Increased uptake of ⁶⁸Ga-DOTA-FAPI-04 in bones and joints: metastases and beyond

To describe the uptake of ⁶⁸Gallium-labelled fibroblast activation protein inhibitor (⁶⁸Ga-FAPI) in the bones and joints for better understanding of the role of ⁶⁸Ga-FAPI PET in benign and malignant bone lesions and joint diseases.

All 129 ⁶⁸Ga-FAPI PET/MR or PET/CT scans from June 1, 2020, to February 20, 2021, performed at our PET center were retrospectively reviewed. Foci of elevated ⁶⁸Ga-FAPI uptake in the bones and joints were identified. All lesions were divided into malignant and benign diseases. Benign lesions included osteofibrous dysplasia, periodontitis, degenerative bone diseases, arthritis, and other inflammatory or trauma-related abnormalities. The number, locations, and SUVmax of all lesions were recorded and analyzed. The detectability of ⁶⁸Ga-FAPI PET and ¹⁸F-FDG PET in patients who had two scans was also compared.

Elevated uptake of ⁶⁸Ga-FAPI in/around the bones and joints was found in 82 cases (63.57%). A total of 295 lesions were identified, including 94 (31.9%) malignant lesions (all were metastases) and 201 (68.1%) benign lesions. The benign lesions consisted of 13 osteofibrous dysplasia, 48 degenerative bone disease, 33 periodontitis, 56 arthritis, and 51 other inflammatory or trauma-related abnormalities. The spine, shoulder joint, alveolar ridge, and pelvis were the most commonly involved locations. Bone metastases were mainly distributed in the spine, pelvis, and ribs. Among benign diseases, periodontitis and arthritis are site-specific. The mean SUVmax of bone metastases was significantly higher than that of benign diseases (7.14 ± 4.33 vs. 3.57 ± 1.60, p < 0.001), but overlap existed. The differences in SUVmax among subgroups of benign diseases were statistically significant (p < 0.001), with much higher uptake in periodontitis (4.45 ± 1.17). ⁶⁸Ga-FAPI PET identified much more lesions than ¹⁸F-FDG PET (104 vs. 48) with higher uptake value.

⁶⁸Ga-FAPI accumulated in both bone metastases and some benign diseases of the bones and joints. Although the uptake of ⁶⁸Ga-FAPI was often higher in bone metastases, this finding cannot be used to distinguish between benign and malignant lesions. ⁶⁸Ga-FAPI PET also has the potential to locate and evaluate the extent of both malignant tumor and benign diseases in bones and joints.

NCT04554719, NCT04605939. Registered September 8, 2020 and October 25, 2020-retrospectively registered, http://clinicaltrails.gov/show/NCT04554719 ; http://clinicaltrails.gov/show/NCT04605939.