Clear Cell Sarcoma of Kidney : Differential

Comments:
Clear Cell Sarcoma of Kidney (CCSK) : Differential Diagnosis: Given the heterogeneity of its microscopic features, CCSK can mimic other pediatric renal neoplasms, including Wilms tumor, cellular congenital mesoblastic nephroma, primitive neuroectodermal tumor (PNET), and metanephric stromal tumor. The demonstration of YWHAE-NUTM2 gene fusion or BCOR internal tandem duplication would be diagnostic of CCSK. Alternatively, a selected panel of immunohistochemical markers would confirm the diagnosis in most cases.

The cellular pattern of CCSK closely resembles undifferentiated, small, round, blue cell tumors, and the differential includes blastemal component of Wilms tumor and PNET. Any entrapped renal tubules may be mistaken for the epithelial component of Wilms tumor.

Features favoring CCSK: finely dispersed chromatin in the nuclei, lack of nuclear overlap or molding, fibrovascular septae, and the lack of heterologous elements.

Features favoring blastemal Wilms tumor and PNET: aggressive infiltration of kidney with entrapment of entire chunks of renal parenchyma as opposed to single tubules entrapped by CCSK, coarse chromatin, and membranous CD99 positivity. Epithelioid pattern in CCSK may be mistaken for tubular differentiation in Wilms tumor. Cytokeratin is consistently positive in Wilms tumor and negative in CCSK.