Postoperative cognitive decline (POCD) is a common and impactful outcome of surgical procedures in older adults. The pathophysiology and causative mechanisms for POCD are poorly understood. The robot-assisted radical cystectomy (RARC) is increasingly utilized. In patients undergoing RARC, Although prolonged Trendelenburg position and pneumoperitoneum can increase the cerebral blood flow, the excessive cerebral perfusion can lead to encephalemia, which reduce the oxygen uptake of brain tissue and cause insufficient oxygenation of brain tissue at the cellular level. POCD may take place due to cerebral hemodynamic changes. The goal of the current study is to investigate the combined effect of this position and CO2 pneumoperitoneum on POCD during RARC with the monitor of cerebral oxygen.