Response assessment of bevacizumab in patients with recurrent malignant glioma using [¹⁸F]Fluoroethyl-l-tyrosine PET in comparison to MRI

To investigate prospectively the potential of O-(2-[¹⁸F]fluoroethyl)-l-tyrosine (¹⁸F-FET) PET in comparison to MRI for the assessment of the response of patients with recurrent high-grade glioma (rHGG) to antiangiogenic treatment.

Ten patients with rHGG were treated biweekly with bevacizumab/irinotecan (BEV/IR). MR images and dynamic ¹⁸F-FET PET scans were obtained at baseline and at follow-up after the start of treatment (median 4.9 weeks). Using MRI treatment response was evaluated according to RANO (Response Assessment in Neuro-Oncology) criteria. For ¹⁸F-FET PET evaluation, a reduction >45 % of the metabolically active tumour volume was considered as a treatment response, with the metabolically active tumour being defined as a tumour-to-brain ratio (TBR) of ≥1.6. The results of the treatment assessments were related to progression-free survival (PFS) and overall survival (OS). For further evaluation of PET data, maximum and mean TBR were calculated using region-of-interest analysis at baseline and at follow-up. Additionally, ¹⁸F-FET uptake kinetic studies were performed at baseline and at follow-up in all patients. Time–activity curves were generated and the times to peak (TTP) uptake (in minutes from the beginning of the dynamic acquisition to the maximum uptake) were calculated.

At follow-up, MRI showed a complete response according to RANO criteria in one of the ten patients (10 %), a partial response in five patients (50 %), and stable disease in four patients (40 %). Thus, MRI did not detect tumour progression. In contrast, ¹⁸F-FET PET revealed six metabolic responders (60 %) and four nonresponders (40 %). In the univariate survival analyses, a response detected by ¹⁸F-FET PET predicted a significantly longer PFS (median PFS, 9 vs. 3 months; P = 0.001) and OS (median OS 23.0 months vs. 3.5 months; P = 0.001). Furthermore, in four patients (40 %), diagnosis according to RANO criteria and by ¹⁸F-FET PET was discordant. In these patients, PET was able to detect tumour progression earlier than MRI (median time benefit 10.5 weeks; range 6–12 weeks). At baseline and at follow-up, in nonresponders TTP was significantly shorter than in responders (baseline TTP 10 ± 8 min vs. 35 ± 9 min; P = 0.002; follow-up TTP 23 ± 9 min vs. 39 ± 8 min; P = 0.02). Additionally, at baseline a kinetic pattern characterized by an early peak of ¹⁸F-FET uptake followed by a constant descent was more frequently observed in the nonresponders (P = 0.018).

Both standard and kinetic imaging parameters derived from¹⁸F-FET PET seem to predict BEV/IR treatment failure and thus contribute important additional information for clinical management over and above the information obtained by MRI response assessment based on RANO criteria.