Metastatic renal cell carcinoma to the thyroid gland: a clinicopathologic study of 36 cases.

Clear cell tumors of the thyroid gland in general are uncommon. Metastatic renal cell carcinoma (RCC) to the thyroid gland is a rare occurrence but must be considered in the differential diagnosis of a thyroid gland clear cell neoplasm to prevent misclassification, potentially resulting in inappropriate clinical management. Thirty-six cases of metastatic RCC to the thyroid were retrospectively retrieved from the files of the Endocrine Registry of the Armed Forces Institute of Pathology. The tumors occurred in 22 women and 14 men, ages 53-80 years (mean, 64.9 years). Symptoms were present for a mean of 13.0 months. The tumors generally affected a single lobe of the thyroid gland as a solitary mass (n = 30; 83%), measuring 1.0-15.0 cm in diameter (mean, 3.8 cm). Histologically, the tumors were composed of polygonal cells with clear cytoplasm, distinct cell membranes, and small compact eccentric nuclei within a rich vascular network. Diastase-sensitive, periodic acid-Schiff-positive material (n = 22; 61%) and/or Oil Red O-positive material (n = 5; 14%) were noted. Thyroglobulin immunohistochemistry was negative in the foci of metastatic RCC. Although the majority of the patients had documented previous evidence of an RCC (n = 23; 64%) as remotely as 21.8 years before the thyroid metastases (mean, 9.4 years), the metastatic tumor to the thyroid gland was the initial manifestation of RCC in 13 patients. The majority of patients (n = 23; 64%) died with disseminated disease (mean, 4.9 years), but 13 patients (36%) were alive or had died without evidence of disease (mean, 9.1 years). In the presence of a clear cell tumor of the thyroid gland, the diagnostic considerations must include metastatic RCC. The light microscopic features may suggest this possibility and the diagnosis can be established by supplemental histochemical and immunohistochemical studies. Surgical treatment of the metastatic disease is suggested, as this may result in prolonged patient survival.