Langherans Cell Histiocytosis of the Thyroid Gland Detected by 18FDG-PET/CT

Focal accumulation of fluorodeoxyglucose (FDG) into the thyroid is detected in 4% of patients examined for nonthyroid tumors by positron emission tomography (PET) imaging. Up to 40% of these patients have malignant tumors, mainly arising from follicular cells (1). Benign nodules (i.e. follicular adenomas) may accumulate FDG also, but FDG-negative nodules are very unlikely to be malignant (2, 3). Langherans cell histiocytosis (LCH) is a rare monoclonal disease of histiocytes with an overall 3% mortality in adults, rarely involving the thyroid gland alone or as part of multisystemic disease (4). We observed a 43-yr-old female affected by breast carcinoma with an incidentally discovered thyroid enlargement. Clinical examination revealed a 20-mm hard nodule in the left thyroid lobe. Serum TSH was normal and ultrasoundguided fine-needle aspiration biopsy was directly performed by a dedicated cytopathologist (21-gauge needle, three passes). The cytological examination showed few thyrocytes without malignant findings, some lymphocytes and macrophages, and was classified as nondiagnostic (thy1) according to the British Thyroid Association 2002 recommendations. The role of fineneedle aspiration biopsy repetition in these cases is debated and, based on its high negative predictive value for both thyroid and breast carcinomas, an 18FDG-PET/computed tomography scan was performed (5, 6). A focal FDG uptake was shown in the left thyroid nodule (SUVmax 8.45) (Fig. 1) without other pathological FDG-positive areas. Left lobectomy was then performed, and thyroid LCH was demonstrated by histological examination and immunostaining against S-100 protein. Thyroid involvement by LCH is rare, and the diagnosis may often be extremely difficult (7). Neither ultrasound nor scintigraphy provides specific findings for LCH, and the final diagnosis is generally reached through cytological or histological examination (8). The high FDG uptake found in the present case reflected the enhanced metabolic rate of LCH cells. Clearly, FDG uptake is not specific for LCH; however, according to recently reported data, malignant disease should be excluded in FDGpositive thyroid nodules (9). To our knowledge, we first reported here the FDG uptake by LCH; this is of interest for nuclear medicine physicians involved in PET imaging and, additionally, may suggest further evaluations of FDG imaging in LCH assessment.