^18-F-FDG PET/CT in Localizing Additional CNS Lesion in a Case of Langerhans Cell Histiocytosis: Determining Accurate Extent of the Disease

A 6-year-old boy came to the pediatrics OPD with chief complaints of generalized bodyache with a stinging-like sensation throughout the body for 6 months. He had also developed burning micturition since 3 months with increased frequency of micturition. On further examination, generalized erythematous skin rashes were noted. The patient was referred for skin biopsy that revealed features of Langerhans cell. Immunohistochemistry was positive for CD1. The patient was referred for an 18-F-Fludeoxyglucose Positron Emission Tomography-Computed Tomography (FDG PET/CT) scan that revealed multiple FDG avid lytic lesions involving skull bones and tibia with increased FDG uptake. Incidentally, a focus of FDG uptake was noted in the hypothalamus in brain [Figure 1]. For further confirmation of the lesion, MRI was performed.

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Figure 1 18-F-Fludeoxyglucose Positron Emission Tomography–Computed Tomography (FDG PET/CT) scan revealed multiple FDG avid lytic lesions involving skull bones and tibia with increased FDG uptake. Incidentally, a focus of FDG uptake was noted in the hypothalamus in brain

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Langerhans cell histiocytosis (LCH) is a rare neoplastic disease of antigen-presenting cells, with an incidence rate of 4.0-5.4/1 million individuals.[1] The most frequent sites of occurrence are skin, bone, and central nervous system (CNS).[2] CNS involvement in LCH can be classified into tumorous, non-tumorous, and atrophic lesions. Tumorous lesions can include involvement of hypothalamus, hypothalamic-pituitary axis such as enhancement and thickening of pituitary stalk, enlargement of pineal gland, extra-axial space involvement, and intra-cerebral granulomatous lesions. Non-tumorous lesions include involvement of dentate nucleus and deep white matter changes.[3] Because the lesions are characterized by proliferation of histiocytes, they commonly show increased 18F-FDG uptake.[4] Thus, 18F-FDG PET/CT helps in detecting the extent of involvement (staging) of the disease. It helps in choosing the treatment options and is also useful in prognostication of the disease.[5] Previous reports of LCH in FDG PET/CT imaging mostly show bone, lung, lymph node, or skin invasion. To the best of our knowledge, only one case reported by Kruljac et al.[6] described hypothalamic involvement of LCH on FDG PET-CT. Thus, the present case gives an insight into the appearance of a rare disease, which can serve as a baseline study for the assessment of treatment response later and further establish the role of FDG PET-CT as a whole body imaging modality in staging of LCH.