Answer: B. Oncocytoma

Histological Description: Focally the tumor is composed of nests with large, round-to-polygonal cells with abundant eosinophilic cytoplasm in fibromyxoid stroma. In this area well-preserved nuclei that have fine chromatin are uniformly round with variably sized nucleoli. The majority of the tumor is composed of cords and irregular nests of pleomorphic cells in a fibromyxoid stroma. The pleomorphic cells have degenerative nuclear atypia with multinucleation, pseudoinclusions, and smudgy chromatin. Mitotic figures and necrosis are absent.

Discussion: Oncocytomas are typically composed of cells are arranged in nests, acini, tubules or microcysts set in a fibromyxoid stroma. In some cases lacking fibromyxoid stroma, there are still back-to-back nests; a diffuse growth pattern rules out oncocytoma. In the typical case of oncocytoma, well-preserved nuclei that with fine chromatin are uniformly round with variably sized nucleoli. Pyknotic dark nuclei with crenate irregular edges should be discounted. Although oncocytomas are benign, there are certain histological features that can mimic renal cell carcinoma. Extension into perirenal adipose tissue can be seen in 20% of cases and not an indication of malignancy. Invasion of renal sinus and renal vein branches can rarely be seen in typical oncocytomas and still consistent with a benign clinical course. Cells with degenerative atypia, as seen in this case, can also mimic malignancy. Typically, the cells with degenerative atypia occur in clusters surrounded by more classic appearing oncocytoma. Unusually, as in this case, the degenerative atypia is more diffuse and even more closely can be confused with renal cell carcinoma. Despite the prominent pleomorphism, there are no mitotic figures or necrosis, which would be expected if this was a pleomorphic grade 4 renal cell carcinoma. If one needed reassurance, a ki67 would be negative in the pleomorphic nuclei indicating a degenerative phenomenon.