Short-term volumetric tumor response as predictor for long-term tumor control after Gamma Knife radiosurgery of vestibular schwannoma

Introduction
Gamma Knife radiosurgery (GKRS) is an effective treatment strategy for vestibular schwannomas (VSs), obtaining long-term tumor control rates of 90%. Currently, predicting long-term tumor control is not possible, making lifelong follow-up mandatory and prospective studies on tumor control difficult to conduct. Therefore, predicting tumor control at earlier stages would be beneficial. We explored the predictability of long-term tumor control employing short-term volumetric tumor responses.

Methods
All VS patients with minimum follow-up of 3 years were included. Tumor volumes were obtained from follow-up MRIs. Tumor volume reduction (TVR) was calculated at 6, 12, 24 and 36 months following GKRS. Correlations between TVRs and long-term tumor control were evaluated.

Results
We included 795 patients, of which 96 showed loss of tumor control. The median follow-up time was 61 months. TVRs at 6, 12, 24, and 36 months were based on MRIs from 186, 693, 541 and 399 patients, respectively. Cox regression analyses indicate that TVRs at 6, 24 and 36 months are significantly related to tumor control, indicating a proportional correlation. Kaplan-Meier analyses, when stratifying the cohort in 4 groups based on the TVRs, confirmed this relation: tumors with pronounced TVRs in the first years following GKRS had significantly lower risks of treatment failure, as compared to increasing tumors. For example, if tumor volumes were stable or decreasing between 6 and 24 months after GKRS, loss of long-term tumor control was observed rarely compared to tumors with increasing volumes in this follow-up period: 10-year tumor control probabilities are 92.0% and 67.1%, respectively.

Conclusions
Results from this research clearly indicate that TVRs at 6, 24 and 36 months following treatment are predictive for long-term tumor control in GKRS-treated VS. These findings can help in determining an individual follow-up schedule for VS patients and make prospective studies on long-term tumor control more feasible.