P15.06.a hybrid [18f]fet pet-mri; a valuable advanced imaging tool to discriminate between tumor recurrence and pseudoprogression in glioma patients

BACKGROUND One of the well-known challenges in the response assessment of glioma patients is differentiating tumor recurrence from treatment-related effects, i.e. pseudoprogression. The PET tracer O-(2-18F-fluoroethyl)-L-tyrosine ([18F]FET) visualizes increased amino acid metabolism and can potentially aid differentiation between vital tumor tissue and reactive changes, and can herewith guide clinical decision-making and surgical planning. This study aimed to assess the diagnostic yield of hybrid [18F]FET PET-MRI in a series of pre-treated patients suspected of glioma recurrence. MATERIAL AND METHODS This retrospective study included histologically-confirmed glioma patients who had undergone standard-of-care therapy consisting of gross-total resection surgery and/or radiotherapy. [18F]FET PET-MRI scans were performed between 04/2022 and 03/2023. Patient received 204 MBq (median, range: 180-242) [18F]FET intravenously, followed directly by a 45 minute dynamic PET and simultaneous MRI including DWI and gadolinium-based sequences (PWI). Time-activity-curves (TACs), showing the mean tissue radioactivity in tumor regions of interest as a function of time, were compared to the TAC in healthy tissue. Static PET images for qualitative visual analysis were reconstructed by accumulating the 20-40 minute timeframes. Tumor recurrence (i.e., a positive scan) was defined as focal uptake exceeding uptake in healthy brain parenchyma according to the joint EANM/EANO/RANO practice guideline/SNMMI procedure standard. Outcomes were compared to previously-obtained MRIs, post-operative pathology, and clinical/radiological follow-up. RESULTS Thirteen patients, 31% female, median age 39 (range: 28-50), with astrocytoma (grade 2_n=2; grade 3_n=1; grade 4_n=2), oligodendroglioma (grade 2_n=6; grade 3_n=1), and glioblastoma (n=1) were included. [18F]FET PET-MRI was positive in 9/13 patients (70%), which in 5/9 (55%) was confirmed by pathology (n=3) or clinical follow-up (n=2). In 6/9 patients with a positive scan (66%), the previously-obtained MRI (time interval range: 0.5-4.1 months) showed negative (n=4) or unclear (n=2) DWI and PWI findings. Four/13 (30%) patients had a negative [18F]FET PET-MRI and did not show signs of tumor progression after an average follow-up of 3, 5, 11 and 12 months, respectively. CONCLUSION Simultaneous [18F]FET PET-MRI shows high potential for accurate differentiation between true tumor progression and treatment-related pseudoprogression in glioma patients with equivocal findings on MRI including DWI and PWI. The diagnostic yield of this hybrid imaging technique exceeded that of MRI-only in our case series, and thus provides a promising advanced imaging tool in the follow-up of glioma patients. We aim at prospective studies to validate the clinical value of this multimodal approach.