Purpose

FAPI ligands (fibroblast activation protein inhibitor), a novel class of radiotracers for PET/CT imaging, demonstrated in previous studies rapid and high tumor uptake. The purpose of this study is the head-to-head intra-individual comparison of ⁶⁸Ga-FAPI versus standard-of-care ¹⁸F-FDG in PET/CT in organ biodistribution and tumor uptake in patients with various cancers.

Material and methods

This international retrospective multicenter analysis included PET/CT data from 71 patients from 6 centers who underwent both ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT within a median time interval of 10 days (range 1-89 days). Volumes of interest (VOIs) were manually drawn in normal organs and tumor lesions to quantify tracer uptake by SUVmax and SUVmean. Furthermore, tumor-to-background ratios (TBR) were generated (SUVmax tumor/ SUVmax organ).

Results

A total of 71 patients were studied of, which 28 were female and 43 male (median age 60). In 41 of 71 patients, the primary tumor was present. Forty-three of 71 patients exhibited 162 metastatic lesions. ⁶⁸Ga-FAPI uptake in primary tumors and metastases was comparable to ¹⁸F-FDG in most cases. The SUVmax was significantly lower for ⁶⁸Ga-FAPI than ¹⁸F-FDG in background tissues such as the brain, oral mucosa, myocardium, blood pool, liver, pancreas, and colon. Thus, ⁶⁸Ga-FAPI TBRs were significantly higher than ¹⁸F-FDG TBRs in some sites, including liver and bone metastases.

Conclusion

Quantitative tumor uptake is comparable between ⁶⁸Ga-FAPI and ¹⁸F-FDG, but lower background uptake in most normal organs results in equal or higher TBRs for ⁶⁸Ga-FAPI. Thus, ⁶⁸Ga-FAPI PET/CT may yield improved diagnostic information in various cancers and especially in tumor locations with high physiological ¹⁸F-FDG uptake.