Answer: Clear cell sarcoma of the kidney (CCSK)

Histology: The tumor is composed of bland, plump epithelioid to spindled cells that are set within a myxoid background. These are the characteristic “cord” cells of clear cell sarcoma of the kidney. The cord cells are focally associated with dense strands of collagen, and focally the tumor acquires a more hyalinized, spindle cell appearance. The cord cells are separated by fibrovascular septa, which are focally thickened and cellular. The cord cells feature bland nuclei with fine chromatin; nucleoli are never prominent. These cells have indistinct cytoplasmic borders.

Discussion: The differential diagnosis centers around spindle cell tumors of the pediatric kidney. Metanephric stromal tumor is a prime consideration here, but can be excluded by the absence of “onion-skinning” around entrapped tubules, absence of angiodysplasia, and absence of heterologous elements such as glial tissue. Immunohistochemistry could be useful here since most MSTs stain for CD-34 while CCSK are negative. This distinction is crucial since MSTs are benign and cured by simple surgical excision. Stromal predominant Wilms tumor usually does not demonstrate sclerosis unless the lesion has been previously treated. The absence of blastema or epithelial differentiation even focally within the tumor also argues against Wilms tumor. Congenital mesoblastic nephroma either resembles an infiltrative fibromatosis (classic type), or the highly cellular infantile fibrosarcoma (cellular type). The myxoid background, epithelioid cord cell morphology, and cellular septa seen in the current lesion are not characteristic of CMN.

It is important that the pathologist recognize CCSK, since this tumor is responsive to Adriamycin therapy. With the addition of Adriamycin to chemotherapy protocols, overall survival for this tumor has risen from 20% to 70%. Failure to recognize a tumor as a CCSK can therefore deny a child potentially life-saving therapy. CCSK is also distinguished by its tendency to metastasize to sites (such as bone and brain) which are unusual for a Wilms tumor, and its ability to present with metastases late after diagnoses (ten years or more). Hence, prolonged surveillance is required for these children.