NIMG-77. MAGNETIC RESONANCE IMAGING AND AMINO ACID POSITRON EMISSION TOMOGRAPHY IN DIFFERENTIATING TUMOR RECURRENCE FROM RADIATION NECROSIS IN HIGH GRADE GLIOMAS

Abstract BACKGROUND AND PURPOSE Differentiating tumor recurrence (TR) from radiation necrosis (RN) in high grade glioma (HGG) following radiotherapy can be challenging. The ability of conventional magnetic resonance imaging (MRI) to differentiate TR from post-therapeutic effects is often limited. Multiparametric advanced MRI (perfusion) and positron emission tomography (PET) with amino acid tracers, specifically 18F-Fluoroethyl-Tyrosine (FET) can provide relevant additional information on tumor metabolism, which allows for a more accurate diagnosis to differentiate tumor recurrence from radiation necrosis in high grade gliomas. MATERIALS AND METHODS Prospective analysis of 62 lesions in 62 patients with HGG who underwent both MRI and FET-PET imaging within three weeks intervals was done independently by a neuroradiologist and nuclear medicine physician. The study was conducted in a tertiary care oncology center between July 2018 and August 2021. Manually segmented regions of interest were placed over the areas of maximum enhancement/suspicion on MRI and FET uptake, which were used to calculate the relative cerebral volume (rCBV) and tumor to background ratios, respectively. Definitive diagnosis (TR versus RN) was made on clinico-radiological follow-up or histopathological report (wherever available). RESULTS Out of the 62 lesions that were studied, 46 and 16 had TR and RN, respectively. The sensitivity and specificity for determination of TR in HGG with conventional MRI were 98% and 62.5% respectively, while with FET-PET it was 91% and 87.5% respectively. The PPV , NPV and accuracy for MRI were 88%, 91% , 89% , while for FET-PET it was 95%, 78% and 90% respectively. A combination of MRI and PET parameters (mean target-to-background ratio), demonstrated an increase in diagnostic accuracy to 97%. CONCLUSION Cumulative imaging with MRI and FET-PET offers a multiparametric assessment of glioma recurrence that is correlative and complimentary, with higher accuracy and clinical value.