Metastases of renal cell carcinoma to the thyroid gland with synchronous benign and malignant follicular cell-derived neoplasms.

Clear cell renal cell carcinoma (CCRCC) is the most common origin for metastasis in the thyroid. A 51-year-old woman was referred to our hospital for a subcarinal lesion. Ten years before, the patient had undergone a nephrectomy for CCRCC. Whole-body fluorodeoxyglucose positron emission tomography revealed elevated values in the thyroid gland, while the mediastinum was normal. An endoscopic ultrasonography-guided fine-needle aspiration biopsy of the mediastinal mass was consistent with CCRCC, and this was confirmed after resection. The thyroidectomy specimen also revealed lymphocytic thyroiditis, nodular hyperplasia, one follicular adenoma, two papillary microcarcinomas, and six foci of metastatic CCRCC involving both thyroid lobes. Curiously two of the six metastatic foci were located inside two adenomatoid nodules (tumor-in-tumor). The metastatic cells were positive for cytokeratins, CD10, epidermal growth factor receptor, and vascular endothelial growth factor receptor 2. No BRAF gene mutations were found in any of the primary and metastatic lesions. The patient was treated with sunitinib and finally died due to CCRCC distant metastases 6 years after the thyroidectomy. In CCRCC patients, a particularly prolonged survival rate may be achieved with the appropriate therapy, in contrast to the ominous prognosis typically found in patients with thyroid metastases from other origins.