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Case 1: Proliferation of spindled cells with elongated nuclei, similar to fibromatosis.

There are some lymphocytes streaming between the fascicles.

Fascicles of spindle cells with little atypia or mitotic activity.

The classic variant often has an interdigitating tumor-normal interface, entrapping normal elements.

Case 2: This case has frequent mitotic figures, suggesting it may be the cellular variant. The cellular variant may have cystic areas.

Case 3: A uniform proliferation of spindle cells are present, arranged in fascicles. The spindle cells are myofibroblastic and fibroblastic by immunohistochemistry.

The spindle cells stream between normal structures. There is no desmoplasia.

A bland sheet of spindle cells.

There is a collection of lymphocytes adjacent to some poorly-formed tubular elements.


Congenital mesoblastic nephroma (CMN) is the most common renal tumor in the first 3 months of life. Over 90% occur within the first year of life, and these tumors do not occur after age 2.1

Grossly, the tumor is solitary, unilateral with a cut surface that is firm, bosselated, whorled and light-tan, resembling a leiomyoma. Foci of cystic change and hemorrhage is not uncommon. The tumor is not encapsulated and interdigitates with the normal kidney, thus the tumor-kidney interface is often blurred. Most lesions are centered on the hilus, involving the renal sinus.

CMN comes in two forms, the "classic" variant and the "cellular" variant.1,2

In up to 20% of CMNs, the two forms coexist. In these instances, nodules of the cellular variant are seen in a background of the classic variant. It is thought that the cellular variant may arise from the classic variant.

The differential diagnosis consists of other pediatric tumors such as Wilms tumor, clear cell sarcoma and rhabdoid tumor. A Wilms tumor, especially a stromal-predominant variant, may mimic the cellular variant of CMN. However, Wilms tumor occurs at a later age than CMN, exhibits a circumscribed border and one should see blastema histologically. Clear cell sarcoma also occurs at a later age than CMN and are negative for vimentin, smooth muscle actin and desmin, whereas CMN is positive for desmin, actin and SMA. Careful microscopic examination should distinguish CMN from rhabdoid tumor, as the latter has characteristic cytoplasmic inclusions and prominent nucleoli.1,2,3


The tumor is associated with polyhydramnios and prematurity. It presents as a palpable abdominal mass and often discovered on prenatal ultrasound.


Excellent; surgical excision is generally curative. If incompletely excised, the tumor may recur. Therefore, the surgical pathologist must carefully examine the margins, as this tumor (especially the classic variant) tends to have infiltrative borders and the kidney-tumor interface is hard to delineate.


→There are two variants: classic and cellular. The cellular variant is more cellular, with high mitotic activity, a sarcomatous appearance and a pushing border. The classic variant has little or no mitotic activity and has a interdigitating border with entrapped elements.

→The cellular variant exhibits the same translocation as congenital fibrosarcoma t(12;15) forming the ETV6-NTRK3 fusion gene. This is not seen in the classic variant.


1 Murphy WM, Grignon DJ, Perlman EJ. Tumors of the Kidney, Bladder and Related Urinary Structures. AFIP Atlas of Tumor Pathology. Fourth series, Fascicle 1. Washington DC; AFIP: 2004; 57-64.

2 Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 518-9.

3 Zhou M, Magi-Galluzzi, C. Genitourinary Pathology: Foundations in Diagnostic Pathology. Philadelphia, PA: Elvesier; 2006: 321-3.

Last updated: 2012-01-09
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