Gamma Knife radiosurgery following partial resection of large vestibular schwannomas: evaluation of long-term tumor control

IntroductionRecently, we have reported the influence of pretreatment growth rates on Gamma Knife radiosurgery (GKRS) of vestibular schwannomas (VS): fast growing tumors are less likely to obtain tumor control.1 This indicates that biological aspects intrinsic to VS are influencing the GKRS treatment outcome. Also, various papers have reported that GKRS treatment for large VS is significantly less effective compared to small-to-medium-sized VS. These findings suggest that large VS may have biological properties similar to those of fast-growing VS. To evaluate this, we assessed the long-term GKRS tumor control of remnants of large VS after partial resection. Furthermore, we compared this to the outcome of large tumors (>10cc) that were not resected, but received primary GKRS. MethodsPatients with a minimum follow-up of 4 years were included. Loss of tumor control was defined as radiological progression. All patients were uniformly treated according to protocol. We employed Kaplan-Meier survival analyses to determine differences in tumor control probability rates following GKRS between small-to-medium-sized VS and large VS, either after partial resection or after primary GKRS treatment.ResultsIn our center, 736 patients received primary GKRS treatment, while 77 patients were first subjected to partial resection. Kaplan-Meier analyses showed significantly lower tumor control rates for large tumors, both after partial resection and after primary treatment, compared to the small-to-medium-sized VS (log-rank, p<0.001). The obtained 10-year tumor control rates were 78.5%, 73.6%, and 89.7%, respectively. These control rates observed in large VS are even worse than the rates observed in fast growing small-to-medium-sized tumors (85.1%).1ConclusionsResults from this research clearly show that tumor control rates of large VS, either after partial resection or primary treatment, show lower tumor control rates following GKRS. The survival curves of these tumors resemble the curve of extremely fast growing VS, suggesting that the intrinsic tumor biology is indeed an important factor influencing tumor control rates of VS after GKRS.References1. Langenhuizen PPJH, Zinger S, Hanssens PEJ, Kunst HPM, Mulder JJS, Leenstra S, et al. (2018). Influence of pretreatment growth rate on Gamma Knife treatment response for vestibular schwannoma: a volumetric analysis. J Neurosurg, ,1-8.