68 Ga-FAPI and 18 F-FDG PET/CT Imaging in Langerhans Cell Histiocytosis for Recurrence and Therapeutic Response Assessment.

A 2-and-a-half-year-old boy with a previous history of Langerhans cell histiocytosis reached remission as assessed by 18 F-FDG PET/CT. Routine follow-up 18 F-FDG PET/CT revealed the appearance of new bone lesions in the right scapula and right ischium with abnormal FDG activity 3 months after the end of treatment, and Langerhans cell histiocytosis recurrence was considered. Meanwhile, these lesions also showed increased FAPI uptake. After adjusting the chemotherapy regimen, both FDG and FAPI uptake almost disappeared in these lesions.