Tumor-to-tumor metastasis of renal cell carcinoma into oncocytic carcinoma of the thyroid. Report of a case and review of the literature.

A case of a multifocal oncocytic carcinoma of the thyroid with intratumoral metastases of renal cell carcinoma in a 52-year-old male is reported. Thirteen months before the presentation of the thyroid lesion, the patient underwent nephrectomy for a renal cell carcinoma. The thyroid tumor clinically presented as a palpable nodule. Preoperative fine-needle aspiration cytology showed two cell types: oncocytes and multivacuolated clear cells. The cytologic features were interpreted as a coincidence of an oncocytic tumor of the thyroid and metastasis of a clear cell renal carcinoma. The diagnosis was confirmed by histologic and immunohistochemical examinations. Interestingly, except for metastases within multiple foci of the oncocytic carcinoma, there were no metastatic deposits in nontumoral thyroid. Although the occurrence of tumor-to-tumor metastasis in thyroid gland is exceptionally rare, with only 12 such cases reported to date, one should be aware of this phenomenon to avoid an incorrect diagnosis. To the best of our knowledge, this is the first report of a metastasis into tumor of the thyroid gland, with oncocytic carcinoma being the recipient.