Fine-needle aspiration cytology in a case of isolated involvement of thyroid with Langerhans cell histiocytosis.

We present an unusual case of isolated thyroid involvement with Langerhans cell histiocytosis (LCH), diagnosed presumptively by fine-needle aspiration (FNA) cytology and subsequently confirmed by immunohistochemistry in a 13-yr-old boy. The cytologic findings include high cellularity in a hemorrhagic background, the presence of characteristic mononucleated and multinucleated Langerhans cells with prominent nuclear grooves, and abundant foamy cytoplasm in a background of mixed eosinophilic and aggregates of benign thyroid follicular cells. Further diagnostic confirmation was obtained by positive S-100 protein immunohistochemistry of the Langerhans histiocytes on paraffin-embedded sections of open thyroid biopsy of the prominently enlarged left lobe. We present the complete clinicopathologic features of this case, along with ultrasound, computerized axial tomographic, and technetium-99 scan findings. We also discuss possible differential diagnostic consideration in light of a review of the literature and the role of FNA cytologic diagnosis in such a rare yet cytomorphologically characteristic case.