Nimg-05. characterization of long-term metabolic changes of brain metastases after treatment with stereotactic radiosurgery using serial fet pet imaging

Abstract BACKGROUND In the present study, we characterized the long-term metabolic changes of brain metastases after treatment with stereotactic radiosurgery (SRS) by serial PET imaging using the radiolabeled amino acid O-(2-[18F]-fluoroethyl)-L-tyrosine (FET). We hypothesized that this approach is of considerable clinical value in differentiating between radiation-induced changes and local brain metastasis relapse. Patients and METHODS From 2010-2020, we retrospectively identified patients with brain metastases from solid extracranial primary tumors who (i) were treated with SRS with or without concurrent immunotherapy using checkpoint inhibitors, (ii) had equivocal MRI findings after SRS, and (iii) subsequently underwent two or more additional FET PET scans during follow-up for long-term evaluation. Mean tumor-to-brain ratios (TBR) were obtained. Diagnostic performances were calculated using receiver operating characteristic curve analyses. Diagnoses were confirmed by neuropathological evaluation (29%) or clinicoradiologically (71%). RESULTS We identified 14 patients with 42 FET PET scans (median number, 3; range, 2-4). Concurrent to SRS, 4 patients (29%) were treated with checkpoint inhibitors. Following SRS, equivocal MRI findings occurred after a median time of 7 months (range, 2-62 months). Subsequently, FET PET scans were acquired over a median 16 months (range, 7-99 months). Nine patients (64%) had radiation-induced changes, and five had local brain metastases relapse. TBRs calculated from the last available FET PET scan showed the highest accuracy (93%) in identifying radiation-induced changes (threshold, 1.95; P=0.011). In lesions consistent with local brain metastases relapse, TBR increased considerably over time (median increase, 17%; range, 11% - 32%). In contrast, in lesions considered as radiation-induced changes, TBR remained stable or even decreased (median change, -4%; range, -44% - 11%). The TBR evolution over time significantly differed between the two groups (P=0.011). CONCLUSIONS Serial FET PET imaging has a high diagnostic accuracy for characterizing long-term changes of brain metastases after treatment with stereotactic radiosurgery.