Introduction. Surgery is first-line therapy for glioblastoma, and there is evidence that gross total resection is associated with improved survival. Gross total resection, however, is not always possible, and relationships among extent (percent) of resection (EOR), residual volume (RV), and survival are unknown. The goals were to evaluate whether there is an association between EOR and RV with survival and recurrence and to establish minimum EOR and maximum RV thresholds. Methods. Adult patients who underwent primary glioblastoma surgery from 2007 to 2011 were retrospectively reviewed. Three-dimensional volumetric tumor measurements were made. Multivariate proportional hazards regression analysis was used to evaluate the relationship between EOR and RV with survival and recurrence. Results. Of 259 patients, 203 (78%) died and 156 (60%) had tumor recurrence. The median survival and progression-free survival were 13.4 and 8.9 months, respectively. The median (interquartile range) pre-and postoperative tumor volumes were 32.2 (14.0-56.3) and 2.1 (0.0-7.9) cm 3 , respectively. EOR was independently associated with survival (hazard ratio [HR], 0.995; 95% confidence interval [CI]: 0.990-0.998; P ¼ .008) and recurrence (HR [95% CI], 0.992 [0.983-0.998], P ¼ .005). The minimum EOR threshold for survival (P ¼ .0006) and recurrence (P ¼ .005) was 70%. RV was also associated with survival (HR [95% CI], 1.019 [1.006-1.030], P ¼ .004) and recurrence (HR [95% CI], 1.024 [1.001-1.044], P ¼ .03). The maximum RV threshold for survival (P ¼ .01) and recurrence (P ¼ .01) was 5 cm 3. Conclusion. This study shows for the first time that both EOR and RV are significantly associated with survival and recurrence, where the thresholds are 70% and 5 cm 3 , respectively. These findings may help guide surgical and adjuvant therapies aimed at optimizing outcomes for glioblastoma patients.

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