MR IMAGING FINDINGS AFTER GAMMA KNIFE RADIOSURGERY FOR VESTIBULAR SCHWANNOMAS: A SINGLE-CENTER STUDY

Introduction: Vestibular schwannomas (VS) are benign intracranial tumors arising from the vestibular division of the VIII cranial nerve. Several treatment options are currently used, such as, observation, stereotactic radiosurgery (SRS), conventional fractionated radiotherapy (CFRT), and microsurgery (MS). Due the complex anatomy of the cerebellopontine angle, surgical treatment of the VSs may cause significant morbidity (1). Large series reported excellent control rates and significantly lower complication rate with SRS than with MS (2). Methods: In this study, we present a retrospective analysis of our institution patients diagnosed with VS and treated with SRS, between 2014 and 2018. All patients were submitted to magnetic resonance imaging (MRI) pre and post surgical treatment and tumors were evaluated by their dimensions, volume and presence or absence of necrosis. Results: A total of 77 patients, 26 (33%) have done MRI follow-up in our institution. Thus, 26 patients were included in our retrospective analysis. 19 (73%) patients were treated exclusively with SRS, other 7 (27%) were treated with combined modality, microsurgery followed by SRS. 04 (15%) were diagnosed with neurofibromatosis type 2 (NF2). The target and the ipsilateral cochlea were defined on T1, T2 and fiesta series and adapted to bone window CT scans. The mean prescribed dose with GK was 12Gy and mainly, we targeted the isodose line of 50%. The mean maximum cochlear dose was 7,03Gy and the used constraints were, 08 Gy as maximum dose and 04Gy as median dose. The mean interval follow up was 448 days with 04 (15%) patients demonstrating stable imaging findings in the control exam. 15 (57%) patients demonstrated tumor growth and 06 (23%) exhibited lesion reduction volume. All lesions that increased demonstrated necrotic areas in the follow up exam. Tumors that were stable or had volume reduction demonstrated variable imaging pattern in the follow up exam. Discussion: The purpose of this study is to describe the main MRI findings related to SRS of VS. Radiologists should be aware that tumor growth in control exam is not always related to tumor progression, specially if necrotic areas appear in the follow up exam.