The thyroid gland is a rare site of clinically detectable tumor metastasis and kidneys are frequently the site of the parent malignancy. In the present review on thyroid metastases from renal cell carcinoma, cases were searched on PubMed by entering the strings: “renal carcinoma [or“hypernephroma”] AND thyroid metastasis/metastases”. Thus, we retrieved a total of 111 cases that were published between 1964 and 2007, a total that became 113 by adding two patients observed by us. The female to male ratio was 1.35:1. The primary renal cancer was almost always unilateral (90%) (with no significant side preference) and only rarely bilateral (9% in men, 4% in women), whereas bilaterality of thyroid metastases was relatively more frequent (28%). Thyroid metastasis from renal cancer was commonly single with a unique node that appears solid and hypoechoic at ultrasonography, approximately 9 years after nephrectomy. Concordance of lateralization was insignificantly greater for the right kidney/right thyroid lobe pattern (54%) than for the left kidney/left thyroid lobe pattern (40%), regardless of gender. Finally, survival was longer for women. Thyroid metastases, even if rare in the clinical practice, must be considered in the differential diagnosis of a thyroid nodule, particularly in patients who have a history of malignancies.

Renal cell carcinoma can recur many years after diagnosis and nephrectomy metastasizing even in uncommon sites, including thyroid gland. Thyroid metastases are extremely rare, the most frequent site of origin are renal tumors. Metastases in thyroid gland appear as painless nodules or masses, "cold" at scintiscan. We report the case of a 67-year-old male patient affected by clear cell renal carcinoma, diagnosed by fine-needle aspiration cytology procedures, and treated with anticancer medical therapy, who noticed after some months a mass in the neck-thyroid region requiring deeper medical investigations. By this way, thyroid fine-needle aspiration cytology reported a lesion made of malignant epithelial cells compatible with metastases of renal carcinoma (clear cell). Clinical and pathological data, together with immunostaining, supported the diagnosis of metastatic clear cell renal carcinoma. The diagnosis of metastatic disease, although difficult clinically and pathologically, should be suspected in patients with a clinical history of cancer, particularly in case of renal cell carcinoma, but fine-needle aspiration cytology can provide the clue for diagnosis.

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