Histological analysis
Two independent investigators (KS and KO) observed pathological hematoxylin-eosin staining samples under the microscope in 12 of 19 cases. Immunohistochemical staining of calcitonin was conducted in 2 of 12 cases, while no positive case was found. The pattern of the epithelium, structure of the wall, and contents in the lumen of the fistula were evaluated. Moreover, the characteristics of inflammation cells and thyroid, thymic, and parathyroid tissues around the fistula were also analyzed.