Identification of prognostically relevant residual tumor burden in glioblastoma after surgery: A comparative analysis of MR-based RANO resect classes vs [ ¹⁸ F]FET PET.

2068 Background: Accurate assessment of postoperative residual tumor burden is critical in IDH -wildtype glioblastoma. The MRI-based RANO resect classification stratifies patients according to postoperative contrast-enhancing and non-contrast enhancing tumor volume but may underestimate metabolically active residual disease. The additional prognostic value of postoperative [¹⁸F]FET PET beyond MRI is yet unclear. Methods: This retrospective bicentric study included 140 patients with newly diagnosed IDH -wildtype glioblastoma and evaluable postoperative MRI and [¹⁸F]FET PET. Residual tumor volumes were segmented on MRI and classified according to the RANO resect system. PET-derived residual metabolic tumor volumes were defined using PET RANO 1.0 criteria. Spatial agreement between MRI- and PET-based volumes was quantified using Dice coefficients. Associations with overall survival were analyzed using Cox regression. Incremental prognostic value of PET was assessed using an imaging-only model (contrast-enhanced T1 (T1CE) vs. T1CE + PET) and a fully adjusted model including clinical covariates. Results: PET-derived residual tumor volumes exceeded MRI-defined volumes and showed low spatial concordance with MRI (Dice coefficient: 0.01–0.25) across all RANO resect classes. Postoperative PET-, T1CE-, and T2/FLAIR-derived tumor volumes were each associated with overall survival. Increasing PET-derived residual tumor burden was associated with a continuous increase in mortality risk. In multivariable Cox regression, PET-derived residual tumor volume remained independently associated with overall survival (adjusted hazard ratio 1.021 per cm³, 95% CI 1.008–1.033; ** p = 0.0024). In the imaging-only model, addition of PET improved time-dependent discrimination between 6 and 24 months (ΔAUC(t) 0.03–0.07) and increased Harrell’s C (ΔC ≈ 0.03). In the fully adjusted model, PET provided significant non-redundant prognostic information (likelihood ratio test, * p = 0.0049). Conclusions: Postoperative [¹⁸F]FET PET identifies metabolically active residual tumor not adequately captured by MRI and provides additional prognostic information beyond MRI-based assessment. Integration of PET into postoperative evaluation may refine prognostic stratification in IDH-wildtype glioblastoma.