Positron emission tomography(PT)/computed tomography (CT) scans utilizing radioactive fluorine [18F]-labeled fibroblast activation protein inhibitor-42 ([18F]FAPI-42) detected more recurrent or metastatic gastrointestinal stromal tumors (GISTs) than PET/CT imaging using labeled fluorodeoxyglucose ([18F]FDG) according to a new study published in the European Journal of Nuclear Medicine and Molecular Imaging.

During the study, a total of 106 lesions were detected in 35 patients. Imaging with [18F]FDG did not discover 38 (35.8%) of these tumors which were detected using [18F]FAPI-42. The undiscovered tumors included 26 liver metastases, 10 peritoneal metastases, 1 gastrointestinal recurrence, and 1 bone metastasis.

Positive FDG uptake without FAPI uptake was observed in 10 (9.4%) of the lesions, which were all peritoneal metastases, and 11 lesions were negative to both tracers. Overall, [18F]FAPI-42 imaging detected 87.5% of tumors compared to 53.8% (P <.001) using [18F]FDG. The difference was even larger when comparing tumor metastases with [18F]FAPI-42, detecting 87.5% compared to 33.3% using [18F]FDG (P <.001).


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“Our study showed that [18F]FAPI-42 PET/CT for imaging of recurrent or metastatic GISTs could be superior to [18F]FDG PET/CT in specific organs, specifically for liver metastases,” the authors said.

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The tumor-to-background ratio of the maximum standardized uptake value (SUVTBR) for liver metastases was also significantly higher using [18F]FAPI-42 than [18F]FDG (2.4 vs 0.9; P <.001). No significant differences in SUVTBR were observed for other locations, however.

Histological samples taken from 3 patients showed that [18F]FAPI-42 uptake in lesions corresponded with increased FAP expression in those tumors. The uptake was found to be independent of tumor size, the degree of imaging enhancement, the type of gene mutation, and targeted treatment.

In contrast, the longest diameter of tumors was significantly higher in FDG-positive tumors than in FDG-negative tumors. The SUVTBR of FDG was significantly different between different mutations and whether patients received targeted therapy or not.

Reference

Wu C, Zhang X, Zeng Y, et al. [18F]FAPI-42 PET/CT versus [18F]FDG PET/CT for imaging of recurrent or metastatic gastrointestinal stromal tumors. Eur J Nucl Med Mol Imaging. Published online August 30, 2022. doi:10.1007/s00259-022-05955-x