FET PET in Differentiating Tumour Progression From Pseudoprogression in High Grade Glioma

The goal of this clinical trial is to evaluate the performance characteristics of O-(2-\[18F\]fluoroethyl)-L-tyrosine (FET) PET in differentiating pseudoprogression from tumour progression in patients with equivocal conventional imaging and determine the sensitivity and specificity of \[18F\]FET-PET in delineating disease. The main question\[s\] it aims to answer are: * whether 18F-FET-PET will demonstrate high diagnostic accuracy to detect true tumour progression * whether we can optimise the threshold cut-offs for TBRmax and other relevant parameters in discriminating pseudoprogression and disease progression Participants will undergo a limited 18F-FET PET/CT of the brain in SGH.