Gross: classically described as "mahogany brown with a central stellate scar"; hemorrhage and necrosis are rare.
Microscopic:
cells are arranged in diffuse sheets or may be "packeted" in small nests within a loose edematous stromal background -- some have noticed a similarity to a chain of islands in the ocean, and have referred to this as an "archipelaginous architecture"
the cytoplasm is finely granular and very eosinophilic
nuclei are round with inconspicuous nucleoli (rare occasional bizarre nuclei can be seen) -- in fact, the very bland nuclei in oncocytoma help distinguish it from the chromophobe variant of RCC which typically has much more pleomorphic nuclei.
mitotic figures are absent or rare
EM: cytoplasm filled with mitochondria
Atypical oncocytoma: microscopic vascular invasion or microscopic extension into the perirenal fat
Features impermissible in an oncocytoma:
clear cells, numerous mitoses, papillary architecture, positive Hale’s colloidal iron stain, gross vascular invasion, gross extension into the perirenal fat; chromophobe-like vesicles by EM
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