⁶⁸Ga-FAPI PET/CT features of thoracic sarcoidosis: a prospective observational study

Sarcoidosis is a multisystem granulomatous disease that most commonly affects the lungs. ¹⁸F-2-fluoro-2-deoxyglucose (¹⁸F-FDG) PET/CT is a sensitive imaging modality for assessing of sarcoidosis. However, fasting before ¹⁸F-FDG PET/CT is generally required to improve image quality and diagnostic accuracy. In this context, our study aims to explore the utility of ⁶⁸Ga-fibroblast activation protein inhibitors (⁶⁸Ga-FAPI) PET/CT in sarcoidosis, with particular emphasis on lesion uptake patterns and intensity, as well as the correlation with disease activity.

This is a prospective, single-center, observational study conducted at the Peking Union Medical College Hospital. Eight patients with newly diagnosed thoracic sarcoidosis were consecutively recruited for ⁶⁸Ga-FAPI PET/CT between January 2024 and December 2024. The follow-up ⁶⁸Ga-FAPI PET/CT imaging was scheduled 6-12 months after standard treatment. In addition, we retrospectively analyzed ¹⁸F-FDG PET/CT scans from 16 sarcoidosis patients. For both imaging groups, the maximum and mean standardized uptake values (SUVmax and SUVmean, respectively) and corrected lesion-to-background ratio (LBR) were calculated.

⁶⁸Ga-FAPI PET/CT imaging detected FAPI-avid lesions in all 8 patients, including enlarged lymph nodes (8 patients, SUVmax = 7.78 ± 3.98, LBR = 7.4 ± 5.9), pulmonary infiltrates (6 patients, SUVmax = 5.35 ± 1.15, LBR = 10.4 ± 3.8), spleen nodules (2 patients) and hepatic nodules (1 patient). Following treatment, the follow-up ⁶⁸Ga-FAPI PET/CT scans showed lesion shrinkage and decreased uptake parameters. However, some pulmonary lesions showed non-significant changes of SUV or LBR. Addtionally, the average SUVmax, SUVmean, and LBR for ⁶⁸Ga-FAPI of lymph nodes tended to be lower than ¹⁸F-FDG uptake parameters in the retrospective cohort.

⁶⁸Ga-FAPI PET/CT can be useful for evaluating both active and chronic granulomatous inflammation. It is also potentially valuable for determining disease extent, detecting occult disease, and monitoring treatment response in sarcoidosis.