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Metanephric Adenoma

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  • Uncommon benign neoplasm composed of small primitive cells or small blue cells that look like immature metanephric tubules.
  • Grouped with metanephric adenofibroma (MA + with stroma) and metanephric stromal tumor (pure stromal tumor in pediatric patients); both also benign.
  • Possible tumor suppressor gene located on chr 2p13.
  • Wide age range (11 months to 83 years old; mean 41 years).
  • Female>male.
  • Majority are incidental, and if symptomatic presents with hematuria, flank pain and abdominal mass; a minority may have polycythemia.
  • Gross: Well-circumscribed, solid, unencapsulated, yellow tan homogenous cut surface and may have some necrosis and calcifications.

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  • Histology:
    • Proliferation of small cells with very little cytoplasm (thus appears blue) arranged in tubules and less often in papilla (image A), (image B), (image C) & (image D).
    • Neoplastic cells proliferate inside tumor tubules creating a "glomeruloid" appearance.
    • Nuclei may show folds, indistinct nucleoli and absent or rare mitosis.
    • Degenerative changes including psammoma calcifications may be present.
  • Immunohistochemistry: Nuclear WT1+ (or Wilm's tumor 1), AMACR+ and CD57+.
  • Benign course.
  • DDX:
    • Wilm's Tumor: Similar primitive appearing cells but is more atypical with mitosis, is triphasic (also has mesenchymal and blastemal components) and is CD57-.
    • Papillary RCC, type 1 or basophilic type: More cytoplasm than MA, usually WT1- or CD57-, and has +7, +17 and -Y (cytogenetics usually not necessary).