Abstract
PURPOSE: Radiolabelled fibroblast activation protein inhibitors (FAPI) are promising molecular imaging tracers for the diagnosis and staging of pancreatobiliary cancer. To enable treatment response assessment with [(68)Ga]Ga-FAPI-46, the day-to-day variability of its uptake must be defined. This study aimed to determine the repeatability of semi-quantitative [(68)Ga]Ga-FAPI-46 PET/CT measurements in pancreatobiliary cancers. METHODS: Patients with pathologically confirmed pancreatobiliary cancer (pancreatic ductal adenocarcinoma (PDAC) or cholangiocarcinoma (CCA)) were included. Patients underwent two [(68)Ga]Ga-FAPI-46 PET/CT scans, within 7 days, without any treatment before or between scans. Static long axial field-of-view (LAFOV) PET/CT scans were performed 60 min post-injection of [(68)Ga]Ga-FAPI-46 (EARL2-compliant reconstruction). Suspected malignant lesions were semi-automatically delineated (local background-adjusted 50% isocontour of the peak standardised uptake value (SUV)) and semi-quantitative measurements were extracted (SUV(mean), SUV(peak) and SUV(max)). Tumour-to-background ratios (TBR) were also calculated. Repeatability was assessed using the repeatability coefficient (RC) and the intraclass correlation coefficient (ICC). RESULTS: Twelve patients were included (seven PDAC, three perihilar CCA and two intrahepatic CCA). The median injected dose was 216 MBq (interquartile range (IQR) 167-266 MBq) and the median uptake time was 60 min (range 60-68). In total, 70 FAPI-positive lesions were delineated. The RCs of SUV(mean), SUV(peak) and SUV(max) were 23.7%, 23.9% and 29.8%, respectively. For the blood pool adjusted TBR(mean), TBR(peak) and TBR(max), the RC's were 21.6%, 20.8% and 28.0%, respectively. All ICCs were above 0.98. CONCLUSION: Semi-quantitative measurements of [(68)Ga]Ga-FAPI-46 PET/CT have an excellent repeatability and can potentially be used in future studies to assess treatment response in pancreatobiliary cancers.