Abstract INTRODUCTION Glioblastoma is highly resistant to radio- and chemotherapy and its heterogeneity is a therapeutic challenge. O6-methylguanine–DNA methyltransferase (MGMT) promotor methylation is an established biomarker for favorable effect of chemotherapy. Supramaximal resection has proven predict the best overall survival (OS), but whether this benefit applies equally to patients with different MGMT-promotor methylation status is still to be debated. MATERIAL AND METHODS Population based retrospective study that examined all adult patients in South-Eastern Norway that underwent resection for IDH wildtype glioblastoma between January 2019 and December 2021. Extent of resection (EOR) was classified according to RANO resect classification and MGMT-promotor methylation status by pyrosequencing-qPCR. RESULTS The study included 281 patients. MGMT-promotor methylated patients (MGMT+) had a median OS of 18.4 months compared to 13.1 months in MGMT-unmethylated (MGMT-) patients (p&amp;lt;0.0001). Patients with supramaximal resection had a median OS of 20.4 months compared to 14.9 months (p&amp;lt;0.01) with maximal resection. For patients with supramaximal resection we found no significant difference in median OS (25.5 vs. 18.3 months (p=0.24)) dependent on MGMT-status. Separated analysis between MGMT-status and EOR showed a median survival of 18.3, 13.4 (p&amp;lt;0.001), and 8.5 (p&amp;lt;0.001) months for MGMT-, and 25.5, 18.4 (p=.99), and 9.0 (p&amp;lt;0.001) months for MGMT+ in patients where supramaximal-, maximal-, and submaximal resection was achieved, respectively. CONCLUSIONS Patients with supramaximal resection and MGMT+ have a 50% 2-years survival probability. In patients where supramaximal resection was achieved, survival benefit was not dependent on MGMT-status. There was a significant improved survival in MGMT- patients between supramaximal and maximal resection, that was not present for MGMT+ patients. Intraoperative molecular diagnosis has the potential to guide the surgical strategy dependent on MGMT-promotor methylation status, especially in tumors where supramaximal resection has a higher risk of new neurological deficits.

SURG-47. SUPRAMAXIMAL RESECTION IMPROVES OVERALL SURVIVAL IN PATIENTS WITH MGMT-UNMETHYLATED GLIOBLASTOMA