Does Radiation Dose to the Labyrinth Predict Change in Balance Function and Patient Perceived Dizziness following Stereotactic Radiotherapy for Vestibular Schwannoma?

Objective: Radiation dose to the cochlea has been identified as an important prognostic factor in hearing outcomes following stereotactic radiosurgery for vestibular schwannoma. To the authors' knowledge, associations between radiation dose parameters, objective vestibular testing and patient perceived dizziness have not yet been explored. The primary objective of the current study is to determine the influence of radiation dose to the vestibule on change in balance function and patient reported dizziness handicap following treatment.

Design: Prospective observational pilot study

Setting: Vanderbilt University, Skull Base Center

Participants: All subjects who underwent linear accelerator-based stereotactic radiotherapy for sporadic vestibular schwannoma that completed pre- and post-treatment vestibular testing.

Main Outcome Measures: The radiation dose to the vestibule including maximum point dose and volume receiving 5, 10, 15, and 20 Gy were reviewed. Pre- and post-treatment balance function test results and Dizziness Handicap Inventory (DHI) scores were obtained for each patient.

Results: Ten adult subjects (5 females; median age 70.5 years) met inclusion criteria. The median time between treatment and most recent vestibular testing was 9.1 months. Two tumors were purely intracanalicular, while the median posterior fossa tumor diameter of the remaining 8 patients was 1.9cm. All subjects underwent linear accelerator based stereotactic radiotherapy; 9 received a fractionated scheme consisting of 5 × 4.5Gy sessions, while one patient received a single unfractionated dose of 12Gy. The median vestibular max and mean doses were 16.0Gy (16.3; range 5.9–29.6) and 8.4Gy (mean 10.5; range 3.3–20.0) respectively. The median pre- and most recent post-treatment DHI scores were 12 (mean 8.4; range 0–16) and 20 (mean 21.8; range 2–58) respectively. The median ipsilateral pre- and most recent post-treatment percent caloric weakness was 65% (mean 65.2; range 0–100) and 73% (mean 64.3; range 0–100) respectively. There were no statistically significant associations, or identifiable trends, between radiation dose and change in vestibular function or DHI scores. Notably, the four ears receiving the highest radiation dose to the vestibule had minimal changes in balance function testing and DHI scores. The single patient that received unfractionated therapy experienced the largest decline in caloric function (65%) and a statistically significant deterioration in DHI score (28 points).

Conclusions: Previous studies have demonstrated that dizziness is a powerful predictor of quality of life in patients with vestibular schwannoma. Based on these preliminary data, radiation dose to the vestibule does not reliably predict change in labyrinthine function or patient perceived dizziness handicap and the delivery of fractionated radiotherapy may potentially pose less risk of vestibular function loss compared with single fraction treatment. Future studies with larger patient numbers will be needed to further clarify these preliminary findings.