Diagnostic difficulties in oncocyte-containing renal carcinoma

Two cases of renal tumor presenting diagnostic difficulties are reported. In one case, fine-needle aspiration biopsy from a large neoplasm yielded only oncocytes, but histologic examination showed also islets of renal cell carcinoma. The tumor caused pain, and computed tomography demonstrated inhomogeneity of its inner composition. In the second case there was a silent, small neoplasm, and both cytologic and histologic specimens showed a mosaic pattern of renal cell carcinoma and oncocytes. The diagnostic difficulties are discussed on the basis of these two cases and reports in the literature.