Malignant Transformation of a Vestibular Schwannoma Following Gamma Knife Radiosurgery: Case Report and Review of the Literature

Introduction: Whether used in a primary or adjuvant fashion for treatment of brain neoplasms, external beam radiation therapy is associated with the development of secondary malignancy and malignant transformation of benign tumors. Radiosurgery is designed to minimize these risks by delivering a highly targeted, cytotoxic dose of radiation to the tumor. Nonetheless, there is a growing body of literature describing patients with malignant transformation of primary benign tumors after radiosurgery, including vestibular schwannoma (VS).

Case Report: We report the case of a 46-year-old woman who presented with right facial paresthesias and imaging consistent with a right-sided vestibular schwannoma (volume ∼18.5 cm³). She underwent subtotal resection followed by gamma knife radiosurgery (GKRS) at 6 months postoperatively. Initial histology demonstrated benign vestibular schwannoma with a MIB-1 labeling index of 5.7%. At 59 months following GKRS, repeat resection demonstrated frank malignant transformation on histology (MIB-1 index, 33.8%).

Discussion: Malignant vestibular nerve tumors are an extremely rare entity, with only 18 reports in the literature. Our report describes the sixth pathologically confirmed case of malignant transformation following radiosurgery, supporting the contention that radiosurgery itself plays a causative role in transformation. The presence of an elevated MIB-1 labeling index in a histologically benign lesion may predispose toward malignant transformation in the setting of adjuvant radiosurgery.