Role of 18F-Fluoro-Ethyl-Tyrosine (FET) Positron Emission Tomography (PET) for Grading Glioma

The management and prognosis of patients with glioma is highly dependent on the tumour grade according to the new 2016 classification of the World Health Organization (WHO), which incorporates molecular characteristics. Standard magnetic resonance imaging (MRI) enhanced by contrast is the basis of imaging primary brain tumours including gliomas. Nevertheless MRI specificity to type glioma is limited. Recently, positron emission tomography (PET) molecular imaging using radiolabeled amino acids or their analogues has been recommended by the Neuro-Oncology Response Assessment (RANO) working group for differential diagnosis of brain lesions, non-invasive classification of glial tumours, prognostic value, tumour delineation, stereotactic biopsy radiotherapy planification and treatments follow-up, to provide additional informations beyond MRI on biological processes such as cell proliferation, membrane biosynthesis, glucose consumption and absorption of amino acid analogues. Among the radiotracers used in PET, radiolabeled amino acids or their analogues are increasingly used in clinical routine for glioma imaging. Although most previous PET studies focused on brain gliomas used L-[methyl- 11 C] -methionine (11C-MET), the fluorinated amino acid analogue O - (2-[ 18 F] fluoroethyl) -L-tyrosine (18F-FET) appeared to be a favorable marker for clinical routine due to his longer half-life than Carbone 11. Recent european guidelines attempt to provide some guidance on the performance and interpretation of molecular imaging. The authors recommend a static (20-40 mn after injection (Pi)) or dynamic PET acquisition (40-50 mn from injection). A visual analysis can be completed by a quantitative analysis which consists to measure mean and maximal tumour activity uptake values (SUVmean and SUVmax) and their respective tumour to background ratios (TBRmean and TBRmax). Although the mean physiological brain activity uptake is well defined, the measurement of mean glioma activity uptake is less clear. Indeed, TBRmean depends on the delineation of the tumour ROI and/or VOI. Most often previously, VOI was determined by a 3D contouring process using a tumour-to-brain ratio of at least 1.6 at the beginning of the scan, threshold defined on a brain gliomas biopsy-controlled study. Moreover, Albert et al. emphasized the interest of early TBRmax.To our knowledge, none study evaluated others parameters as SUVmax, SUVmean and TBRmean in early period. In this context, the aim of this study was to compare quantitative and qualitative PET parameters between Low Grade Glioma and High Grade Glioma.