Incidental Pulmonary Metastases Revealing Subcentimeter Papillary Thyroid Carcinoma

ABSTRACT Objective Here we present 2 cases of papillary thyroid microcarcinomas (PMCs) that had metastasized at presentation. The 2015 American Thyroid Association and the American College of Radiology Thyroid Imaging Reporting and Data System (TI-RADS) criteria do not recommend biopsy of the majority of subcentimeter thyroid nodules, as PMCs are mostly indolent with excellent prognosis. However, the paradigm of active surveillance presents a conundrum on how to identify the rare patient with distant metastatic disease while avoiding unnecessary intervention in the majority. Methods After initial discovery of incidental lesions on chest computed tomography, core or wedge biopsies of the lung lesion were performed. Thyroid nodules on ultrasound were classified by TI-RADS. Tumor DNA was sequenced, annotated, filtered on 119 known cancer genes, and filtered for variants with an exome allele frequency of <0.001. Results A 70-year-old woman and a 29-year-old woman presented with incidental pulmonary lesions on computed tomography scan. Lung biopsy revealed lung metastases from papillary thyroid carcinoma. The thyroid nodules in both patients were TI-RADS 3 and American Thyroid Association low-suspicion. Molecular testing showed a c.1721C>G mutation (p.Thr574Ser) in the TSHR gene in patient 1 and a codon 61 mutation in the NRAS gene in patient 2. Both patients were iodine-avid, with complete structural remission in one patient and ongoing treatment with evidence of structural response in the other. Conclusion The 2 presentations demonstrate unexpected and concerning behavior of PMCs. Both thyroid tumors were subcentimeter in diameter, meaning they would have escaped detection using traditional risk-stratification algorithms in active surveillance. Further knowledge of tumor genetics and microenvironment may assist in predicting tumor behavior in PMCs. Abbreviations ATA American Thyroid Association CT computed tomography PMC papillary thyroid microcarcinoma PTC papillary thyroid carcinoma TI-RADS Thyroid Imaging Reporting and Data System