Answer: Renal synovial sarcoma

Histology: The tumor has the classic appearance of a biphasic synovial sarcoma. It is composed of bland highly cellular, non-pleomorphic spindle cells associated with glands with hypochromatic nuclei. Stromal collagen is also evident.

Discussion: The biphasic Wilms tumor could resemble this lesion although untreated Wilms tumors do not typically show extensive collagen deposition, and the age is more typical of synovial sarcoma than Wilms tumor. The nuclei are also wrong for Wilms tumor; instead of having a primitive blastemal appearance, they are more open and hypochromatic. Metanephric adenofibroma (MAF) is essentially a hyperdifferentiated Wilms tumor related lesion. The stroma of metanephric adenofibroma is identical to that of metanephric stromal tumor, featuring bland thin spindled cells with abundant collagen, concentric “onion skinning” growth around native tubules and vessels, angiodysplasia, and heterologous differentiation. The epithelial component of MAF is essentially that of an inactive metanephric adenoma. These features are not typical or characteristic of the current case. Mixed epithelial stromal tumor is a recently described lesion that usually occurs in females and involves the renal pelvis. The stromal component of this lesion typically shows muscular differentiation, while the epithelial component may be cystic but more typically has complex tubular patterns. This lesion expresses its hormone receptors (estrogen and progesterone receptors), which may explain its predilection for females. It lacks the high cellularity of the current lesion.

Renal synovial sarcoma is recently recognized entity that has likely been miscategorized in the past as Wilms tumor. It typically affects older patients than Wilms tumor does, and may present at advanced stage. Recognition of this entity is important, since therapy designed for Wilms tumor is inadequate for synovial sarcoma, and synovial sarcoma is responsive to specific chemotherapy regimens. Most renal synovial sarcomas have proven to be monophasic spindle cell type, where the differential diagnosis includes blastemal Wilms tumor, cystic partially differentiated nephroblastoma, and sarcoma arising in cystic nephroma. Primary renal synovial sarcoma occupies a significant subset of cases previously referred to as “embryonal sarcoma of the kidney.”