# 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

## TL;DR

A rare kidney tumor case with misleading test results and high PD-L1 expression is reported, offering insights into diagnosis and immunotherapy.

## Contribution

This case report highlights a rare tumor with a misleading diagnostic test and high PD-L1, suggesting potential for immunotherapy.

## Key findings

- Primary undifferentiated pleomorphic sarcoma of the renal pelvis is extremely rare and challenging to diagnose.
- Urine FISH can produce misleading positive results in non-urothelial malignancies.
- High PD-L1 expression in this case suggests potential for immunotherapy.

## Abstract

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.