[⁶⁸Ga]Ga‑FAPI PET/CT in the evaluation of Langerhans cell histiocytosis: comparison with [¹⁸F]FDG PET/CT

Purpose: We aimed to explore the value of [⁶⁸Ga]Ga‑FAPI PET/CT for the evaluation of Langerhans cell histiocytosis (LCH) in comparison with [¹⁸F]FDG PET/CT.

Methods: Thirty-two patients pathologically diagnosed with LCH were enrolled in this study. [⁶⁸Ga]Ga‑FAPI and [¹⁸F]FDG PET/CT were performed within 1 week to identify disease extent and status. The detectability and intensity of the involved organs were compared between these two tracers.

Results: Thirty patients had active disease while two had non-active disease. In patients with active disease, the most commonly involved organ was bone (27/30), and [⁶⁸Ga]Ga-FAPI PET/CT detected more osseous lesions (106/106) than [¹⁸F]FDG PET/CT (52/106). [⁶⁸Ga]Ga-FAPI also identified liver, skin, and salivary gland involvement, which were often missed by [¹⁸F]FDG. Although both tracers detected thymus and pituitary gland involvement, [⁶⁸Ga]Ga-FAPI demonstrated higher image contrast and more diagnostic confidence. Lymph node involvement, however, was not visualized by [⁶⁸Ga]Ga-FAPI. Due to the superior sensitivity of [⁶⁸Ga]Ga-FAPI, approximately 30% (10/30) of patients experienced reclassification in disease status or subtype. Furthermore, [⁶⁸Ga]Ga‑FAPI appeared to be advantageous in response assessment.

Conclusion: [⁶⁸Ga]Ga-FAPI PET/CT outperforms [¹⁸F]FDG PET/CT in detecting osseous and extra-nodal lesions in LCH, providing a valuable tool for precise disease evaluation and treatment planning.