Performances of 68Ga-PSMA PET-CT compared to 18F-FDOPA PET-CT to discriminate inflammatory radiation related changes from early recurrences in Glioblastoma's patients: Preliminary analysis

Discriminating recurrence from post-radiation related modification in patients with glioblastomas still remains challenging with morphological and functional imaging. The purpose of this pilot study was to assess the feasibility of 68Ga-PSMA PET/CT in detecting early recurrence, by comparison with 18F-FDOPA PET/CT. This was a non-randomized, prospective, multicentric feasibility study. Patients followed up for glioblastomas, with a MRI performed since the end of the radiotherapy until 12 months of follow up after the end of radiotherapy, were referred for both 68Ga-PSMA and 18F-FDOPA PET-CT, whatever the conclusion of the MRI (post radiation modifications, relapse or doubtful MRI). Nine patients from 2 centers were included. They were referred for both PET/CT which were realized the same day with a 6 hours delay. SUVmax, lesion-to-striatum ratio (TBR), lesion-to-normal parenchyma ratio (TNR), and lesion-to-salivary glands ratio (TSGR) were calculated. Among our nine patients, five patients demonstrated good correlation between 18F-FDOPA and 68Ga-PSMA PECT/CT findings. For four of these five patients, both examinations were consistent with recurrence, better visualized with PSMA, thanks to a better lesion-to-background ratio. Examinations of the fifth patient were suggestive of post radiation related changes. It was better analyzed with PSMA which displays a very faint uptake compared to DOPA. Conversely, four of our nine patients showed conflicting results. A recurrence was not detected by the PSMA because of a previously introduced bevacizumab treatment (false negative). For another patient, both examinations were consistent with recurrence, but there was an uptake mismatch regarding suspected lesion sites. This highlights distinct pathophysiological processes. Eventually, two patients presented inconsistent examinations, one patient with post radiation related remodeling (DOPA + PSMA−) and one suspected of recurrence (DOPA + PSMA−). Investigations are ongoing to try to explain these findings. Despite some scarce discrepancies between 68Ga-PSMA and 18F-FDOPA examinations, and the small number of patients included, this pilot study highlights the potential role of 68Ga-PSMA in helping to discriminate post-radiation inflammatory related modifications from glioblastoma recurrence in patients whithout recent bevacizumab treatment. The discrepancies are probably related to not fully understood distinct pathophysiological processes. False positives and false negatives could be minimised by following certain rules before carrying out the examination. 68Ga PSMA PET/CT seems useful because of its excellent lesion to background ratio. In case of positive PSMA recurrence, 177Lu PSMA could be considered as a new therapeutic alternative. However, more powerful prospective studies are needed to validate these results.