Gliomas are the most common malignant primary central nervous system (CNS) tumours. When high-grade gliomas (HGG) recur, subsequent magnetic resonance (MRI) imaging, with additional sequences is required.The Positron Emission Tomography (PET) radiotracer [18F]-fluorodeoxyglucose (FDG) will be used in this study to distinguish between changes seen on MRI which can be a reflection of pseudoprogression, radiation necrosis, or recurrence.

Molecular imaging has been used to distinguish recurrent tumor from post-treatment changes through the use of positron emission tomography (PET) as well as other techniques. The best-studied PET radiotracer for this application is [18F]-fluorodeoxyglucose (FDG). Normal brain matter is very FDG-avid, making it more difficult to identify lesions and in addition, inflammation associated with radiation injury has been shown to be FDG avid.

In light of this, variations of the standard FDG protocols have been proposed in order to increase overall accuracy, including dual time point imaging (DTPI), consisting of injecting the patient with the standard radiotracer and acquiring two sets of images several hours apart, typically the normal initial images in addition to a delayed acquisition set.

There is good reason to suspect that DTPI FDG-PET would be useful a technique for characterizing lesions in the brain. It's been shown that FDG uptake by normal brain parenchyma initially increases then decreases with time, while tumor uptake typically increases and then plateaus. This pattern of increasing and then decreasing FDG activity has also been seen in inflammatory tissue. The difference in FDG uptake at different times is what allows for a better distinction between malignant and benign tissue.

Inclusion Criteria:

• Age ≥ 18 years old
• Able and willing to comply with the study procedures
• The patient must be followed at The Ottawa Hospital for a tissue-proven, grade III or IV glioma.
• The patient must have been treated in the past with radiotherapy for glioma.
• A disease recurrence is suspected based on clinical symptoms and/or imaging results.

Exclusion Criteria:

• Missing information regarding tumor type and grade
• Brain biopsy in the ten days preceding DTPI FDG-PET
• Breastfeeding or pregnancy
• Claustrophobia or inability to lie still in a supine position
• Unwillingness or inability to provide informed consent

Participants will be patients who have previously been treated with radiochemotherapy for high grade gliomas who are clinically assessed with a brain MRI for suspected disease recurrence.