Observational, retrospective study of real-world second-line anti–PD-(L)1 immunotherapy in advanced/metastatic upper-tract urothelial carcinoma. Intervention: immune checkpoint inhibitors (monoclonal antibodies) against PD-1 (e.g., pembrolizumab, nivolumab) or PD-L1 (e.g., atezolizumab, durvalumab, avelumab). Mechanism: block PD-1/PD-L1 interaction to release inhibitory signaling, restoring cytotoxic T-cell activation, proliferation, and antitumor function. Targets/pathways: PD-1 on T cells and PD-L1 on tumor/immune cells, PD-1/PD-L1 checkpoint axis (reducing SHP2-mediated inhibition of TCR signaling). Contextual biology: UTUC more often harbors MSI-high and FGFR pathway alterations, which may influence immunotherapy responsiveness.