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

• The classic variant was the first to be described, however, less than one-third of tumors have this histology. It consists of intersecting fascicles of elongated spindled cells. These fascicles interdigitate with the normal renal parenchyma and sometimes infiltrate into the perirenal soft tissues. Often, there are entrapped kidney elements within the proliferating fibroblasts (or myofibroblasts). Mitotic figures are seen, but are less frequent that the cellular variant. Thin-walled blood vessels are prominent. The histology is essentially identical to infantile fibromatosis.
• The cellular variant is more commonly seen and consists of tightly-packed, plump rounded spindle cells with vesicular nuclei and less abundant cytoplasm. Compared to the classic variant, the overall picture is that of increased cellularity and mitotic activity, imparting a sarcomatous appearance. This variant tends not to interdigitate with the normal renal parenchyma, but rather, forms a pushing circumscribed border. Slight to moderate pleomorphism may be present. This histology is identical to infantile fibrosarcoma and in fact, bears the same cytogenetic lesion. Both infantile fibrosarcoma and the cellular CMN demonstrate trisomy 11 and a t(12;15)(p13:q25) translocation. Note that the classic variant of CMN does not share this cytogenetic aberration.

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.