Solitary thyroid metastasis of renal clear cell carcinoma: report of a case.

A case of solitary thyroid metastasis of renal clear cell carcinoma is described. The patient was a 77-year-old Japanese woman, who was referred to our department after a thyroid tumor was identified in May 1999. She had a history of renal clear cell carcinoma of the left kidney, which had been partially resected 3 years previously. Ultrasound sonography demonstrated that a well-demarcated hypoechoic mass containing high-echo spots representing small calcifications, which measured 45 x 34 x 31 mm in size, occupied the left lobe. Computed tomography revealed a low-density mass containing small calcifications. The results of preoperative fine-needle aspiration cytology strongly suggested a clear cell carcinoma metastasizing to the thyroid. A left hemithyroidectomy was performed on July 27, 1999. A histological examination revealed that the neoplasm was composed of tumor cells with abundant clear cytoplasm and round nuclei. The histological characteristics of this thyroid tumor were virtually identical to the renal cell carcinoma resected 3 years previously. Thyroglobulin stained negatively in the clear cells of the resected thyroid tumor in an immunohistochemical analysis. Clinically, the thyroid gland is a rare site of tumor metastasis; however, we should also consider the possibility of metastasis in the case of thyroid tumor patients with a history of renal cell carcinoma.