A 23-year-old woman presented with morning dizziness for 10 days. Brain MRI (a-d, arrow) revealed a solid and cystic mass in the right frontal lobe without peritumoral edema. The mass was obvious enhanced after contrast-enhancement. The patient then underwent [18F]FDG PET/CT for further differentiation of the brain mass and was included in a clinical trial of [68Ga]Ga-FAPI (NCT04499365). Written informed consent was obtained from the patient. In [18F]FDG PET/CT (e-f, arrow), the right frontal mass was hypometabolic (SUVmax 1.6), suggesting a low-grade tumor. However the mass showed significantly intense uptake of [68Ga]Ga-FAPI (SUVmax 30.9) (g-h, arrow). The patient then underwent surgical resection of the tumor, and the pathologic diagnosis was solitary fibrous tumor (SFT) /hemangiopericytoma (HPC) (WHO grade II). Hematoxylin-eosin staining (i) showed that the lesion was composed of spindle cells. Immunohistochemistry (IHC) revealed positivity for STAT6(j). FAP-staining revealed prominent positive staining of tumor stromal cells (k).

[68Ga]Ga-FAPI is a novel PET agent developed for solid tumor that specifically targets fibroblast activation protein overexpressed in cancer-associated fibroblasts [1]. SFT/HPC is a rare fibroblastic mesenchymal neoplasm, and SFT/HPC of central nervous system (CNS) usually arises from the cranial or spinal dura [2]. Recent cases [3, 4] reported intense [68Ga]Ga-FAPI uptake in SFTs arising from retroperitoneal region and lung and confirmed the high expression of FAP in pulmonary SFT by IHC. In this case, the CNS SFT/HPC also showed high uptake of [68Ga]Ga-FAPI, and IHC confirmed high expression of FAP in tumor stromal cells. Therefore, [68Ga]Ga-FAPI may be a promising tracer in detecting SFT/HPC.

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