Renal cell carcinoma metastatic to the thyroid.

Metastatic renal cell carcinoma to the thyroid is an uncommon occurrence that can cause clinical and pathologic problems in diagnosis. The authors report seven cases from the files of The University of Texas M.D. Anderson Cancer Center, Houston, Texas. Each presented clinically as a palpable thyroid nodule months or years after the primary renal cell carcinoma had been resected. Although renal cell carcinoma is more common in men, we found a female predominance of 6:1 in this series. These lesions appear as solitary "cold" nodules on iodine 131 scans and may be misdiagnosed as primary thyroid neoplasms, especially if the renal primary is still unrecognized. A correct diagnosis is important because surgical management can be curative. The diagnosis is facilitated by the use of fat stains, electron microscopic study, and immunoperoxidase stains.