Comparison of [⁶⁸Ga]Ga-Fibroblast Activation Protein Inhibitor–04 and [¹⁸F]FDG PET Imaging for Solitary Fibrous Tumor and Preliminary Application of FAP-Targeted Radiopharmaceutical Therapy

Solitary fibrous tumor (SFT) is a rare sarcoma of mesenchymal origin. Although generally benign, SFTs carry the risk of recurrence and metastasis, with limited effective treatment options. The aims of this study are to compare the performance of fibroblast activation protein inhibitor (FAPI), [⁶⁸Ga]Ga-DOTA-FAPI-04 (denoted as [⁶⁸Ga]Ga-FAPI-04), and conventional [¹⁸F]FDG PET/CT in patients with recurrent or metastatic SFTs head to head and to preliminarily explore the value of FAP-targeted radiopharmaceutical therapy with ¹⁷⁷Lu for SFT patients. <b>Methods:</b> Thirty-one participants (21 men, 44 ± 13 y) with suspected recurrent or metastatic SFTs underwent both [¹⁸F]FDG and [⁶⁸Ga]Ga-FAPI-04 PET/CT within 1 wk. The positive-lesion rates of the 2 PET/CT scans in the different organs involved and the uptake values (SUV_max) were compared. Four patients with high [⁶⁸Ga]Ga-FAPI-04 uptake received single-cycle therapy of 2.22 GBq of a [¹⁷⁷Lu]Lu-labeled, FAP-targeted radiopharmaceutical, [¹⁷⁷Lu]Lu-Evans blue-FAPI, and were followed up for 4 mo. <b>Results:</b> In 522 local recurrences and distant metastases in the 31 patients, [⁶⁸Ga]Ga-FAPI-04 PET detected significantly more lesions than did [¹⁸F]FDG (87.0% vs. 45.4%, <i>P</i> < 0.001). In terms of lesion uptake values, [⁶⁸Ga]Ga-FAPI-04 PET showed a mean SUV_max higher than that of [¹⁸F]FDG in most recurrence or metastatic organs (bone, lung, central nervous system, pancreas, and pleura, <i>P</i> < 0.001; kidney and abdominopelvic cavity, <i>P</i> = 0.001; muscle and pericardium, <i>P</i> < 0.05). Four patients tolerated [¹⁷⁷Lu]Lu-Evans blue-FAPI well. The total-body absorbed dose and the effective dose were 4.02E-01 ± 3.54E-02 Gy and 4.01E+02 ± 4.18E+01 mSv, respectively. Subsequent follow-up with [⁶⁸Ga]Ga-FAPI-04 PET showed that these patients were in stable condition. <b>Conclusion:</b> [⁶⁸Ga]Ga-FAPI-04 may be a promising PET agent for the assessment of SFTs. Given the lack of effective treatments for advanced SFTs, high FAP expression in this type of tumor is expected to become a potential treatment target.