Distant metastatic potential in primary renal sarcoma with MEIS1-NCOA2 gene fusion.

Recently, two cases of a distinct sarcoma characterized by a novel MEIS1-NCOA2 gene fusion arising in the kidney were described (1). Grossly, both these tumors were characterized by a distinctive nodular appearance, one of which was predominantly cystic with multiple excrescences projecting within cyst lumens, while the other demonstrated solid architecture, with perirenal fat and renal sinus vasculature invasion. Morphologically, the most distinctive feature of these neoplasms was their nodular growth pattern, produced by monomorphic plump spindle cells arranged in vague fascicles with a whorling pattern. A small component of a more primitive small round cell component typically centered on capillary vasculature was also present. In addition, both cases had variant patterns, including well-formed fascicles composed of elongated spindled cells in collagenous background, reminiscent of monophasic synovial sarcoma in one and epithelioid-round cell component with Homer Wright pseudorosette-like structures in the other. The latter tumor also showed aggressive features including necrosis and renal sinus vascular and perinephric fat invasion. The immunoprofile was not specific. Given the very limited clinical follow up in both these patients, the malignant potential of these tumors remains to be confirmed. To our knowledge, no additional cases have been reported. Herein, we report one such case with biopsy proven metastatic disease, broadening the histologic spectrum and confirming the distant metastatic potential of this emerging group of tumors.