P16.09VALUE OF FET-PET EVALUATION IN PATIENTS TREATED WITH AXITINIB

INTRODUCTION: VEGF/VEGFR targeted therapy for recurrent glioblastoma (rGB) normalizes tumour vasculature and restores the blood-brain barrier (BBB), resulting in decreased contrast enhancement and reduced peritumoral oedema. MRI-T2/FLAIR images are useful to document non-contrast-enhancing tumor progression, but its specificity is limited. Since the uptake of Fluoroethyltyrosine in GB reflects tumor metabolism and not BBB-integrity, the extent of (18)F-fluoroethyltyrosine (FET) uptake, as assessed by positron emission tomography (PET), may be of value in assessing the tumor response to treatment with small molecule VEGFR-inhibitors such as axitinib., MATERIAL AND METHODS: We evaluated patients with rGB treated with axitinib with MRI and FET-PET (static images acquired 45 min. after tracer administration) at baseline. In patients demonstrating a response on MRI (using RANO criteria), FET-PET was repeated. For 18F-FET PET, reduction of the metabolically active volume (Sum of tumour associated pixels with target-to-normal brain ratio of ≥1.5) was considered as a metabolic response to treatment. RESULTS: 15 patients treated with axitinib were evaluated with MRI and FET-PET. On MRI, 2 pts had CR and 4 PR. All responses on MRI were also characterized by a decrease in FET-active tumor volume. The extent of reduction of metabolically active volume did not correlated with the extent of tumor regression on Gd- MRI: the 2 patients with CR on MRI had a 65% and 43% decrease on FET-PET, respectively, whereas the decrease on FET-PET ranged between 26 and 100% in the patients with PR on MRI. A trend was observed between a higher baseline FET-active tumor volume and an increased progression-free survival (PFS) and overall survival (OS). A greater reduction of the metabolically active volume did correlate with longer PFS and OS (logrank p > 0,001 and p < 0,025, respectively). CONCLUSION: In patients with rGB treated with the VEGFR-inhibitor axitinib, baseline FET-PET has prognostic value. Response evaluation based on FET-PET is complementary to MRI based assessment and may prove superior in predicting survival on treatment with axitinib. Our preliminary findings deserve further evaluation in a larger series.