Gamma knife radiosurgery for cystic vestibular schwannomas: Morphological and dosimetric correlation in a...

Cystic vestibular schwannomas (CyVS) have unique biological and morphological characteristics which presents the development of specialized Stereotactic Radiosurgery (SRS) treatment protocols. This retrospective study addresses the relationship between linear and volumetric parameters of CyVS treated with Gamma Knife (GKRS) radiosurgery and evaluates the long-term tumor control predictive ability of the cyst-to-tumor volume ratio (CVR).

Thirty-eight patients treated from 2012 to 2019 were included in the study. A Pearson analysis was performed to correlate the tumor diameter/volume with cyst diameter/volume. The CVR was evaluated as the ratio of the cyst volume to total tumor volume and classified as solid-dominant (<10 % cyst volume), mixed (10-50 % cyst volume), or cyst-dominant (>50 % cyst volume). Dosimetric analysis included conformity index (CI), gradient index (GI), target coverage and selectivity, as well as hearing outcomes assessed using the AAO-HNS and Gardner-Robertson scales. Local tumor control was evaluated using Kaplan-Meier survival analysis.

The mean tumor diameter and volume was 20.0 +/- 6.2 mm and 5.25 +/- 3.51 cc, respectively. The mean cyst diameter and volume were 10.9 +/- 4.7 mm and 0.97 +/- 1.21 cc, respectively. There was strong correlation between tumor diameter and volume (r = 0.86, p < 0.0001) as well as between cyst diameter and volume (r = 0.85, p < 0.0001), indicating linear measures are appropriate substitutes for volumetric measurements in evaluating treatment response. Actuarial local control following GKRS was 84.8 %, with over 20 % of patients achieving a ≥ 20 % volume reduction at last follow-up.

These findings demonstrate that CVR serves as a useful imaging biomarker, supporting individualized radiosurgical planning and enabling reliable long-term management of patients with CyVS.