Occult Pulmonary Micrometastases From a Papillary Thyroid Carcinoma Undetectable by Imaging

Pulmonary metastases from papillary thyroid carcinoma (PTC) are usually detectable by computed tomography (CT) or radioactive iodine (RAI) scanning. Pathologically proven pulmonary micrometastases that are completely undetectable by imaging have rarely been reported. A 39-year-old woman underwent surveillance CT four years after total thyroidectomy for PTC. A solitary right pulmonary nodule, which had been present since before thyroid surgery and had gradually enlarged, was clinically suspected to be metastatic. Wedge resection of the lung revealed the nodule to be a benign hemangioma. However, careful histopathological examination incidentally identified multiple tiny nodules measuring 200-1,000 μm in the surrounding lung parenchyma. These lesions exhibited characteristic features of PTC and were positive for thyroglobulin on immunohistochemistry, confirming pulmonary micrometastases. A retrospective CT review and postoperative RAI scintigraphy failed to detect these lesions. The patient received RAI therapy and remains disease-free 126 months after pulmonary resection. This case demonstrates that pulmonary micrometastases from PTC may exist despite the absence of radiological or scintigraphic findings. Occult micrometastatic disease may be more prevalent than clinically appreciated, highlighting a diagnostic limitation of imaging-based surveillance.