Case Report: A rare case of primary undifferentiated pleomorphic sarcoma of the renal pelvis with high PD-L1 expression and a misleading positive urine FISH.

Primary undifferentiated pleomorphic sarcoma (UPS) of the renal pelvis is an extremely rare and aggressive mesenchymal malignancy. The diagnosis remains challenged, and its clinical characteristics and treatment strategies remain unclear. Urine fluorescence in situ hybridization (FISH) is a highly sensitive tool for detecting urothelial carcinoma (UC), but 'false'-positive results can be observed in non-UC malignancies. We present a challenging case of primary UPS of renal pelvis with positive urine FISH and high programmed death receptor ligand 1 (PD-L1) expression, which complicates the initial diagnosis and opens avenues for potential immunotherapy.