Renal-cell carcinoma with intranuclear inclusions metastatic to thyroid: a diagnostic problem in aspiration cytology.

A case of renal-cell carcinoma (RCC) exhibiting prominent intranuclear inclusions in a metastasis to the thyroid is presented. RCC had been treated in this patient by right nephrectomy 7 yr earlier and by left partial nephrectomy 5 yr earlier; the patient had also received radiation to the neck 4 yr earlier for carcinoma in situ of the left true vocal cord. Aspiration performed on a rapidly enlarging mass in the left lobe of the thyroid revealed both clusters of tumor cells and single tumor cells with prominent intranuclear inclusions. Definitive distinction between a primary and metastatic neoplasm was difficult on the aspiration specimen. Subsequent thyroidectomy confirmed the diagnosis of metastatic renal-cell carcinoma with intranuclear inclusions by histologic, immunohistochemical, and ultrastructural analysis. The differential diagnosis by aspiration cytology of thyroid neoplasms that contain intranuclear inclusions is discussed.