NIMG-16. ASSESSMENT OF RESPONSE TO LOMUSTINE-TEMOZOLOMIDE CHEMOTHERAPY IN ADDITION TO RADIOTHERAPY IN PATIENTS WITH NEWLY DIAGNOSED GLIOBLASTOMA USING FET PET

Abstract BACKGROUND We examined the value of O-(2-[18F]fluoroethyl)-L-tyrosine (FET) PET for early response assessment in patients with newly diagnosed glioblastoma and methylated O6-methylguanine-DNA methyltransferase promoter treated with lomustine-temozolomide chemotherapy in addition to radiotherapy. METHODS Thirty-one patients (29-74 years) were treated according to the CeTeG/NOA-09 phase-3 trial. FET PET and anatomical MRI were performed at baseline and follow-up after the third cycle. We obtained mean and maximum tumor-to-brain ratios (TBR) and metabolic tumor volumes (MTV). Thresholds derived from FET PET were defined using receiver operating characteristic analyses to predict progression-free survival (PFS) of ≥12 months. The predictive value of FET PET parameters was subsequently evaluated using univariate and multivariate survival estimates. MRI response assessment was based on the RANO 1.0 criteria. FET PET data were also used to estimate response according to the PET RANO 1.0 criteria. RESULTS Patients received a median number of 6 cycles of lomustine-temozolomide chemotherapy (3-6 cycles). After treatment initiation, the median follow-up was 17.2 months (7.2-59.4 months). As of April 2024, we confirmed tumor progression according to the RANO 1.0 criteria in 24 patients (77%). After three cycles of lomustine-temozolomide chemotherapy, a reduced mean TBR value by ≥10% predicted a significantly longer PFS (31.0 vs. 10.4 months; P=0.039). In contrast, changes in maximum TBR values, MTV, the RANO 1.0 criteria, and the PET RANO 1.0 criteria were not predictive for response (P>0.05). Multivariate survival analysis revealed that changes in mean TBR values predicted a significantly prolonged PFS independent of age, the extent of resection, and the Karnofsky Performance Status (P=0.036; HR, 3.892; 95% CI, 1.242-17.230). CONCLUSION Our results suggest that FET PET is clinically valuable for identifying responders to lomustine-temozolomide chemotherapy in addition to radiotherapy early after treatment initiation. The evaluation of FET PET’s value in predicting a significantly longer overall survival is ongoing.