68GaGa-FAPI versus 2-18FFDG PET/CT in patients with autoimmune thyroiditis: a case control study

Abstract Purpose Radiolabelled fibroblast activation protein inhibitors (FAPIs) are becoming increasingly important for imaging various tumour diseases. However, it is essential to be aware of potential pitfalls. Here, we investigate FAP expression in the thyroid gland in autoimmune thyroiditis (AIT). Methods AIT patients with pathological thyroid uptake on 68 GaGa-FAPI PET were compared with glucose metabolism on 2- 18 FFDG PET in terms of SUV max /SUV peak /SUV mean /tissue-to-background ratio (TBR), and with a healthy control group. Results Between September 2019 and July 2021, 6 patients presented with a visually increased thyroid uptake and TBR on 68 GaGa-FAPI PET. In the retrospective clinical work-up, all patients had known or newly diagnosed AIT. Compared to a matched healthy control group, FAP expression and glucose metabolism were significantly increased ( 68 GaGa-FAPI (SUV peak ): 7.0 vs. 1.7; p = 0.004/(TBR bloodpool ): 6.8 vs. 1.7; p = 0.002; 2- 18 FFDG (SUV peak ): 3.9 vs. 1.4; p = 0.004/(TBR bloodpool ): 4.0 vs. 1.2; p = 0.041). However, there was no significant difference in median uptake between 68 GaGa-FAPI and 2-18FFDG PET (SUV peak : 7.3 vs. 5.6; p = 0.104). Conclusion Patients with AIT show higher thyroid uptake on 68 GaGa-FAPI and 2- 18 FFDG PET. Incidental thyroid uptake is another pitfall in the interpretation of 68 GaGa-FAPI PET and should prompt a clinical work-up.