Pitfalls and Common Findings in 68 Ga-FAPI PET: A Pictorial Analysis

Lukas Kessler, Justin Ferdinandus, Nader Hirmas, Fadi Zarrad, Michael Nader, David Kersting, Manuel Weber, Sandra Kazek, Miriam Sraieb, Rainer Hamacher, Katharina Lueckerath, Lale Umutlu, Wolfgang P Fendler, Christoph Rischpler

Abstract: Fibroblast activation protein inhibitor (FAPI) PET/CT is a new tool in the diagnostic workup of cancer. With a growing volume of applications, pitfalls and common findings need to be considered for ⁶⁸Ga-FAPI PET/CT image interpretation. The aim of this study was to summarize common findings and report pitfalls in ⁶⁸Ga-FAPI PET/CT.

Methods: Ninety-one patients underwent whole-body PET/CT with either FAPI-04 (n = 25) or FAPI-46 (n = 66). Findings were rated in a consensus session of 2 experienced readers. Pitfalls and common findings were defined as focal or localized uptake above the background level and categorized as unspecific or nonmalignant and grouped into degenerative, muscular, scarring/wound-healing, uterine, mammary gland, and head-and-neck findings. The frequency of findings was reported on a per-patient and per-group basis, and SUV_max, SUV_mean, and SUV_peak were measured.

Results: Non-tumor-specific uptake was found in 81.3% of patients. The most frequent finding was uptake in degenerative lesions (51.6%), with a mean SUV_max of 7.7 ± 2.9, and head-and-neck findings (45.1%). Except for the salivary glands, the uptake values did not differ between 10 and 60 min after injection in most findings. Uterine uptake was found in most women (66.7%), with a mean SUV_max of 12.2 ± 7.3, and uptake correlated negatively with age (SUV_max, r = -0.6, P < 0.01; SUV_peak, r = -0.57, P < 0.01; SUV_mean, r = -0.58, P < 0.01).

Conclusion: Pitfalls include non-tumor-specific ⁶⁸Ga-FAPI uptake in degenerative lesions, muscle, the head and neck, scarring, the mammary glands, or the uterus. Here, we summarize the findings to help readers avoid common mistakes at centers introducing ⁶⁸Ga-FAPI PET/CT.