Papillary thyroid microcarcinoma with multiple pulmonary metastases following lung cancer surgery: a case report

BACKGROUND: Distant metastasis is extremely rare for papillary thyroid microcarcinoma (PTMC) without lymph node metastasis or extrathyroidal extension, for which active surveillance (AS) is indicated. The evaluation of distant metastases in low-risk PTMC is controversial. A case of PTMC in which AS would have been performed if chest CT and lung surgery had not been performed is reported. CASE PRESENTATION: The patient was a 71-year-old woman undergoing follow-up in the Department of Thoracic Surgery at our hospital for multiple frosted glass shadows in both lung fields for one and a half years. To make a definitive diagnosis, thoracoscopic right middle lobectomy and left upper partial lobectomy were performed 4 and 6 months earlier, respectively. In both resected specimens, lung adenocarcinoma and small metastasis of papillary thyroid carcinoma (PTC) were found. The patient was transferred to our department for a thorough examination for PTC. Ultrasonography was performed to search for the primary lesion, and it showed an irregular hypoechoic mass of 4 mm and 6 mm in the middle of the right lobe of the thyroid gland. The patient was diagnosed with PTC. Its clinical stage was T1a (m) N0 M1 (stage IVC). Total thyroidectomy and prophylactic central node dissection were performed. The pathological diagnosis was PTC (typical type) pT1a (m) N0. Postoperatively, she received radioactive iodine therapy. CONCLUSIONS: We experienced an extremely rare case and struggled to determine a treatment plan. We might be aware that lung metastases could develop in low-risk PTMC.