Clear Cell Sarcoma of the Kidney Workup

No specific laboratory study confirms the diagnosis of clear cell sarcoma of the kidney (CCSK); therefore, most testing pertains to the workup of an abdominal mass.

• CBC count: CBC counts should be performed to evaluate for evidence of anemia.
• Creatinine levels: Serum creatinine levels are tested to assess the patient's renal function.
• Standard preoperative laboratory studies: Prothrombin time and activated partial thromboplastin time should be checked in preparation for surgery, and urinalysis should be performed.

CT scans of the chest (before surgery) and abdomen should be performed initially to define the extent of the tumor. Examples are shown below.

Abdominal ultrasound should be performed to evaluate the status of the inferior vena cava and any gross extension into the renal vein. Tumor thrombus in the renal vein is present in approximately 5% of patients with clear cell sarcoma of the kidney.

Bone scan and brain CT scan or MRI are also part of the workup.

Clear cell sarcoma of the kidney usually presents as a large unicentric mass markedly distorting or almost completely replacing the kidney. The mean diameter of measured tumors in the NWTSG series was 11.3 cm, with a range of 2.3-24 cm. The mean weight of the kidney tumor was 661 g. When an epicenter could be determined, the renal medulla was the most common location. No case of multicentric origin was identified in the NWTSG series. Sections of tumors appear grossly as tan-gray, soft, and mucoid. Cystic foci are almost universally present and occasionally represent the dominant feature, so that the diagnosis of multilocular renal cyst is made. Discrete foci of necrosis and hemorrhage may be present.

Histologically, clear cell sarcoma of the kidney shows 3 components, namely, (1) cord cells, which are small round-to-oval cells with deceptively bland cytologic features, including mitotic figures; (2) septal cells, which are spindle-shaped cells along the fibrovascular septa (fibrovascular septa can be demonstrated more convincingly using reticulum stain); and (3) an intercellular matrix composed of mucopolysaccharide, which ranges from minute indiscernible droplets to large pools imparting the clear appearance of clear cell sarcoma of the kidney.

Depending on the amount, distribution, and variation in morphology of the 3 components, the tumor shows a classic clear cell sarcoma of the kidney pattern or the variant histologic patterns. Variant histologic patterns may also be observed in the metastases. The classic pattern usually represents the predominant pattern in most clear cell sarcoma of the kidney tumors; the patterns blend smoothly with one or more of the following variant patterns:

• Myxoid pattern (50%)
• Sclerosing pattern (35%)
• Cellular pattern (26%)
• Epithelioid pattern (trabecular or acinar type) (13%)
• Palisading (Verocay body) pattern (11%)
• Spindle cell pattern (7%)
• Storiform pattern (4%)
• Anaplastic pattern (2.6%)

The anaplastic pattern is defined by nuclear hyperchromasia, nuclear gigantism, and atypical mitoses. Overexpression of p53 (>75% of the nuclei) has been demonstrated in 2 of 3 anaplastic clear cell sarcoma of the kidney lesions. The classic histologic pattern of clear cell sarcoma of the kidney is characterized by cord cells arranged in cords, nests, or groups surrounded by thin fibrovascular septa. A moderate amount of clear intercellular matrix separates the cord cells, giving a clear appearance, hence the designation clear cell sarcoma of the kidney. The term clear cells is doubly a misnomer because the clear cell appearance is caused by loose spacing of the round or oval cord cells with intervening intercellular clear mucoid matrix and because the clear appearance may be absent in many cases.

The diagnosis of clear cell sarcoma of the kidney should be considered, even if no real or apparent clear appearance is noted in the tumor cells in an unusual renal tumor. The classic pattern described is observed at least focally in most patients. However, in a minority of cases, such as in resected tumors or in small biopsy specimens, the classic pattern is absent and only the variant pattern is seen. Therefore, the practicing pathologist must be familiar with the variant pattern.

Unfortunately, no diagnostically useful immunohistochemical features are available. Tumor cells usually test positive for vimentin and negative for cytokeratin, factor VIII–associated antigen, epithelial membrane antigen, desmin, S100, factor XIIIa, c-kit, polyclonal carcinoembryonic antigen (CEA), and MAC387. Positive staining results for cytokeratin, α ₁ -antitrypsin, and α ₁ -antichymotrypsin have been described.

Electron microscopy reveals features of primitive mesenchymal cells with abundant pale extracellular matrix, containing scant collagen fibers, and occasional septa, containing myofibroblasts or pericytes. The main contribution of immunohistochemistry and electron microscopy is to exclude other diagnostic possibilities.

Staging for renal tumors is as follows:

• Stage I: The tumor is limited to the kidney and is completely resected. The renal capsule is intact, and no evidence of rupture is observed. The vessels of the renal sinus are not involved, and no evidence of tumor at or beyond the margins of resection exists.
• Stage II: The tumor extends beyond the kidney but is completely resected. Regional extension of tumor has occurred. Blood vessels outside the renal parenchyma (including those of the renal sinus) may contain tumor. Biopsy is performed on tumors (except by fine needle aspiration), or spillage of the tumor occurs before or during surgery; spillage is confined to the flank and does not involve the peritoneal surface. No evidence of tumor at or beyond the margins of resection is noted.
• Stage III: Residual tumor is nonhematogenous and is confined to the abdomen. Stage III criteria are (1) the presence of lymph nodes within the abdomen (renal hilar, para-aortic, or beyond) that demonstrate positive results for tumor, (2) the tumor penetrates the peritoneal surface, (3) the tumor implants on the peritoneal surface, (4) gross or microscopic evidence of the tumor is present after resection, (5) resection is incomplete because of involvement of vital structures, or (6) tumor spillage is not confined to the flank.
• Stage IV: Hematogenous metastases (eg, lung, liver, bone, brain) or lymph node metastases extend outside of the abdominopelvic region.
• Stage V: Bilateral renal involvement is discovered at diagnosis. Each side is staged individually using the above criteria.