A patient safety system (PSS) (100) uses optical tracking in a linear accelerator treatment room to prevent gross setup errors. A patient (150) undergoes a computed tomography (CT) treatment-simulation scan while a CT ball bearing (BB) is on patients surface. The CT BB is replaced with an infrared reflective marker (IRRM) (160) before radiotherapy treatment starts. Coordinates of the CT BB relative to an isocenter of the treatment room are used as reference for tracking. The coordinate system of an optical tracking system is converted to a coordinate system of the treatment room. The PSS evaluates setup accuracy for a radiotherapy session by comparing real-time position of the single IRRM determined by the optical tracking technology with a predicted reference position, and displays results on a graphical user interface. The PSS stops radiation when a deviation between real-time coordinates and predicted coordinates of the IRRM (160) exceeds a predefined threshold.
