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Mixed Epithelial and Stromal Tumor/Cystic Nephroma

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  • Likely to represent as part of the same spectrum, CN being purely cystic and MEST being solid and cystic.
  • Marked female predominance.
  • Mostly incidental renal mass.
  • Gross: well circumscribed, multilocular cystic tumor, mostly solitary and confined to kidney (image A).
  • Histology:
    • Biphasic epithelial and stromal tumor.
    • Epithelial cells vary flat, cuboidal or "hobnailed".
    • Stroma fibrous or "ovarian-like".

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  • Immunohistochemistry: stromal cells often ER+ and PR+.
  • Vast majority have benign behavior; rare reports of tumors with malignant behavior.
  • DDX:
    • Multilocular cystic RCC: lined by clear cells, no ovarian-like stroma and is CAIX+.
    • Tubulocystic carcinoma:lined by cells with high-grade nuclei and lacks ovarian-like stroma.