Evaluation of Reported Malignant Transformation of Vestibular Schwannoma: De Novo and After Stereotactic Radiosurgery or Surgery

Objective To critically analyze each reported case of malignant transformation of vestibular schwannoma (VS) after either stereotactic radiosurgery (SRS) or microsurgery (MS). Data Sources We searched the Pubmed/Medline database using the relevant key words vestibular schwannoma, acoustic neuroma, malignant, transformation, radiation, induced, stereotactic, radiosurgery, malignancy, GammaKnife, and CyberKnife and combinations thereof. Study Selection Inclusion criteria for malignant transformation of VS after SRS included histopathology of initially benign VS, subsequent histopathology confirming malignant VS, reasonable latency period between malignancy and benign diagnoses. Data Extraction A neurotologist and a skull base neurosurgeon independently assessed each case report for quality, entry, exclusion criteria, and comparability of extracted data. Data Synthesis We calculated median age, latency times, and survival times for each case report. Results Malignant transformation has been documented to occur after either SRS or MS. Eight cases were included that showed histopathologic evidence of malignant transformation after SRS and MS. Four cases of malignant transformation were included that demonstrated malignant transformation after MS only. Malignant transformation of VS can also occur de novo, and de novo malignant VSs are also encountered, which can confound a causal inference from either SRS or MS. Eighteen cases of primary malignant VS were included. Studies that were identified but not included in the review are summarized and tabulated. We found 12 studies of malignant transformation associated with NF2. Conclusion The potential mechanism leading to malignant transformation of VS seems more obvious for SRS and is less understood for MS. Given a low incidence of de novo malignant schwannoma, the possibility that these are spontaneous events in either setting cannot be ruled out. Risk of malignant transformation of VS after either SRS or MS is not zero; however, the magnitude of this risk is probably minimal based on the evidence from eight histopathologically confirmed cases.