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BACKGROUND The CeTeG/NOA-09 trial demonstrated a significant survival benefit of radiotherapy plus lomustine-temozolomide chemotherapy in glioblastoma patients with methylated O6-methylguanine-DNA-methyltransferase (MGMT) promoter compared to standard temozolomide chemoradiation (1). However, data on pseudoprogression following treatment according to the CeTeG/NOA-09 trial is lacking. Due to the limited specificity of conventional MRI we evaluated the value of O-(2-[18F]fluoroethyl)-L-tyrosine (FET) PET for the differentiation of pseudoprogression from actual tumor progression. METHODS Twenty-one patients with newly diagnosed IDH-wildtype glioblastoma and methylated MGMT promoter with conventional MRI findings suspicious for tumor progression according to RANO criteria (2) after radiotherapy completion were investigated using FET PET. The maximum and mean tumor-to-brain ratios (TBRmax, TBRmean) were calculated. Pseudoprogression was diagnosed neuropathologically or clinicoradiologically (i.e, if (i) a stable clinical course, (ii) stable or improved MRI findings, and (iii) no treatment change occurred within the next six months). Diagnostic performance of FET PET parameters was evaluated using receiver-operating-characteristic (ROC) analyses and Fisher’s exact test. Imaging results were also related to progression-free survival (PFS). RESULTS Pseudoprogression was identified in 9 of 21 patients (43%). In the majority of patients (n=8; 89%), pseudoprogression was diagnosed clinicoradiologically. The median time between radiotherapy completion and pseudoprogression was nine weeks (range, 5-25 weeks). ROC analysis yielded an optimal cutoff value of 1.95 for TBRmean to differentiate between pseudoprogression and tumor progression (sensitivity, 89%; specificity, 75%; accuracy, 81%; area under the curve, 0.72±0.13; P = 0.008). CONCLUSIONS In a considerable number of patients, pseudoprogression may occur following radiotherapy plus off-study lomustine-temozolomide chemotherapy according to the CeTeG/NOA-09 trial. FET PET appears of value for the differentiation of pseudoprogression from tumor progression in this group of patients. Literature: 1. Herrlinger et al., 2019 Lancet; 2. Wen et al., 2010 J Clin Oncol

neuro-oncology

NIMG-26. DIAGNOSIS OF PSEUDOPROGRESSION FOLLOWING RADIOTHERAPY PLUS LOMUSTINE-TEMOZOLOMIDE CHEMOTHERAPY IN NEWLY DIAGNOSED GLIOBLASTOMA PATIENTS USING FET PET