Aberrant Positivity for BCL6 Corepressor (BCOR) Immunohistochemistry in Undifferentiated Round Cell Sarcoma of the Kidney With Melanoma of the Ureter

BCL6 corepressor (BCOR)-rearrangement sarcoma, an undifferentiated round cell sarcoma, is rare, and its pathophysiology is diverse. A 14-year-old girl presented with a tumor in the left kidney with invasion of the left iliopsoas muscle, left renal vein, and inferior vena cava. An immunohistochemical analysis of the tumor biopsy specimen was positive for BCOR, but no BCOR gene aberrations were detected. Chemotherapy was administered according to the Ewing's sarcoma protocol. When the residual tumor was resected after chemotherapy, melanoma was found in the ureter, which was removed. At eight months after surgery, the patient relapsed and underwent another round of chemotherapy, followed by additional irradiation. These results suggest that some sarcomas without BCOR gene variants were found to be BCOR-positive by immunohistochemistry (IHC) and that melanoma and sarcoma coexisted.