Answer: Nephroblastoma (Wilms tumor)

Histology: Histologic description: This tumor shows characteristic serpentine blastemal patterns of Wilms tumor and that it features interconnecting elongated cords of primitive cells set in a focally cellular, edematous stroma. The stroma shows focal areas of high cellularity and a primitive appearance that indicate that it is in fact neoplastic, and the epithelium shows focal areas of tubular differentiation. Additional areas of this tumor show characteristic geographic areas of apoptosis, a common finding within nephroblastoma. At high power, the primitive blastemal tumor cells of this neoplasm approximate each other such that there is nuclear molding. This feature is commonly seen in Wilms tumor, and is similar to the nuclear molding classically described in adult small cell carcinoma of the lung.

Discussion: Differential diagnosis: In an adolescent, the diagnoses of renal PNET and renal synovial sarcoma are in fact more likely than blastemal Wilms tumor. A PNET should have rounder nuclei, show rosette formation, and lack the neoplastic stromal component of the current case. These lesions would label in a membranous fashion for 013 (MIC2), which was not true of the current case. Renal synovial sarcomas are usually of the monophasic spindle cell type, and frequently demonstrate large cystic areas. The cysts arise from entrapped native collecting ducts that dilate when surrounded by tumor. Intraabdominal desmoplastic small round cell tumor is suggested by the nested pattern with prominent stroma within this case. Both blastemal Wilms tumor and IADSRCT may label with desmin but not for actin, which can further the confusion. However, IADSRCTs feature smaller, more angulated tumor cell nests. IADSRCT also labels more diffusely with cytokeratin than the current case did.