O-(2-[18F]fluoroethyl)-L-tyrosine (FET) pet vs. stereotactic biopsy in glioma-patients after multimodal therapy: impact on treatment decision

Objective: O-(2-[18F]fluoroethyl)-L-tyrosine (FET) PET has been reported to offer a promising diagnostic tool when differentiation between tumor- and therapy-associated contrast enhancement on MRI in patients with gliomas undergoing multimodal treatment is difficult. However, the impact of FET-PET findings on treatment decision remains uncertain and has yet to be evaluated.

Methods: In a prospective study, a FET-PET investigation was performed in all glioma patients after multimodal therapy with the neuroradiological diagnosis (based on MRI) of a tumor recurrence between December 2003 and January 2005. Criteria for tumor recurrence/progression in FET-PET was a standardized uptake value (SUVmax) of >2.2. This threshold value was suggested for the sake of an optimal diffential diagnosis based on a previously performed comparative evaluation of PET and histological data of patients with brain tumors. To confirm diagnosis as the basis for adequate further treatment, stereotactic serial biopsy with a multimodal (CT,MRT,FET-PET) treatment planning was performed. Specimens were taken from the „hotspot“ area as defined by the SUVmax. Both stereotactic biopsy and FET-PET findings were correlated with clinical follow-up.

Results: 36 glioma patients were included. In 30 Patients (12 WHO grade II, 7 WHO grade III, 11 WHO grade IV) FET-PET results indicating a tumor recurrence were concordant with the biopsy-results as well as the clinical follow up. In 6 patients (4 WHO grade II, 1 WHO grade III, 1 WHO grade IV) in whom FET-PET was highly suspicious for tumor recurrence, histopathologic evaluation failed to reveal significant active tumor. The FET-PET findings were also discordant to the clinical follow-up (lack of any progressive symptoms) in these patients.

Conclusion: FET-PET might be a helpful tool to better distinguish benign therapy-induced side effects from tumor recurrence/progression. However, in our series there was a false positive FET-PET result concerning tumor recurrence/progression in a significant number of patients. Whether this is due to using the SUVmax (threshold of 2.2) as indicator or a general technique-imminent problem of the FET-PET is yet unknown. Therefore – for the time being – further treatment decisions in multimodally treated glioma patients should not be based on FET-PET findings alone.