Abstracts vi236 NEURO-ONCOLOGY • NOVEMBER 2017 or worsened motor deficit was seen postoperatively in 210 cases (30%). Of these 210 cases, there was improvement in the motor function to baseline by 3 months postoperatively in 160 cases (76%), while the deficit remained in 50 cases (24%). Majority (56%) of long-term deficits were mild or moder- ate (antigravity or better). Patients in whom the subcortical motor path- ways were identified during surgery with stimulation mapping were more likely to develop an additional motor deficit postoperatively compared to those in whom the subcortical pathway could not be found (45% vs. 19% respectively, p<0.001). This difference also remained when considering the likelihood of a long-term deficit (i.e. persisting >3 months; 12% vs. 3.2%, p<0.001). A significant region of diffusion restriction around the resection cavity was seen in 20 patients with long-term deficits and was more com- mon in cases when the motor pathways were not identified. Thus, long term deficits that occur in settings where the subcortical motor pathways are not identified seem in large part due to local ischemic injury to descending tracts. CONCLUSION: Stimulation mapping allows surgeons to identify the descending motor pathways during resection of tumors in perirolandic regions and to achieve an acceptable rate of morbidity in these high risk cases. SURG-04. CRANIAL BONE DEFECTS IN PATIENTS TREATED WITH TTFIELDS: A CASE SERIES Niklas Thon 1 , Martin Misch 2 , Marscha Schlenter 3 and Oliver Heese 4 ; 1 Department of Neurosurgery, Ludwig-Maximilians University, Munich, Germany, Munich, Germany, 2 Department of Neurosurgery, Charité- Universitaetsmedizin, Berlin, Germany, 3 Clinic for Radiooncology and Radiotherapy, Uniklinik RWTH Aachen University, Aachen, Germany, 4 Department of Neurosurgery, Helios Hospital Schwerin, Schwerin, Germany BACKGROUND: Glioblastoma multiforme (GBM) is the most abundant primary brain tumor with a devastating prognosis. Treatment of newly diag- nosed GBM is generally based on maximal safe resection and subsequent concomitant chemoradiation followed by adjuvant temozolomide (TMZ). Tumor-treating-fields (TTFields) are a new treatment modality added to the temozolomide maintenance with proven efficacy in a large clinical trial. The therapy is based on anti-mitotic effects of alternating electric fields applied locally through so called transducer arrays being directly attached to the scalp. Patients with cranial bone defects, e.g. missing bone, were excluded from the clinical trial with TTFields and clinical experience regarding safety and feasibility is missing. We here report on cases with cranial defect on TTFields therapy. CASES: We observed four patients with recurrent GBM and missing cranial bone (female=1, male=3) at four different centers in Germany. The median age when starting the therapy was 55 years (range 51 - 69 years). All patients received first-line treatment at best clinical prac- tice and experienced progressive disease. Craniectomy was indicated due to different reasons like inflammation or wound healing deficit. Thereafter, treatment with TTFields started in these patients at first and later recur- rence. There were no severe adverse events observed in this population. The median monthly compliance was 83 % and with a range of 78 – 84 % compliance, all patients exceeded 75 % as suggested by study data to enable most efficient therapy. CONCLUSIONS: This multicentric and retrospec- tive case series describes patients who were treated with TTFields despite a cranial bone defect. All patients were compliant to the therapy and there were no severe or additional side effects observed. Our observation provides first evidence that this setting is feasible and safe. Additional studies are war- ranted to increase clinical evidence for the safe use of TTFields in patients with cranial bone defects. SURG-05. THE IMPACT OF SURGERY IN MOLECULARLY DEFINED LOW-GRADE GLIOMA: AN INTEGRATED CLINICAL, RADIOLOGICAL AND MOLECULAR ANALYSIS Maarten Wijnenga 1 , Pim French 1 , Hendrikus Dubbink 2 , Winand Dinjens 2 , Peggy Atmodimedjo 2 , Johan Kros 2 , Marion Smits 3 , Renske Gahrmann 3 , Geert-Jan Rutten 4 , Jeroen Verheul 4 , Ruth Fleischeuer 5 , Clemens Dirven 6 , Arnaud Vincent 6 and Martin van den Bent 1 ; 1 Department of Neurology, Erasmus MC Cancer Institute, Rotterdam, Netherlands, 2 Department of Pathology, Erasmus MC Cancer Institute, Rotterdam, Netherlands, 3 Department of Radiology and Nuclear Medicine, Erasmus Medical Center, Rotterdam, Netherlands, 4 Department of Neurosurgery, St. Elisabeth Hospital, Tilburg, Netherlands, 5 Department of Pathology, St. Elisabeth Hospital, Tilburg, Netherlands, 6 Department of Neurosurgery, Erasmus MC Cancer Institute, Rotterdam, Netherlands INTRODUCTION: The WHO classification of gliomas has been revised completely and is now predominantly based on molecular criteria. This requires re-evaluation of the impact of surgery in molecularly defined low- grade glioma subtypes. We performed a retrospective study to assess the interaction between molecular markers and postoperative tumor volume on overall survival in patients with low-grade glioma. METHODS: We included 228 adult patients who underwent surgery for a