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Case 354 - Discussion

Uploaded: 2009-10-16, Updated: 2009-10-16


Diagnosis: cellular congenital mesoblastic nephroma

Tumor cells positive for ETV6 (12p13) rearrangement.

Congenital Mesoblastic Nephroma (CMN)


  • Congenital mesoblastic nephroma (CMN) is a stromal neoplasm confined to infancy.

  • Median age at diagnosis 2 months, >90% within the first year of life.

  • Hypercalcemia and hyperreninism commonly seen.


  • Most tumors are centered near the hilus of the kidney and nearly all involve the renal sinus.
  • 0.8 to 14 cm in greatest dimension, mean, 6.2 cm.
  • Solitary, unilateral masses with soft or firm, bulging cut surfaces often indistinguishable from a nephroblastoma.
  • Cysts, hemorrhage, and necrosis are common features.


  • Predominantly monomorphic neoplasms composed of spindled mesenchymal cells of fibroblastic or myofibroblastic lineage.
  • Classic type CMN
    • <1/3 of CMN.
    • Closely resembles infantile fibromatosis, characterized by intersecting fascicles of spindle cells, resembling fibroblasts or myofibroblasts, interspersed with scant collagen fibers.
    • Dilated, thin-walled vascular spaces are often prominent.
    • Mitotic activity is variable but generally less conspicuous than in the cellular pattern.
    • The tumor margins are highly irregular, with radiating bands of cells extending into the renal parenchyma and often into the perirenal soft tissue.
    • Abnormal metaplastic changes in tubules or glomeruli adjacent to or entrapped by the lesion are present in many specimens, including papillary hyperplasia and small nodules of hyaline cartilage. Extramedullary hematopoiesis is common. Skeletal muscle differentiation is not a feature of CMN.
  • Cellular type CMN
    • Most common.
    • Characterized by increased cellular density and a high proliferative rate, imparting a sarcomatous appearance to the tumor.
    • Most commonly consist of plump cells with vesicular nuclei and a small to moderate amount of cytoplasm.
    • Lesions are sharply circumscribed grossly, without the interdigitating margins of classic lesions.
    • Slight to moderate nuclear pleomorphism may be present and the cells often grow in sheets of somewhat elongated cells.

    • Rare tumors contain cells with prominent nucleoli as well as areas of necrosis, closely resembling the features of a rhabdoid tumor.

    • In some cellular CMNs, a prominent capillary vasculature that mimics the vasculature of a clear cell sarcoma of the kidney (CCSK).





      Interface with kidney

      Circumscribed, unencapsulated

      Highly irregular, interdigitating

      Genetic features

      t(12;15)(p13;q25), trisomy chromosome 11

      None known


      Plump, slightly spindled cells

      Markedly elongated

      Extrarenal equivalent

      Infantile fibrosarcoma

      Infantile fibromatosis


  • Mixed mesoblastic nephroma: cellular and classic patterns coexist in ~20% of CMNs.


  • Positive for antibodies to myofibroblast.
  • Positive for vimentin, desmin, and actin.
  • Negative for laminin, cytokeratins, S-100 protein, and WT1.


  • CCSK
  • Rhabdoid tumor

Mesoblastic Nephroma

Clear Cell Sarcoma of Kidney


   Age less than 6 months
   Increased renin, calcium

Age more than 1 year
Metastases (except lung)

Light Microscopy

   Classic mesoblastic pattern

Classic CCSK pattern

   Renal dysplasia (e.g., cartilage)

Most variant CCSK patterns

   Coarse chromatin

Fine chromatin

   High mitotic rate

Low mitotic rate

   Extensively infiltrating margins

Grossly well demarcated, microscopically infiltrative

   Staghorn vessels in tumor

Extensive sclerosis

   Tumor surrounds groups of nephrons

 Tumor entraps isolated nephrons


   Positivity for desmin and/or actin

Negativity for desmin and actin




Clear Cell Sarcoma


   Nephroblastoma syndromes
   Bilateral/multicentric tumors

Metastases to bone, brain, or other sites
(except lung, lymph nodes, liver)

Gross and Light Microscopy

   Heterologous cell types (skeletal muscle, etc.)

Classic or variant CCSK patterns

   Classic blastemal patterns (serpentine, etc.)

Homogeneous, pale H&E appearance

   Nodular growth pattern

Tumor surrounds, isolates nephrons

   Botryoid intrapelvic growth

Prominent collagen

   Nephrogenic rests



  WT1+, CD56+, epithelial +, muscle+, or neural +

Vimentin +



  • t(12;15) (p13;q25). ETV6 on 12p13, ETS transcription factor family, neurotrophin-3 receptor (NTRK3) gene on 15q25, a membrane-bound protein with tyrosine kinase activity. Also seen in infantile fibrosarcoma.
  • Trisomy for chromosome 11. Also seen in infantile fibrosarcoma.


  • CMNs are treated by complete surgical excision without adjuvant chemotherapy unless gross residual tumor remains.
  • Recurrences and metastases occur in 5-10% of patients overall and are confined to tumors containing cellular histology.
  • The most significant factors associated with local recurrence and metastases are: 1) the presence of cellular histology; 2) tumors of stage III or greater; and 3) involvement of intrarenal or sinus vessels.


  • AFIP, tumor of the kidney, bladder and related urinary structures, series 4