Final Diagnosis -- Clear Cell Sarcoma of the Kidney



Differential Diagnosis:

Most clear cell sarcomas of the kidney (CCSK) have the classic histologic pattern, as in this case, and their recognition is straightforward. However, it is important that variant patterns of CCSK be distinguished from other pediatric renal neoplasms because of its more aggressive clinical course and different therapy. The epithelial pattern of CCSK can be confused with Wilms' tumor. The tumor cells can condense into epithelioid ribbons and cords resembling Wilms' tumor. These areas are usually nodular and are adjacent to areas of classic CCSK. Pseudorosettes may be found in these epithelioid areas, usually along vascular septa. Occasionally these epithelioid ribbons are associated with abundant extracellular myxoid material which can become confluent and create cystic tubules (Image 13, 14). The cells lining these tubules do not express cytokeratin or epithelial membrane antigen (EMA) unlike entrapped renal tubular epithelium. Tubule formation in Wilms' tumor is EMA and cytokeratin positive. Furthermore, the presence of a mesenchymal and blastemal component helps to differentiate Wilms' tumor from CCSK.

CCSK may have a prominent spindle cell component arising from the fibrovascular septa, and, when extreme, may resemble the broad intersecting fascicles of a mesoblastic nephroma (MN)( Image 15). This variant of CCSK with prominent spindle cell areas may also be difficult to distinguish from a cellular variant of mesoblastic nephroma (CMN) because of its more sarcomatous appearance. Often, cellular areas of MN are focal and may be adjacent to classic areas (Image 16). The nuclei in these cellular foci of CMN have coarser chromatin, conspicuous nucleoli, more cytoplasm and express vimentin, desmin and actin. MN is frequently diagnosed within the first 6 months of life. Microscopically, CMNs have interlacing spindle cell fascicles, randomly distributed irregular "staghorn" vessels, and may show coexisting renal dysplasia. Conversely, CCSK is usually diagnosed between ages 1 and 2.

Histologically, CCSK has characteristic fine arborizing vessels within septa, and "staghorn" vessels are not seen. It is not associated with renal dysplasia, and tumor nuclei have fine evenly dispersed chromatin. The arborizing vasculature is a key feature in distinguishing CCSK from MN. Mesoblastic nephroma has very little metastatic potential and an excellent overall prognosis, unlike CCSK which has a propensity for early metastasis to bone and worse prognosis. In the palisading pattern of CCSK the nuclei palisade in cellular and acellular areas and may resemble a schwannoma. However, these areas are S100 negative and are ultrastructurally distinct.

In addition to the epithelial, spindled and cystic variants, other patterns show prominent sclerosis, hyalinization, myxoid change, pericytomatous and anaplastic changes. Metastatic foci of CCSK will often but not always resemble the primary tumor, underlining the importance of recognizing the variant patterns. The most helpful histologic features identifying CCSK are the characteristic delicate arborizing vessels in septa, monotonous clear cells and uniform nuclei with dispersed chromatin and characteristic invasion of the renal pelvis.


The patient underwent a left nephrectomy with lymph node dissection with out complications, and was receiving chemotherapy and radiation 1 month post surgery.

Clear cell sarcoma of the kidney (CCSK) is now recognized as a unique renal neoplasm of childhood with distinctive histologic and clinical features. First recognized by Kidd et al in 1970 as a more aggressive tumor, subsequent investigation has confirmed it as a separate entity from Wilm's tumor, which it was once classified with.

CCSK is a rare tumor comprising less than 5% of pediatric renal neoplasms. Its peak incidence is in the second year with a slight male predominance. Patients usually present with an abdominal mass.

The cell of origin is unknown. Many ultrastructural studies have failed to trace the histogenesis to a specific cell type; some have suggested a renomedullary interstitial cell, but support for this has been lacking. It is generally accepted that it arises from primitive nephrogenic mesenchyme.

Grossly, these tumors are bulky, white, lobulated and often centered on the medulla. The average size is over 500g and they are always unilateral. Hemorrhage and necrosis are uncommon. Most often, these tumors are confined to the renal capsule at presentation.

Microscopically, most CCSKs have the classic histologic pattern, as in this case. Typically, there is a which is a monomorphic population of clear, round to polygonal cells separated by delicate vascular septa. Nuclei are uniform with inconspicuous nucleoli and evenly dispersed chromatin and minimal clear to eosinophilic cytoplasm. Nuclear grooves are not well appreciated in tissue sections, but are prominent and very helpful in making this important diagnosis in touch imprints or fine needle aspirates.

These tumors appear to arise in the medulla and diffusely infiltrate the renal sinus soft tissue and commonly entrap individual non neoplastic nephrons and collecting tubules, which may result in cystic dilitation.

Ultrastructurally, the cord like arrangements of cells are joined by primitive junctions, and basement membrane material is not found within the tumor. Extracellular collagen is often prominent. Glycogen is absent, and lipid is rarely seen. Immunohistochemical stains may be of use excluding other pediatric renal neoplasms. CCSKs express vimentin; keratin, EMA and PAS are negative.

CCSK has a marked propensity to metastasize early to bone, most commonly involving the skull, spine, ribs, femur. In early reports, as many as 70% of CCSKs metastasized to bone, however, more recent reports show a lower incidence of metastasis, perhaps due to newer chemotherapeutic regimens. Local radiotherapy and aggressive chemotherapy regimens including Doxorubicin have improved the 6 year survival rate to 63%, versus 25% without doxorubicin. However, late recurrence remains a problem with these tumors; 30% of relapses occur after 2 years, and prolonged follow up is required.


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Contributed by Peter Christakos, MD and Paul S Dickman, MD


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