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Renal Oncocytoma

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  • Benign kidney tumor characterized by cells with eosinophilic granular cytoplasm and low grade nuclei growing in nests.
  • Shows some overlap with chromophobe RCC, eosinophilic variant, and not uncommonly are difficult to distinguish.
  • Usually asymptomatic and detected on radiologic work-up.
  • Cytogenetics: chr -1 and -Y and alterations in chr 11q13.
  • Gross:
    • Classically described as "mahogany brown with central stellate scar" (image A) (Beware that stellate scar can also be seen in chromophobe RCC).
    • Hemorrhage and necrosis are rare.

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  • Histology:
    • Cells arranged in small nests either "packeted" or within a loose stromal background (image B).
    • Cytoplasm is finely granular and very eosinophilic (image C) & (image D).
    • Nuclei are round with inconspicuous nucleoli (may have occasional bizarre or "degenerative-type" nuclei); absent or rare mitosis.
  • Electron microscopy: Cytoplasm filled with abundant mitochondria.
  • Immunohistochemistry: CD117+; unlike chromophobe RCC, shows only scattered or absent CK7+ (Currently the best immunostain to distinguish).
  • DDX: (3 main differentials for solid tumors with eosinophilic granular cytoplasm including renal oncocytoma).
    • Chromophobe RCC, eosinophilic variant: in contrast shows larger nests or alveolar growth, have nuclear irregularity and perinuclear halo and diffuse CK7+. (Distinction mainly morphologic)
    • Clear cell RCC, with eosinophilic granular cytoplasm: in contrast shows arborizing "chicken-wire" vasculature and with diffuse membranous CAIX+ and CD10+.