110 Use of advanced imaging modalities to provide added value in longitudinal surveillance of pilocytic astrocytoma

Abstract Introduction Pilocytic Astrocytoma is the most common paediatric brain tumour, accounting for approximately 20% of all childhood brain tumour diagnoses. While gross total resection confers a significant survival benefit, complete resection is often not feasible due to tumour location, leaving residual disease that increases the risk of recurrence or progression. Surveillance of these cases relies on serial MRI to identify signs of progression. However, accurately measuring small volumes of residual tumour within a resection cavity is challenging, and subtle changes may occur before detectable measurements of progression are possible. Quantitative advanced imaging techniques, such as diffusion weighted imaging (DWI) and proton MR Spectroscopy (1H-MRS), may therefore provide a valuable tool to support response assessment, by providing early objective biomarkers of changes in tumour characteristics indicative of progression. Methods Longitudinal analysis of imaging data acquired from paediatric patients recruited to the CCLG Imaging of Tumours study with a diagnosis of pilocytic astrocytoma and residual tumour post resection was conducted (n = 17). Quantitative metabolite concentrations derived from 1H-MRS were analysed to characterise the variability of metabolites during periods of radiological stability and compare metabolite concentrations between stability and progression. Results Within this cohort, readily detectable metabolites, including myo-inositol, total choline, and total creatine, demonstrated low variability over time within patients with stable disease. In contrast, statistically significant changes in these metabolite concentrations were observed at progression (p < 0.01). Conclusions The results of this study have shown the potential for MRS to distinguish slow tumour progression from radiological stability. Subsequent analyses will be undertaken of quantitative features extracted from DWI data within the same cohort, aiming to evaluate their complementary value alongside MRS to improve assessment of tumour response and early detection of progression.