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Kidney tumor - cysts, children, adult benign

Childhood neoplasms

Congenital mesoblastic nephroma

Reviewers: Mandolin Ziadie, M.D. (see Reviewers page)
Revised: 5 January 2012, last major update December 2011
Copyright: (c) 2003-2012, PathologyOutlines.com, Inc.


● Congenital myofibroblastic tumor that resembles infantile fibromatosis / leiomyoma (classic) or fibrosarcoma (cellular)
● Also called fetal mesenchymal / leiomyomatous hamartoma
● First described in 1967 (Pediatrics 1967;40:272)


● Most common pediatric renal tumor in infancy
● 5% of all pediatric renal tumors (Cancer 2006;106:2275)
● Age at presentation varies with type (classic, 16 days; cellular, 5 months; mixed, 2 months)
● Rarely occurs in children older than 2 years of age
● Frequency: 66% cellular, 24% classic and 10% mixed

Age distribution curve

Clinical features

● May present in utero with fetal hydrops / polyhydramnios
● Excellent prognosis (5 year survival of 96%, Pediatr Blood Cancer 2008;50:1130)
● 5-10% recur or metastasize (usually cellular type), usually by age 1 year to lung, brain or rarely bone
● Poor prognostic factors include: cellular variant (Pediatr Surg Int 2007;23:205) and advanced stage or sinus / vascular involvement (margins and sinus / vascular involvement should be reported)

Case reports

● Newborn with classic type (Arch Pathol Lab Med 2002;126:103)
● 3 week old infant with flank mass and mixed type (Arch Pathol Lab Med 2004;128:929)
● 2 month old with cellular variant conceived by in vitro fertilization (J Pediatr Surg 2008;43:E27)
● 3 month old girl with cellular type (Case of Week #57)


● Nephrectomy with wide margins
● Chemotherapy if resection is incomplete in infants 3 months or older, or if tumor ruptures during surgery

Gross description

● Unilateral
● Extensive sampling of margin should be performed
Classic: mean 5 cm, variably circumscribed, white / yellow whorled mass near hilum, involving over half of kidney with indistinct tumor-kidney interface; resembles leiomyoma with whorled cut surface; may be cystic; may involve renal sinus; hemorrhage and necrosis uncommon
Cellular: mean 9 cm, necrosis and large cystic areas and hemorrhage
Mixed: mean 10 cm, cysts, hemorrhage and necrosis in cellular areas

Gross images

Tan fleshy masses with hemorrhage, necrosis and cystic degeneration

Rubbery solid homogenous tumor invades the renal parenchyma

Micro description

Classic: resembles infantile fibromatosis or leiomyoma with fascicles and whorls of bland spindled myofibroblasts and thin collagen fibers; tumor surrounds tubules and glomeruli, has irregular borders; chondroid metaplasia / dysplasia of the entrapped tubules is common; mitoses are rare; necrosis / desmoplasia are not present
Cellular: resembles infantile fibrosarcoma with a sheet-like proliferation of plump, atypical spindle cells with abundant cytoplasm, vesicular nuclei and nucleoli; frequent mitotic figures (25-30 / 10 HPF) and necrosis; the tumor has a pushing border
Mixed: tumors with a combination of the above features

Micro images

Classic pattern:

Cellular areas have mitotic figures, cystic degeneration, hemorrhage and necrosis

Prominent interlacing bundles

Resembles fibromatosis


Tumor interdigitates with renal parenchyma

Perirenal invasion with characteristic angiomatous vascular proliferation at advancing edge

Extension into renal sinus and along structures medial to the kidney is an important feature of most lesions


Embryonal metaplasia of Bowman capsule and entrapped tubules; #2-accompanied by epithelial hyperplasia

Mitotic figures

Cellular pattern:

Cellular and classic appearance

Cellular variant has monomorphous sheets of closely packed spindle cells imparting a small blue cell appearance and prominent irregular vascular spaces

Mixed pattern:

Mixed congenital mesoblastic nephroma

Case of the Week #57:
Left to right: well circumscribed tumor; medium power; high power

Left to right: infiltrative margin; glomerular entrapment; angiomatoid area

Cases resembling other tumors:

Left-mesoblastic nephroma; right-consistent with infantile fibrosarcoma

Fig A: classic mesonephric nephroma features, fig B: resembles clear cell sarcoma

Fig A: typical of cellular mesoblastic nephroma; B: invasive edge shows more hyperchromatic, epithelioid-appearing cells with prominent nucleoli; C: resembles rhabdoid tumor, but t(12;15) fusion product was present

Virtual slides

Mesoblastic nephroma

Cytology description

Classic: cellular clusters of spindle cells, mild nuclear pleomorphism, mitotic activity and no blastema (Diagn Cytopathol 2009;37:377)

Positive stains

● Smooth muscle actin, vimentin
● Also renin (within tumor vessels or vessels in trapped cortex, Hum Pathol 1988;19:1347)
● Occasionally WT1 (J Pediatr Hematol Oncol 2008;30:768), INI1

Negative stains

● Keratin (except for entrapped epithelium), laminin, desmin and CD34

Molecular description

Classic and mixed: no identified recurrent genetic aberration, though cases have been reported with trisomy 11, duplication of paternal IGF2 allele and increased expression of IGF2 mRNA (Genes Chromosomes Cancer 2007;46:929, Histopathology 2006;48:748)
Cellular: shares the same translocation with infantile fibrosarcoma: t(12;15); results in ETV6-NTRK3 fusion protein (Mod Pathol 2000;13:29, Mod Pathol 2001;14:1246)

Molecular images

Ideogram and FISH

Gel for fusion transcript

Differential diagnosis

● Adult mixed epithelial and stromal tumor: mean age in 50’s, usually women, similar morphology and staining pattern, also positive for estrogen and progesterone receptors
● Clear cell sarcoma: similar age but has clear cells and chicken-wire vasculature, tumor cells isolate single nephrons, fine nuclear chromatin and low mitotic rate; negative for smooth muscle markers
● Metanephric stromal tumor: older age, nodular low power pattern, onion-skin cuffing around entrapped renal tubules, heterologous differentiation and vascular changes, CD34+
● Rhabdoid tumor: more invasive margins, usually epithelioid cells with cytoplasmic inclusions and prominent nucleoli, usually presents with metastases, INI1-
● Wilms’ tumor: older age, also has blastema and nephrogenic rests; previously treated tumors may have well differentiated spindle cell stroma

Additional references

Hum Pathol 1989;20:682, eMedicine

End of Kidney tumor - cysts, children, adult benign > Childhood neoplasms > Congenital mesoblastic nephroma

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