Objective: To explore prospectively the positive predictive value of O-(2-[(18)F]fluoroethyl)-L-tyrosine (FET)-PET in selected patients with a magnetic resonance imaging (MRI)-based suspicion of a glioma recurrence or progression. Methods Patients with a supratentorial glioma (initial World Health Organization (WHO) grade II, III or IV) were considered eligible if they had both an MRI-(new/progressive contrast-enhancing lesion) and FET-PET-based diagnosis of a recurrence/progression after various forms and combinations of irradiation and chemotherapy. Criterion for tumour recurrence/progression in FET-PET was a standardized uptake value (SUVmax)/Background (BG) ratio of >2.0 in the late uptake phase. All patients underwent multimodal (MRI, FET-PET) imaging-guided stereotactic biopsy. The positive predictive value was defined as the proportion of MRI and FET-PET findings indicating glioma recurrence/progression that also tested positive for tumour recurrence/progression after stereotactic biopsy.
Results: Thirty-one patients with initially WHO grade II (17), WHO grade III (6), and grade IV glioma (8) were included. In 26 patients FET-PET results indicating tumour recurrence/progression were concordant with the biopsy results. In five patients histopathologic evaluation failed to reveal a "vital" tumour. FET-PET findings were also discordant with the radiographic and clinical follow-up in these five patients. The positive predictive value of FET-PET was 84%.
Conclusion: The positive predictive value of FET-PET using the standard ratio method is high, but not high enough to replace stereotactic biopsy in this highly selected study cohort. Whether the calculation of FET uptake in the early phase and/or the evaluation of uptake kinetics will improve the positive predictive value of FET-PET deserves prospective evaluation.