F-18 FDG PET/CT imaging in staging and response assessment in children with langerhans cell histiocytosis.

Objectives: Langerhans cell histiocytosis (LCH) is a rare disease which involves the clonal proliferation of activated dendrocytes and macrophages, infiltrating a variety of organs. LCH has a wide spectrum of clinical manifestations, ranging from spontaneous resolution to rapid progression and death. It may occur at any age but generally affects children between 1 and 15 years. Skeletal survey and bone scintigraphy are initial investigations for skeletal evaluation. FDG PET/CT is a powerful tool for making an early diagnosis; it allows higher diagnostic confidence for measuring the extent of disease (staging) and assessing disease activity, and is consequently useful for evaluating the response to therapy in patients with LCH. However, its clinical implication has not been well elucidated, mainly due to very low incidence of LCH. The aim of this study was to evaluate the clinical usefulness of F-18 FDG PET/CT in LCH patients.

Methods: Thirty-eight children (mean age= 3.15 ± 1.95 years; range- 9 months to 12 years) who were diagnosed histopathologically as LCH were enrolled in the study. For initial diagnosis and evaluating extent of disease, skeletal survey and F-18 FDG PET/CT were included in the work-up for diagnosis of LCH. These patients underwent F-18 FDG PET/CT before and after the first line chemotherapy. Results: Eighteen children (47.3%) had single-system (single site and multiple sites) and 20 children (52.6%) multisystem involvements, respectively. Seven children had single site involvement, five of them showed complete metabolic response at the end of treatment. In 15 children, F-18 FDG PET/CT detected additional active LCH lesions not identified on conventional imaging modalities. At the end of treatment imaging, F-18 FDG PET/CT showed partial response in 19 children (50 %), complete metabolic response in 14 (36.8 %), stable disease in 2 and progression in 3 children. In follow-up F-18 FDG PET/CT scans, complete resolution was identified in 10 patients and reactivation in another 4 patients. Conclusions: PET has been shown to be superior to bone scintigraphy and skeletal survey for overall lesion detection. Patients with LCH involving only the bones have a significantly better outcome than those with other organ involvement. Results of this study suggest that F-18 FDG PET/CT is useful for identification of active lesions, stratification of disease stages, monitoring of therapeutic response, and detection of reactivation in LCH patients.