Determining a cut-off residual tumor volume threshold for patients with newly diagnosed glioblastoma treated with temozolomide chemoradiotherapy: A multicenter cohort study

•Glioblastoma volumetric analysis of extent of resection can predict overall survival.•Achieving an extent of resection of 84% or more imparts an overall survival benefit.•A residual tumor volume of less than 3.50 cc is independently associated with longer overall survival.•Residual tumor volumes may be a more accurate survival predictor than extent of resection.•Glioblastoma promoter MGMT methylation remains a robust survival prognosticator. Standard-of-care treatment of glioblastomas involves maximal safe resection and adjuvant temozolomide chemo-radiotherapy. Although extent of resection (EOR) is a well-known surgical predictor for overall survival most lesions cannot be completely resected. We hypothesize that in the event of incomplete resection, residual tumor volume (RTV) may be a more significant predictor than EOR. This was a multicenter retrospective review of 147 adult glioblastoma patients (mean age 53 years) that underwent standard treatment. Semiautomatic magnetic resonance imaging segmentation was performed for pre- and postoperative scans for volumetric analysis. Cox proportional hazards regression and Kaplan-Meier survival analyses were performed for prognostic factors including: age, Karnofsky performance score (KPS), O(6)-methylguanine methyltransferase (MGMT) promoter methylation status, EOR and RTV. EOR and RTV cut-off values for improved OS were determined and internally validated by receiver operator characteristic (ROC) analysis for 12-month overall survival. Half of the tumors had MGMT promoter methylation (77, 52%). The median tumor volume, EOR and RTV were 43.20 cc, 93.5%, and 3.80 cc respectively. Gross total resection was achieved in 52 patients (35%). Cox proportional hazards regression, ROC and maximum Youden index analyses for RTV and EOR showed that a cut-off value of