# Vestibular dose predicts toxicity in stereotactic radiosurgery for vestibular schwannomas

## TL;DR

Higher radiation doses to the vestibular system during radiosurgery for vestibular schwannomas are linked to worsened symptoms and function, but dose reduction is feasible without affecting tumor treatment.

## Contribution

This study identifies specific radiation dose thresholds to the vestibular system that predict symptom worsening and demonstrates feasible dose reduction strategies.

## Key findings

- A Dosemean >4 Gy to vestibular sensory organs strongly predicts worsened vestibular symptoms (OR = 27.3).
- Dosemax >8 Gy to vestibular sensory organs is also associated with increased symptom risk.
- Optimizing vestibular dose in treatment plans can reduce radiation exposure without compromising tumor or cochlear outcomes.

## Abstract

•Vestibular sensory organs can be delineated on high-resolution planning CT.•Vestibular sensory organs include the vestibule (saccule and utricle) and ampullae.•Higher vestibular dose predicts symptom worsening and vestibular function loss.•Dosemean >4 Gy and Dosemax >8 Gy to vestibular sensory organs increase symptom risk.

Vestibular sensory organs can be delineated on high-resolution planning CT.

Vestibular sensory organs include the vestibule (saccule and utricle) and ampullae.

Higher vestibular dose predicts symptom worsening and vestibular function loss.

Dosemean >4 Gy and Dosemax >8 Gy to vestibular sensory organs increase symptom risk.

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 Dosemean to VSO (6.45 Gy vs 2.92 Gy, p < 0.0001). A Dosemean > 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 Dosemax, with an 8 Gy threshold. Higher Dosemean 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.