae. Vestibular function was assessed with bithermal, bilateral caloric testing, video head impulse testing, and vestibular evoked myogenic potentials. Two illustrative treatment plans were generated to assess feasibility of vestibular dose reduction without compromising planning target volume (PTV) or cochlear constraints.

Among 45 VS patients (median age 61.4 years), 14 (31 %) reported worsened vestibular symptoms at six months. These patients had higher Dose_mean to VSO (6.45 Gy vs 2.92 Gy, p < 0.0001). A Dose_mean > 4 Gy was strongly associated with symptom worsening (OR = 27.3, 95 % CI [3.4-301.8], p = 0.0002). Similar associations were observed for Dose_max, with an 8 Gy threshold. Higher Dose_mean to the lateral ampulla was correlated with a greater percentage change in caloric weakness (slope = 7.77, R² = 0.38, p = 0.04). In two illustrative plans, VSO optimisation lowered vestibular dose without compromising PTV coverage or cochlear dose.

Higher vestibular radiation dose is strongly associated with worsened vestibular symptoms and possibly with functional decline. Dose reduction to vestibular subunits is feasible without compromising the tumor dose, helping mitigate these effects.