Answer: Congenital mesoblastic nephroma (CMN)

Histology: The tumor is essentially a myofibroblastic one, characterized by fascicles of spindle cells that dissect apart the native kidney, leaving islands of entrapped nephrons within. The spindled cells have pale to pink cytoplasm with ill-defined cell borders, and the mitotic rate is high. At the leading edge of the tumor, there is prominent reactive vascular proliferation. Also noted are foci of extramedullary hematopoiesis, and embryonal hyperplasia of entrapped nephrons. The subcortical glomeruli within this kidney are immature for a term infant; a few glomeruli are still forming, a process which should have ceased by 36 weeks gestation. This impaired maturation likely reflects the effects of the tumor in utero.

Discussion: This tumor mimics a stromal predominant Wilms tumor, particularly if one considers the area of embryonal hyperplasia and immature glomeruli to be an immature epithelial component. However, the absence of intralobar nephrogenic rests, heterologous differentiation, or a true blastemal or epithelial component argue against Wilms tumor. A spindled clear cell sarcoma of the kidney is a prime consideration; however, the dissecting architectural pattern seen in this case is not typical of CCSK, and one does not see any of the other variant histologic patterns that are so characteristic of CCSK. The cytology is also distinctive; the nuclei in the current case are uniformly elongated and hyperchromatic, as opposed to the ovoid hypochromatic nuclei characteristic of CCSK.

The metanephric stromal tumor is a recently defined entity that may mimic congenital mesoblastic nephroma. Unlike CMN, MST typically features nodular variations in cellularity, juxtaglomerular cell hyperplasia, angiodysplasia and concentric peritubular growth (“onion skinning”), none of which were seen in the current case.

Congenital mesoblastic nephroma (CMN) is the most common congenital pediatric renal neoplasm. The current case has the architecture of the classic type of CMN in that it morphologically is a fibromatosis centered in the medial kidney (renal sinus) that dissects apart islands of native nephrons. This case is unusual in that it shows a much higher mitotic group than is typically seen. It is now known that cellular type CMN bears the identical t(12; 15) (p13; q25) chromosome translocation and resulting ETV6- NTRK3 gene fusion of infantile fibrosarcoma. It would be interesting to see if the current lesion bears this gene fusion; my guess is that it does not. I would consider it to be a mitotically active classic type CMN, just as infantile fibromatoses may on occasion show high mitotic activity which may overlap with that of infantile fibrosarcoma. Regardless, this patient is best treated by close observation, since this stromal neoplasm has a propensity toward local recurrence but little metastatic capability. Monthly ultrasounds for one year are appropriate; if the child does not recur in this interval, it is fairly safe to assume that the child has been cured.