supratentorial low-grade glioma since 2003. Pre-and postoperative tumor volume were assessed with semi-automatic software on T2-weighted images. Targeted next-generation- sequencing was used to classify the samples according to current WHO 2016 classification. The impact of postoperative tumor volume on overall survival, corrected for molecular profile, was assessed using a Cox proportional-haz- ards model. RESULTS: Median follow-up was 5.8 years. In 39 (17.1%) of cases, glioma subtype was revised after molecular analysis. Complete resec- tion was achieved in 35 patients (15.4%). In 54 patients (23.7%) a small tumor residue (0.1-5.0 cm3) remained. In multivariate analyses, postopera- tive tumor volume was associated with overall survival with a HR of 1.01 (95% CI 1.002-1.02; p=0.016) per 1 cm3 increase in volume. The impact of postoperative volume was particularly strong in IDH mutated astrocytoma patients, where even very small postoperative tumor volumes (0.1-5.0 cm3) already negatively affected overall survival (Log-rank test, p = 0.027). CON- CLUSIONS: Our data provides the necessary re-evaluation of the impact of surgery for all molecular low-grade glioma subtypes. Importantly, even a small postoperative volume has a negative impact on overall survival in IDH mutated astrocytoma, which argues for a second-look operation in this glioma subtype even when minor residual tumor remains. SURG-06. LASER ABLATION IN STEREOTACTIC NEUROSURGERY (LAISE): A MULTI-INSTITUTIONAL RETROSPECTIVE ANALYSIS OF LITT FOR BRAIN METASTASIS Andrew Sloan 1 , Stephen B. Tatter 2 , Alireza Mohammadi 3 , Kevin Judy 4 , Sujit S. Prabhu 5 , Darren Lovick 6 , Roukoz Chamoun 7 , Veronica Chiang 8 and Eric Leuthardt 9 ; 1 University Hospitals of Cleveland, Cleveland, OH, USA, 2 Wake Forest Baptist Medical Center, Winston-Salem, NC, USA, 3 Department of Neurosurgery, Cleveland, OH, USA, 4 Sidney Kimmel Medical College at Thomas Jefferson University, Philadelphia, PA, USA, 5 University of Texas MD Anderson Cancer Center, Department of Neurosurgery, Houston, TX, USA, 6 Saint Luke’s Hospital-University of Missouri at Kansas City, Kansas City, MO, USA, 7 University of Kansas Hospital, Houston, TX, USA, 8 Yale University, New Haven, CT, USA, 9 Washington University, St. Louis, MO, USA INTRODUCTION: Laser Interstitial Thermotherapy (LITT; sometimes called Stereotactic Laser Ablation or SLA,) is a minimally invasive proce- dure increasingly used to treat brain tumors. Previous reports have described small numbers of patients from a few centers. Here we describe the treat- ment and results of 40 patients with 83 brain metastasis treated at 9 cent- ers 2015-2016 and compare this to outcomes in 68 patients with gliomas. METHODS: De-identified data on patients undergoing LITT in a retrospec- tive database were analyzed using standard statistical methods. RESULTS: The median age of patients treated for metastatic brain tumor was older than in patients with gliomas (59.1 yrs. vs. 53.8 yrs.; p =0.0422). Patients were predominately female (55%) and white (75%). Lesions were mostly recurrent (70%), or residual (5%), though 22.5% were newly diagnosed. The most frequent primaries were lung (52.6%), breast (13.2%), colon (5.3%) and melanoma (5.3%). Tumor locations include supratentorial (90.4%), thalamic 2.4%) and brainstem (7.2%). 32.5% were considered to be inoperable, 10% were unable to tolerate radiotherapy, and 2.5 % were unable to tolerate chemotherapy. Median pre-op KPS was 80 (+/-11.2) and median ECOG Performance status was 1.3(+/- 1.5). Previous treatments included steroids (87.5%) RT (71.1%), radiosurgery (54.2%), craniotomy (26.5%) and WBRT (14.5%). Metastasis treated with LITT were more often recurrent compared to gliomas (70% vs. 51.5%; p = 0.047) but were also smaller (7.9 cc vs 12.9cc; p =0.0083). Median lasing time was 38.6 min. Dis- charge status was home (79.5%), rehabilitation (10.3%), and SNF (5.1%). Average follow-up was 300.9 days and median survival was 421 days which was shorter than patients with glioma (568 days), but only 4.8% suffered a neurological death. CONCLUSION: LITT is safe and effective for the treatment of challenging brain metastasis including recurrent and otherwise inoperable tumors with survival and CNS survival equal or better than alter- native treatments. SURG-07. CIRCUMFERENTIAL RESECTION OF GLIOBLASTOMA: A NOVEL SURGICAL TECHNIQUE FOR MAXIMISING EXTENT OF RESECTION AND PROLONGING SURVIVAL Michael Opoku-Darko 1 , Magalie Cadieux 1 , Wajid Sayeed 2 , Jacob Easaw 3 and John Kelly 1 ; 1 Department of Clinical Neurosciences, University of Calgary, Calgary, AB, Canada, 2 Arnie Charbonneau Cancer Institute, University of Calgary, Calgary, AB, Canada, 3 Lahey Hospital and Medical Center, Burlington, MA, USA INTRODUCTION: Evidence suggests that cytoreduction is associated with improved survival in glioblastoma (GBM) and that maximal, or gross total resection of the tumor offers the best chance for prolonged survival. We developed a novel surgical technique for removal of GBM called Circumfer- ential resection during which we identify the interface between the enhanc- ing rim of GBM and surrounding brain structures enabling more complete resection. We evaluate the impact of this method on extent of resection and