Marked Anaplasia Obscuring the Diagnosis of Clear Cell Sarcoma of the Kidney in Pretreatment Needle Biopsy.

Clear cell sarcoma of the kidney is an uncommon pediatric renal malignancy with metastatic potential and marked morphologic heterogeneity. The anaplastic variant is rare and may mimic other high-grade pediatric renal tumors, resulting in diagnostic challenges in limited pretreatment biopsy specimens. A 3-year-old girl presented with progressive abdominal distension. Imaging showed a 15-cm right retroperitoneal mass with tumor thrombus in the inferior vena cava and no distant metastasis. Pretreatment core needle biopsy showed a highly pleomorphic malignant neoplasm with necrosis and fibrous-to-myxoid stroma; however, a definitive diagnosis could not be established. The patient was clinically managed as a nephroblastoma and received preoperative chemotherapy followed by surgical resection. The resected specimen showed features characteristic of clear cell sarcoma of the kidney. Immunoreactivity of BCL6 corepressor and cyclin D1 was observed, whereas Wilms tumor 1 was negative. Molecular testing further identified an in-frame internal tandem duplication of BCOR. Retrospective immunohistochemistry on the pretreatment biopsy demonstrated concordant BCOR and cyclin D1 expression. Marked anaplasia can obscure the typical morphology of clear cell sarcoma of the kidney. Immunohistochemistry for BCL6 corepressor and cyclin D1 at an early diagnostic stage, with molecular confirmation may facilitate accurate diagnosis and timely treatment.