Utility of PET-MRI in Surveillance of Paediatric Brain Tumours

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Brain tumours are the most common solid tumours and the leading cause of death from cancer in children. A particular challenge in their management is detection of recurrent, residual or progressive tumour after initial treatment. This is typically detected on conventional MR surveillance scans. However, it is well documented that post-therapeutic changes can occur on MRI which may not represent disease, particularly after radiotherapy. Erroneously interpreting treatment-related changes as tumor progression may lead to the cessation of an effective treatment or the overestimation of the efficacy of a subsequent treatment. This is particularly important when assessing novel therapies in the context of early-phase clinical trials. A more accurate, non-invasive method of monitoring children after treatment for brain tumours could therefore enhance clinical management and may also lead to a more accurate assessment of novel therapies. Hybrid Positron emission tomography-magnetic resonance imaging (PET/MRI) combines the high contrast and morphological resolution of MRI with the metabolic and physiological resolution from an intergrated PET scan. A variety of PET radiopharmaceutical tracers have been employed but 18F-fluodeoxyglucose (18F-FDG) has evolved over the last two decades to become the most important clinically. Increased glucose metabolism indicated by an increased FDG uptake is commonly seen in proliferating tumors due to the increased glucose transporter expression and the enzyme hexokinase, converting FDG to a phosphorylated product. FDG-PET/CT has been demonstrated to have use for the differentiation of residual or recurrent tumours from post-therapeutic radiotherapy changes in adults 3-5 but its value in children is largely limited to case reports or small case series. The value of PET/MRI in paediatric in paediatric brain tumours is even less certain but in a series of 85 patients it was found to have a significant impact on management in the majority of cases. The accuracy of FDG PET in tumor surveillance may be higher than what has been historically reported because of improved spatial localization with concurrent use of MRI rather than CT. This is a pilot project to explore the utility of PET-MRI in the post-treatment surveillance of high-grade gliomas or medulloblastomas in our institution. These tumours have the highest uptake of 18F-FDG and PET-MRI is therefore more likely to add diagnostic accuracy during their follow-up.