Incidence of rare malignant transformation in vestibular schwannomas treated with stereotactic radiosurgery: a single-institution analysis of 1,061 cases.
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PURPOSE: This study assesses the incidence of malignant transformation (MT) in vestibular schwannomas (VSs) treated with stereotactic radiosurgery (SRS) over a 30-year period, with a median follow-up of 12 years. METHODS: A retrospective analysis was performed on 1,051 individuals who underwent gamma knife surgery (GKS) for VS between May 1991 and March 2023. The cohort comprised 1,013 individuals with sporadic VSs and 38 with neurofibromatosis type 2-related schwannomatosis (NF2-SWN), totaling 1,061 treated VSs. The annual MT risk was calculated based on lesion-years at risk. RESULTS: Among the 1,061 VSs treated with GKS, MT occurred in 2 cases (0.19%), both in individuals with sporadic VSs. Over 12,940 lesion-years from GKS to the latest follow-up, the annual MT risk was 0.016% for all VSs, with no cases observed within the first 5 years post-GKS. However, the risk increased to 0.025% beyond 5 years. No instances of MT were identified in individuals with NF2-SWN. CONCLUSIONS: This study represents the most extensive investigation specifically examining the incidence of MT in VSs treated with SRS, leveraging the largest case series and longest observation period to date. The findings indicate that MT following SRS for VSs is exceedingly rare. Although the overall risk remains minimal, it may increase slightly with extended follow-up. These results support the continued use of SRS as a safe and effective treatment for VS while emphasizing the need for long-term monitoring.