Correlation between pre-treatment growth rate and tumor control of vestibular schwannomas after gamma knife radiosurgery in the dutch database

Introduction: Prognostic factors of tumor control after Gamma Knife Radiosurgery (GKRS) for vestibular schwannoma (VS) are largely unknown. Recently, it has been reported that the growth rate of VS before treatment is indicative of the chance that radiosurgery achieves tumor control. Such findings may have important implications for treatment strategies and may lead to advise for either microsurgery or higher marginal doses for fast growing tumors. However, studies on this important aspect are limited and show conflicting results. Moreover, the available studies are hampered by methodological limitations such as limited patient numbers and followup and two-dimensional assessment of tumor size. The objective of this study is to identify a possible correlation between pre-treatment growth rate and tumor control after GKRS in a large database with sufficient follow-up and volumetric tumor assessments.
Methods: In the prospectively collected database of the Gamma Knife Center Tilburg, 445 patients with VS, treated between 2002 and 2014, that showed documented growth before treatment and who have had a minimum follow-up of 2 years after treatment, were identified. Tumor volumes before, at and after treatment were assessed. GKRS was performed in a uniform way, with a dose of 12-13Gy prescribed to the isodose line covering 90-99% of the target. Failures were defined as tumor progression on 2 consecutive MRI’s beyond 2 years after GKRS, or as judged by the radiosurgical team. Volume doubling times (VDT) before treatment were correlated with the observed tumor control rates and volumetric responses after treatment.
Results: Until now 266 of the 445 patients with documented pre-treatment tumor growth have been analyzed. 25 Patients were lost to follow-up. The median follow-up was 4 years. 25 Patients showed a radiological failure. The 5- and 10-year actuarial control rates were 91% and 78% respectively. VDT varied from 3 to 344 months, with a median of 16 months. Using the Mann-Whitney-U test, the VDT of tumors that showed tumor control is significantly higher than those that failed (p=0.01). After stratifying for VDT at the median, slow growing tumors showed a 5- and 10-year actuarial control rate of 97% and 89%, where the fast growing tumors had a 5- and 10-year control rate of 85% and 68% (p=0.009).
Conclusion: This study clearly shows that the pretreatment growth rate correlates with the observed tumor control after GKRS. Fast growing tumors are less likely to show tumor control. This finding might justify alterations in the management of VS.