Answer: B
Histologic Description: The kidney is overtaken by a small round blue cell tumor. The neoplastic cells have sheet like growth pattern, and are non-pleomorphic small round blue cells that focally seem to form rosettes. The neoplastic cells demonstrate nuclear labeling for NKX2.2 and membranous labeling for CD99, and demonstrated an EWSR1-ERG gene fusion.
Differential Diagnosis: In this age group, a primary Ewing sarcoma would be more common than a blastemal Wilms tumor. The latter would demonstrate nuclear molding, would not show membranous labeling for CD99, and lacks the typical gene fusions of Ewing sarcoma. Neuroblastoma would be a stronger consideration in a patient under the age of 1. Neuroblastoma may grow into the kidney from the adjacent adrenal gland or sympathetic ganglion, mimicking a primary renal neoplasm. Neuroblastoma would label for PHOX2B and not for CD99, and typically would demonstrate elevated serum and urine catecholamines. Poorly differentiated synovial sarcoma may, in areas, be morphologically indistinguishable from Ewing sarcoma; this is why some of these cases were mistakenly included in prior protocols for Ewing sarcoma. Synovial sarcoma would not label for NKX2.2 but would demonstrate labeling for TLE1, and is characterized by an t(X;18) translocation fusing the SS18 with the SSX1 or SSX2 genes.