Distinguishing tumor recurrence from radionecrosis in brain metastases after stereotactic radiosurgery using hybrid FDG-PET and MRI coregistered images

Objectives Dual phase 18 FDG-PET has been proven helpful to assess FDG-avid cerebral tumors as tracer will build up in metastases or tumor recurrences while its retention remains stable within normal tissue or inflammatory process. This is useful when MRI can’t discriminate brain tumor recurrence (TR) from radionecrosis (RN) after stereotaxic radiosurgery (SRS) for brain metastases (BM). Many studies sought to improve diagnostic performance by associating FDG-PET and MRI with interesting results but many bias, mostly within image post-processing. Coregistered MRI and dual phase FDG-PET images could alleviate these biases and be used to extract prognostic biomarkers. Methods We retrospectively evaluated patients treated with SRS for BM which developed a contrast-enhanced MRI lesion with non-conclusive diagnosis for TR or RN. All patients underwent MRI and FDG-PET at least 3 months after their last SRS session. Dual FDG-PET comprised a one hour “based image” (H1) followed by a 4 hours “delayed image” (H4). MRI included contrast enhanced T1 (T1), FLAIR, ASL (BAT,CBF), T1 Perfusion (ktrans, SER) and T2* Permeability (k2, rBF, rBV). PET and MRI data were all coregistered on the T1 images. Semi-automated Regions of Interest (ROI) of the tumor were drawn on the T1 MRI, H1 and H4 FDG-PET images; a reference contralateral white-matter ROI (WM) was drawn for standardization. Imaging metrics were then evaluated for their association with TR or RN based on histological, radiological and clinical criteria after at least 6 months follow-up. Results Eleven patients matched the inclusion criteria; 7 with TR and 4 with RN; no additional treatment was introduced. On visual analysis, FDG-PET and MRI respectively obtained 100% and 83% Sensitivity and 100% and 80% Specificity. The two MRI-false negative diagnoses were subsequently identified on follow-up (1 patient, RN) and histology (1 patient, TR). When standardized on WM, PET SUVmax values were significantly different across groups at H1 (P Conclusion Coregistered PET-MRI images accurately discriminates between TR and RN. With FDG being the most commonly used PET radiotracer, this protocol remains easily transposable and should be encouraged to obtain non-invasive prognostic and clinically relevant biomarkers.