An 18-month-old otherwise healthy girl presented with 1 month of neck swelling. Based on ultrasonography that showed diffusely enlarged heterogeneous thyroid gland, a presumed diagnosis of thyroid cancer was made. Subsequent core needle biopsy revealed Langerhans cell histiocytosis extensively involving the thyroid. 18 F-FDG PET/MR was performed for staging and to evaluate the local extent of the disease in the neck. PET/MR demonstrated a hypermetabolic neck mass inseparable from the thyroid gland. The mass encased the major vessels, trachea, and esophagus without compression or invasion. Osseous involvement was excluded by both skeletal survey and PET/MR.
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