Answer: Clear cell sarcoma of the kidney

Histology: This is a spindle cell proliferation within the kidney, featuring cells with ill-defined cell borders and fine chromatin. These cells entrap native renal tubules, leading to cyst formation and embryonal hyperplasia (the latter refers to the primitive, blue appearance of these entrapped tubules). In some areas, the nuclei of the spindle cell lesion palisade, mimicking the appearance of a schwannoma. Throughout, this spindle cell proliferation is separated by a regular capillary vasculature, and the chromatin of the cells is fine. These are the classic features of a clear cell sarcoma of the kidney (CCSK).

Discussion: A schwannoma is suggested by the focal nuclear palisading; however, the current lesion is S100 protein negative. The lesion also lacks the characteristic thick walled, degenerated blood vessels of a schwannoma. Cogenital Mesoblastic Nephroma is another common spindle cell tumor of the pediatric kidney. However, the current lesion has less well developed fascicles, and lacks the fibromatosis-like or fibrosarcoma-like morphology of congenital mesoblastic nephroma of the classic and cellular types, respectively. Metanephric stromal tumor is a key differential diagnosis, in that it may have myxoid areas and entrap native renal tubules. However, the current lesion lacks the concentric (onion skin) architecture of metanephric stromal tumor, along with the angiodysplasia, and the CD34 immunoreactivity.

Clear cell sarcoma of the kidney comprises approximately 3% of pediatric renal neoplasms. CCSK has a proclivity to metastasize to unusual sites such as brain and soft tissue. The addition of Adriamycin to chemotherapy regimens has improved survival for children with CCSK.

Reference(s):
– Am J Surg Pathol 2000;24:4-18.