Tumor-to-tumor metastasis: lung adenocarcinoma metastasizing to a follicular variant of papillary thyroid carcinoma

Cancer-to-cancer metastasis into a thyroid neoplasm is an uncommon phenomenon with possible diagnostic difficulties. Here, we describe a case of lung adenocarcinoma metastatic into a follicular variant of papillary thyroid carcinoma (FVPTC). A 60-year-old woman with no prior history of malignant neoplasm presented with a nodule in the right lobe of the thyroid gland, some masses in the left lung were found by radiological examination. Histopathological examination of the thyroidectomy specimen demonstrated two different components of carcinoma in a single thyroid nodule; one was FVPTC and the other was high-grade adenocarcinoma. Although both components shared the TTF-1+/CK7+/CK19+/CK20-/SP-A- immunoprofile, only the former was positive for thyroglobulin, and only the latter was positive for CEA. The epidermal growth factor receptor (EGFR) gene mutation at exon21 (L858R) was present only in the latter. The lung biopsy specimen showed cytological, immunohistochemical, and EGFR genotypic features similar to those of the high-grade adenocarcinoma component of the thyroid nodule. These findings resulted in a reliable diagnosis of lung adenocarcinoma metastasizing into an FVPCT and treatment with EGFR-targeted therapy. These results demonstrate that a panel of immunohistochemical staining and molecular analysis is helpful for both diagnosis and appropriate postoperative treatment for a patient with cancer-to-cancer metastasis.