Answer: Clear cell sarcoma of the kidney

Histology: This renal neoplasm is circumscribed at low power; however, on closer examination there are entrapped single nephrons present within the lesion. Some of these nephrons demonstrate embryonal hyperplasia, in that the tubular epithelial nuclei are hyperchromatic and “more blue”, stimulating the epithelium of a Wilms tumor. On closer inspection, the lesion has a regular branching capillary vasculature which is similar to that which is seen in myxoid liposarcoma; the cells between the vessels have bland, fine nuclear chromatin, which is less dense than that of the nuclei of the surrounding capillaries. The cytoplasm appears to be clear, though this is in fact extracellular space between the cells. These are the typical features of a clear cell sarcoma of the kidney.

Discussion: A Wilms tumor would demonstrate true epithelial formations, and usually has a pushing border with the native kidney. Wilms tumors have more hyperchromatic nuclei than those seen in a clear cell sarcoma of the kidney. It should be noted however that Wilms tumors may in areas very closely resemble clear cell sarcomas of the kidney, particularly blastemal areas that are incompletely fixed. Congenital mesoblastic nephroma is essentially a fibroblastic neoplasm of the pediatric kidney. The classic type is a fibromatosis which entraps islands of native nephrons, as opposed to the single nephrons entrapped in this case. The cellular type of congenital mesoblastic nephroma is essentially an infantile fibrosarcoma which pushes aside the native kidney. This neoplasm demonstrates a high mitotic rate, and nuclei which are hyperchromatic in somewhat angulated.

Three reported cases of clear cell sarcoma of the kidney has demonstrated the recurring chromosome translocation, t (10; 17) (q22; p13), though the presumed resulting gene fusion has not been characterized at this time.

Reference(s):
– The American Journal of Surgical Pathology 2000; 24: 4-18.