Thyroid Langerhans Cell Histiocytosis: A Histopathologic Mimicker of Anaplastic Carcinoma.

A 50-year-old man presented with a rapidly growing neck mass. Initial cytology suggested anaplastic thyroid carcinoma (ATC), necessitating urgent surgery. Preoperative imaging revealed a thyroid lesion with a "moth-eaten" appearance; FDG-PET/CT revealed unexpected uptake around the sternocleidomastoid muscle and at the maxilla, with an osteolytic lesion preserving tooth roots. During surgery, atypical infiltration for ATC prompted reevaluation, preventing unnecessary radical procedures. Pathologic examination revealed Langerhans cell histiocytosis (LCH), confirmed by CD1a staining. LCH of adult thyroid is rare, mimicking ATC clinically and cytologically. This case highlights that identifying systemic clues with FDG-PET may enable accurate diagnosis, preventing unnecessary radical surgery.