Volumetric assessment of gliomas: Comparison between F-18-FET and Ga-68-NODAGA-RGDyK

Aim/Introduction Amino acid PET with 18F-FET (FET) is becoming a standard functional imaging technique for the evaluation of glioma [1]. 68Ga-NODAGA-RGDyK (RGD) is a novel radiopharmaceutical targeting the integrin ανβ3 [2], which might provide additional information for stratifying patient prognosis and response to anti-angiogenic treatments. Our aim was to compare the tumor volumes identified by FET and RGD in a series of patients with glioma. Materials and Methods Seven patients with newly diagnosed or recurrent glioma were referred for PET/CT examinations with both FET and RGD, less than one week apart. Images were registered with the software PMOD version 3.903 (PMOD Technologies, Zurich, Switzerland). After image registration, automatic tumour segmentation of both sets of images was performed using the average background signal in a large hemispheric VOI multiplied by 1.6 as a threshold for positivity. Tumour volumes identified by both modalities were compared and their spatial congruence calculated [3]. Results The study population consisted of 7 patients (1F/6 M, mean age: 56y). According to the WHO 2016 classification of brain tumors, there were two glioblastomas (Grade IV, IDH wild-type), one ganglioglioma (Grade II IDH wild-type), two oligodendrogliomas (Grade II, IDH mutant, 1p19q co-deleted), one oligodendroglioma (Grade III, IDH mutant, 1p19q co-deleted) and one newly diagnosed non-enhancing, non-biopsied glioma. Magnetic resonance imaging showed presence of contrast enhancement in 3 out of 7 patients. On visual inspection, 100% (7/7) FET studies and 57% (4/7) RGD studies were positive. The tumour volume delineated using FET (FETvol 1.6) largely exceeded that of RGD (RGDvol 1.6). Median FETvol 1.6 and RGDvol 1.6 were 29.0mL (range, 10.8-207mL) and 2.95mL (range, 0–11.3mL), respectively (p=0.015). Overall, median overlapping volume was 2.95 ml (range 0–8.13mL), resulting in a median spatial congruence of 4.15% (range, 0–59.7%). RGD positivity was mostly located in tumour portions showing contrast enhancement at MRI. In two patients with high-grade tumours, there was RGD uptake extending to FET-negative tumour portions. One patient had a meningioma showing increased RGD uptake and negative FET. Conclusion The information provided by FET and RGD PET are substantially different. FET PET identifies larger volumes than RGD. Some volumetric mismatch exists, whose significance should be further investigated in larger patient series with follow-up.