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Diagnosis: Congenital mesoblastic nephroma (classic)

Congenital renal tumors comprise 2.5-7% of all perinatal tumors. Congenital renal neoplasms include, in decreasing order of frequency, congenital mesoblastic nephroma, Wilms tumor, rhabdoid tumor, clear cell sarcoma, hamartomas, and ossifying tumor of infancy. Congenital mesoblastic nephrom (or simply mesoblastic nephroma) represents 3-10% of all pediatric renal tumors. This tumor was first described as a separate entity by Bolande et al in 1967. Prior to this, it was erroneously confused with congenital Wilms tumor (Singh SP. Mesoblastic Nephroma. In: e-Medicine; visited February 24th, 2010 [Link] - UpToDate, visited February 24th, 2010 [Link])

Congenital mesoblastic nephroma arises from renal mesenchyma and is usually benign. The tumor is considered an hamartoma. About 90 percent present in the first year of life, 50 to 75 percent of cases occur in young infants, and almost none occur after the age of 3 years. After 2 months of age, Wilms tumor is more common than Congenital mesoblastic nephroma. It is presumed to originate from proliferating nephrogenic mesenchyme. The lesion has similar appearance (gross and histologic) to a uterine leiomyoma with spindled cell bundles, but composed of immature renal stromal cells. The tumor lacks renal blastema and neoplastic metanephric elements, thereby differentiating it from Wilms tumor. In addition, it tends to infiltrate the kidney, rather than form the pseudocapsule of classic Wilms tumor (UpToDate, visited February 24th, 2010 [Link]).

There are two pathologic variants: classic and atypical or cellular. The classic form (as our case) is characterized by rare mitoses and absence of necrosis. Entrapped tubules and/or glomeruli are usually seen at the periphery of the tumor. Atypical or cellular variant is characterized by a high mitotic index, hypercellularity, and an atypical growth pattern with necrosis, hemorrhage, and invasion of adjacent structures [See images of the two variantes]. The cellular type accounts for 42-63% of cases.

Congenital mesoblastic nephroma is almost always unilateral and is rarely malignant. It may extend beyond the renal capsule, but rarely metastasizes to distant organs. Cellular variant, older age, and positive surgical margins appear to increase the risk of recurrence and metastasis. Metastases to distant organs such as the brain, bone, and lungs have been reported.

The cellular variant has been shown to bear the t(12;15)(p13;q25) and ETV6 (chromosome 12)-NTRK3 (chromosome 15) gene fusion. These combined genes are thought to activate tyrosine kinase growth signaling. This gene fusion transcript is also reported in congenital or infantile fibrosarcoma (Singh SP. Mesoblastic Nephroma. In: e-Medicine; visited February 24th, 2010 [Link]).

See the chapter Neoplasms (Spanish version) of our Tutorial.

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Bibliography

• Bayindir P, Guillerman RP, Hicks MJ, Chintagumpala MM. Cellular mesoblastic nephroma (infantile renal fibrosarcoma): institutional review of the clinical, diagnostic imaging, and pathologic features of a distinctive neoplasm of infancy. Pediatr Radiol. 2009;39(10):1066-74. [PubMed link]
• Chaudry G, Perez-Atayde AR, Ngan BY, Gundogan M, Daneman A. Imaging of congenital mesoblastic nephroma with pathological correlation. Pediatr Radiol. 2009;39(10):1080-6. [PubMed link]
• Singh SP. Mesoblastic Nephroma. In: e-Medicine: http://emedicine.medscape.com. Visited: February 24th, 2010. [Link]
• Mohanty SK, Parwani AV. Mixed epithelial and stromal tumors of the kidney: an overview. Arch Pathol Lab Med. 2009;133(9):1483-6. [PubMed link] [Free full text]
• Eble, JN, Sauter, G, Epstein, JI, Sesterhenn, IA. Tumours of the urinary system and male genital organs Pathology and genetics. World Health Organization 2004.
• Henno S, Loeuillet L, Henry C, D'Hervé D, Azzis O, Ferrer J, Poulain P, Babut JM, Merlio JP, Jouan H, Dubus P. Cellular mesoblastic nephroma: morphologic, cytogenetic and molecular links with congenital fibrosarcoma. Pathol Res Pract. 2003;199(1):35-40. [PubMed link]

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