¹⁸ F-fluciclovine positron emission tomography uptake is not exclusive to malignant brain tumors: Two cases of low-grade and benign lesions with detailed pathological findings

Background: 18 F-fluciclovine ( 18 F-FACBC) positron emission tomography (PET) is used to visualize malignant brain tumors as part of preoperative surgical planning. We previously reported that the degree of 18 F-FACBC uptake correlated with tumor malignancy; however, data on low-grade and benign brain lesions remain limited. We report two cases of nonmalignant brain lesions with apparent 18 F-FACBC uptake. Case Description: Both cases were suspected to be high-grade gliomas based on preoperative magnetic resonance imaging (MRI) and 18 F-FACBC PET findings, with positive intraoperative 5-aminolevulinic acid fluorescence. Despite these findings, postoperative histopathology revealed pilocytic astrocytoma (World Health Organization grade 1) and capillary hemangioma, respectively. These cases demonstrate that high 18 F-FACBC uptake does not necessarily indicate a high-grade or malignant brain tumor. To further evaluate 18 F-FACBC accumulation, we analyzed lesion-to-normal (L/N) tissue ratios in these cases and our previously reported series, finding higher L/N ratios in high-grade tumors (mean L/N ratio: high-grade vs. low-grade = 8.40 vs. 3.16). Conclusion: Although existing data on the behavior of 18 F-FACBC PET in low-grade and benign lesions are scarce, our cases provide valuable insight into its behavior in these lesions through intraoperative and pathological findings. At the same time, our findings highlight that interpretation of 18 F-FACBC PET should not rely solely on visual assessment and underscore the importance of incorporating quantitative parameters such as the L/N ratio for objective evaluation. Although the clinical application of 18 F-FACBC PET in brain tumors remains limited, further accumulation of clinical data is warranted to establish optimal evaluation strategies and its clinical role in neuro-oncological practice.