Objective: Langerhans cell histiocytosis (LCH) is a rare disorder of clonal proliferation of dendritic cells usually affecting children. However, it may also appear later in life at any age. The severity of the disease varies widely, and multi-system involvement of the lungs, liver and hematopoietic system may be life threatening. This study evaluated the role of 18F-fluorodeoxyglucose positron emission tomography / computed tomography (FDG PET/CT) in patients with single system and multisystem LCHfor disease extent, restaging and response to therapy. Methods:We retrospectively evaluated the data of 37 patients (27 male, 10 female) aged 21.3 ± 18.9 years, comprising 20 children (aged 1-16 years) and 17 adults (aged 20-69 years) with pathologically confirmed LCH. All patients had undergone whole body FDG PET/CT to assess the extent of disease involvement between January 2011 and May 2019. A total of 52 scans were evaluated by two experienced nuclear medicine physicians. Increased FDG uptake with morphological changes on the corresponding CT images were interpreted as PET/CT positive and absence of FDG uptake or any structural lesion were considered PET/CT negative. The metabolic and morphological tumor status and response after chemotherapy were assessed. Out of 52 PET/CT scans, 25 scans were performed for initial work-up and the rest for restaging, follow-up or response evaluation. The results of PET/CT imaging were compared with CT or magnetic resonance imaging (MRI) reports (if available). Results: Of the 37 patients, 20 had multisystem disease (8 high risk, 12 low risk), while single system involvement was noted in 17 patients (9 unifocal, 8 multifocal). FDG PET/CT was positive in 32/37 and detected skeletal (n=28), lymph nodal (n=13), lung (n=5), liver (n=4), pituitary (n=3), ano-cutaneous (n=2) and thyroid (n=2) lesions. In the two groups, FDG PET/CT identified more lesions compared to CT or MRI in 22 patients, including bone, lymph nodal and ano-cutaneous lesions. Six patients with lung lesions had very low FDG avidity. FDG PET/CT upstaged the disease in 8 patients from single system disease to multisystem involvement (2 primary pituitary, 2 primary thyroid and 4 primary skeletal). PET positive lesions had SUVmax of 8.9 ± 3.7 (range 3.8 - 18.8). In patients with follow-up or response evaluation imaging, a complete metabolic response was noticed in the low risk group (n=7), while disease progression was noted in patients with high risk and multisystem involvement group (n=3). Conclusion:Except for lung lesions, FDG PET/CT is a better imaging modality than conventional imaging (CT/MRI) for initial staging as well as restaging and response evaluation in both single system and multisystem LCH. It also helps in management planning by upstaging the disease from single system to multisystem involvement.

Citation Format: ANISH BHATTACHARYA, RAJENDER KUMAR, BHAGWANT RAI MITTAL. Is there a role for 18F-FDG PET/CT imaging in Langerhans cell histiocytosis? [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr B040. doi:10.1158/1535-7163.TARG-19-B040