Answer: Oncocytoma

Histology: The tumor shows in areas the classic pattern of oncocytoma where cells are arranged in nests, acini, tubules or microcysts in a fibromyxoid stroma. The cells are large, round-to-polygonal with abundant eosinophilic cytoplasm. This case is notable for the scattered cells with degenerative nuclear atypia with multinucleation and pseudoinclusions. The atypical cells are in areas clustered and in other areas more diffuse. Apart from the atypical cells, the background cells of the lesion have round nuclei with variably sized nucleoli.

Discussion: The presence of cells arranged in nests, acini, tubules or microcysts in a fibromyxoid stroma pattern is so characteristic that >95% of the time when present, the diagnosis is oncocytoma. Oncocytomas have well-preserved nuclei that have fine chromatin which are uniformly round with variably sized nucleoli. Pyknotic dark nuclei with crenated irregular edges should be discounted. The finding of cells with degenerative nuclear atypia, where in areas the atypical cells are clustered, is typical of some oncocytomas. In contrast to a grade 4 renal cell carcinoma, the atypia consists of large nuclei with smudgy chromatin, pseudoinclusions, lacking mitotic activity. The lack of necrosis also is inconsistent with a true high grade renal cell carcinoma. The tumor lacks prominent, plant-like, thick cell membranes and small crinkly irregular nuclei seen in chromophobe RCC. Extra-renal extension can be seen in 20% of oncocytomas and is not an indication of malignancy. Invasion of renal sinus and renal vein branches can rarely be seen in typical benign oncocytomas. The only helpful immunohistochemical stain is CK7, with patchy focal staining typical. In contrast, clear cell RCC is typically CK7 negative and Chromophobe usually has more diffuse CK7 immunoreactivity.