Fibroblast-Activation Protein PET and Histopathology in a Single-Center Database of 324 Patients and 21 Tumor Entities

We present an overview of our prospective fibroblast-activation protein inhibitor (FAPI) registry study across a 3-y period, with head-to-head comparison of tumor uptake in 68Ga-FAPI and 18F-FDG PET, as well as FAP immunohistochemistry. Methods: This is an interim analysis of the ongoing 68Ga-FAPI PET prospective observational trial at our department. Patients who underwent clinical imaging with 68Ga-FAPI PET between October 2018 and October 2021 were included. Tracer uptake was quantified by SUVmax for tumor lesions and by SUVmean for normal organs. PET tumor volume (40% isocontour) and tumor-to-background ratios were calculated. Correlation between SUVmax and FAP staining in tissue samples was analyzed. Results: In total, 324 patients with 21 different tumor entities underwent 68Ga-FAPI imaging; 237 patients additionally received 18F-FDG PET. The most common tumor entities were sarcoma (131/324, 40%), pancreatic cancer (67/324, 21%), and primary tumors of the brain (22/324, 7%). The mean primary tumor SUVmax was significantly higher for 68Ga-FAPI than 18F-FDG among pancreatic cancer (13.2 vs. 6.1, P < 0.001) and sarcoma (14.3 vs. 9.4, P < 0.001), and the same was true for mean SUVmax in metastatic lesions of pancreatic cancer (9.4 vs. 5.5, P < 0.001). Mean primary tumor maximum tumor-to-background ratio was significantly higher for 68Ga-FAPI than 18F-FDG across several tumor entities, most prominently pancreatic cancer (14.7 vs. 3.0, P < 0.001) and sarcoma (17.3 vs. 4.7, P < 0.001). Compared with 18F-FDG, 68Ga-FAPI showed superior detection for locoregional disease in sarcoma (52 vs. 48 total regions detected) and for distant metastatic disease in both sarcoma (137 vs. 131) and pancreatic cancer (65 vs. 57), respectively. Among 61 histopathology samples, there was a positive correlation between 68Ga-FAPI SUVmax and overall FAP immunohistochemistry score (r = 0.352, P = 0.005). Conclusion:68Ga-FAPI demonstrates higher absolute uptake in pancreatic cancer and sarcoma, as well as higher tumor-to-background uptake along with improved tumor detection for pancreatic cancer, sarcoma, and other tumor entities when compared with 18F-FDG. 68Ga-FAPI is a new tool for tumor staging with theranostic potential.