Answer: Clear cell sarcoma of the kidney

Histology: This tumor is highly cellular and composed of small round cells. The cells are separated by a regular branching capillary vasculature, and the tumor cell nuclei are open with occasional cytoplasmic grooves. Rare entrapped native renal tubules were identified. This is the classic histology of clear cell sarcoma of the kidney (CCSK).

Discussion: A primitive neuroectodermal tumor (PNET) would have more rounded nuclei that lack the open chromatin of a CCSK. PNET would show diffuse membranous staining for MIC2 (CD99) which would not be present in CCSK. A Wilms tumor of the blastemal type can closely mimic CCSK. However, Wilms tumors typically have more coarse chromatin, and the nuclei are packed together and ~{!0~}mold~{!1~} each other. Plump cell cellular congenital mesoblastic nephroma may mimic other small round blue cell tumors of the kidney. These tumors typically have more coarse chromatin, more elongated nuclear shapes, and eccentric pink cytoplasm that may raise the differential diagnosis of rhabdoid tumor.

CCSK comprises approximately 3% of pediatric renal neoplasms. The mean age of presentation is two years, though there are bona fide cases in adults. This tumor has a propensity to metastasize to bone and unusual places such as soft tissue and the central nervous system. CCSK may recur late after diagnosis, so children require prolonged follow-up. The prognosis for children with this tumor is dramatically improved by addition of Doxorubicin to chemotherapy regimens.

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