Papillary Thyroid Carcinoma Cervical Lymph Node Metastasis with Cystic Change Differentiated from Congenital Cystic Lesions with the Assistance of Immunohistochemistry: A Case Study

Diagnosis of cystic papillary thyroid carcinoma (PTC) lymph node metastasis at head neck region can be a challenge in the absence of known PTC history. The congenital cystic lesions of head neck, especially thyroglossal duct cyst (TGDC) and branchial cleft cyst (BCC), are major differential diagnoses in this clinicopathological scenario. The location of cyst and morphology of lining epithelium are critical clues for reaching correct diagnosis. However it is not uncommon that the flattened bland epithelial lining can be seen in both cystic metastases and congenital cystic lesions. Given that Pax8 and TTF-1 are common markers in thyroid follicular epithelium; we applied immunohistochemical stains of those two markers on aforementioned cystic lesions. Here we reported a case of cystic PTC metastasis to lymph node without prior malignancy history and cases of TGDC and BCC. Both Pax8 and TTF-1 stainings highlighted the cyst lining in PTC metastatic lymph node, while they were negative in the lining of TGDC and BCC. Collectively, Pax8 and TTF-1 immunohistochemical studies are very helpful tools for making correct diagnosis of head neck cystic lesions in the challenging clinical cases.