Abstract
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Objectives: Post-treatment imaging can be quite complex for brain tumors but it has great clinical impact. Both amino acid PET and advanced MR imaging showed promising performance for this indication. Our aim was to assess the value of integrated 11C-methionine (MET) PET/MR for the differentiation between tumor progression and treatment related changes for brain tumors.
Methods: Patients with enlarging contrast-enhancing lesions following radiotherapy were consecutively enrolled and evaluated with integrated 11C-MET PET/MR. 10-min PET scan started 20 minutes after injection, together with anatomical MR sequences, diffusion-weighted imaging(DWI), 3D Arterial Spin Labeling (3D ASL) perfusion imaging and magnetic resonance spectroscopy (MRS). Images were analyzed with visual evaluation and quantitative parameters such as SUVmax, SUVmean, T/N, ADC, ADC ratio (lesion/contralateral brain), CBF, CBF ratio (lesion/contralateral brain), Cho/Cr and Cho/NAA. MRS patterns were further classified into three groups (normal, typically abnormal, indeterminate) for each lesion. Pathologic exam or clinical follow-up with more than six months were considered as the reference. Comprehensive diagnosis was made based on both PET and MR information for 11C-MET PET/MR while contrast-enhanced MR mainly relied on RANO criteria.
Results: Twenty-seven patients with long clinical follow-up were enrolled (19 gliomas, 1 germinoma, 1 germinoblastoma, 6 brain metastases). Thirteen patients were finally diagnosed as true progression (12 patients by clinical follow up and 1 patient by pathological exam). And 5 patients were stable and 9 patients were diagnosed as treatment-related changes. Sensitivity, specificity, positive predictive value and negative predictive value for integrated PET/MR were 83.3%, 88.9%, 93.8%, 93.8% and 72.7% versus 83.3%, 22.2%, 68.2%, 40.0% for contrast-enhanced MR. 11C-MET PET/MR was more sensitive for low grade glioma but less sensitive for meningeal lesions. Contrast-enhanced MR was more sensitive for meningeal lesions but less specific for recurrence and radiation necrosis. Comparing the positive lesions (true progression and stable disease) and negative lesions (treatment-related changes), the former group had significantly higher SUVmax (3.91±1.64 vs. 2.11±0.71, p=0.001), T/N (2.26±0.88 vs. 1.38±0.54, p=0.012), and CBF ratio (1.28±0.496 vs. 0.88±0.38, p=0.046). Neither ADC (1.01±0.23 vs. 0.84±0.18 p=0.077) nor ADC ratio (1.09±0.32 vs. 1.05±0.34, p=0.783) had significant difference between two groups. Radiation necrosis can have diffusion restriction more obvious than recurrent lesions. For MR spectroscopy, more cases in the group of positive lesions (11 abnormal, 6 indeterminate, 1 normal) presented with elevated choline peak and depressed NAA peak than the negative group (0 abnormal, 3 normal, 6 indeterminate, p=0.001).
Conclusions: Overall, integrated 11C-MET PET/MR had better diagnostic performance compared with contrast-enhanced MR. Multiple parameters such as SUVmax, T/N, CBF ratio and MRS were valuable for the differentiation between tumor progression and treatment related changes for brain tumors after radiotherapy.