Unsuspected metastatic renal cell carcinoma diagnosed by fine needle aspiration biopsy. A report of four cases with immunocytochemical contributions.

Fine needle aspiration biopsy (FNAB) is currently considered a valid procedure in the diagnosis of various primary and metastatic neoplasms. It is also known that computed tomography (CT)-guided percutaneous FNAB of the kidney is very useful in diagnosing primary renal cell carcinoma (RCC) and has a high accuracy rate. Nonetheless, its usage in the detection of unsuspected metastatic RCC has been described rarely. Below we report four unusual cases of metastatic RCC discovered by FNAB. The patients presented with subcutaneous, pulmonary, adrenal and flank masses with no previous history of RCC. Immunocytochemical (ICC) stains, including cytokeratin, epithelial membrane antigen, vimentin and fat stain, were obtained on two cases and were very helpful in establishing the diagnosis. We suggest that cytopathologists consider metastatic RCC a possibility when evaluating patients with tumors of unknown origin and that FNAB can be useful in diagnosing unsuspected metastatic RCC, especially when assisted by ICC and fat stain.