A case of primary lung adenocarcinoma mimicking metastatic papillary thyroid carcinoma.

A 61-year-old woman, who had a history of total thyroidectomy for follicular variant of papillary thyroid carcinoma (PTC), visited our hospital for assessment of an enlarging nodule which appeared in the lung with multiple metastatic lesions of PTC which had been stable for 17 years. Wedge resection of the lung was performed. Miliary nodules were confirmed to be metastatic PTCs based on their morphological as well as immunohistochemical findings. As for the main nodule, its morphological features suggested a diagnosis of metastatic PTC, while its immunohistochemical findings were identical with primary lung adenocarcinoma. Further genetic analysis provided no definitive information for the diagnosis of the main nodule. The present case shows the need of comprehensive analyses for differentiation between primary lung adenocarcinoma and metastatic PTCs.