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Pathology for Urologists

Congenital Mesoblastic Nephroma (CMN)

Image A
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Image B
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  • Most common renal tumor in first 3 months of life; uncommon after 6 months.
  • Divided into classic variant (1/4), cellular variant (2/3) and the rest is mixed
  • Cellular variant has t(12;15) chromosomal translocation resulting to ETV6-NTR3K gene fusion.
  • Gross:
    • Classic form, relatively large tumor with a firm white whorled cut surface; unencapsulated (image A).
    • Cellular, more fleshy-appearing tumor.
  • Classic CMN
    • Intersecting bundles of spindle cells (image B).
    • Low-grade nuclei with very few mitosis.
    • Margins are infiltrative, with cells extending into the adjacent renal and perirenal tissues.
    • Looks like "benign fibromatosis".

Image C
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Image D
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  • Cellular CMN
    • May be more cellular than the classical form of mesoblastic nephroma (image C).
    • Cells may be polygonal or spindled.
    • Margins may appear to be more "pushing" than infiltrative.
    • High mitotic rate (image D).
    • Looks like "malignant fibrosarcoma".
  • Majority have good prognosis, if completely resected.
  • Cellular suggested to be more aggressive (because morphologically high grade), but so far unsupported.


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