[⁶⁸Ga]Ga-FAPI versus 2-[¹⁸F]FDG PET/CT in patients with autoimmune thyroiditis: a case control study

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).

AIT patients with pathological thyroid uptake on [⁶⁸Ga]Ga-FAPI PET were compared with glucose metabolism on 2-[¹⁸F]FDG PET in terms of SUV_max/SUV_peak/SUV_mean/tissue-to-background ratio (TBR), and with a healthy control group.

Between September 2019 and July 2021, 6 patients presented with a visually increased thyroid uptake and TBR on [⁶⁸Ga]Ga-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 ([⁶⁸Ga]Ga-FAPI (SUV_peak): 7.0 vs. 1.7; p = 0.004/(TBR_bloodpool): 6.8 vs. 1.7; p = 0.002; 2-[¹⁸F]FDG (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 [⁶⁸Ga]Ga-FAPI and 2-[18F]FDG PET (SUV_peak: 7.3 vs. 5.6; p = 0.104).

Patients with AIT show higher thyroid uptake on [⁶⁸Ga]Ga-FAPI and 2-[¹⁸F]FDG PET. Incidental thyroid uptake is another pitfall in the interpretation of [⁶⁸Ga]Ga-FAPI PET and should prompt a clinical work-up.