SU-E-T-543: The Use of a Proportional-Integral-Derivative Design for Optimized Real-Time Head Motion Correction in Frameless SRS

Abstract

Purpose: While less invasive, the immobilization devices used in frameless SRS techniques generally allow for more head movement than a frame, leading to a larger PTV. To maintain traditional SRS accuracies, we have developed a robotic 3D head motion stage to correct for these sub‐ millimeter head deviations occurring during treatment. To achieve optimal head motion management we report on our use of a proportional‐integral‐ derivative (PID) feedback loop to monitor and compensate for real‐time patient motion during frameless SRS. Methods: A PID controller consists of three corrective parameters: a proportional term to account for the error difference between the desired patient position and the actual position; an integral term that accelerates the position correction toward the desired setpoint value; and a derivative term that accounts for the instantaneous rate of change of the patient position error. These parameters were tuned according to manual and heuristic Zielger‐Nichols methods. The PID algorithm was integrated into existing Labview software designed to control patient motion via a computer‐controlled 3D stepper motor stage. For realtime head position feedback either a Polaris external marker tracker or VisionRT system was used. The PID correction was tested both on phantom and healthy volunteers. Results: With volunteers in a relaxed supine position, the PID method maintained head motion to under 0.25 mm as based on the optical monitoring system in all three directions for greater than 90% of the time. Artificial introduction of extreme 1–5 mm head shifts by manual couch moving were corrected 2–3 times faster by PID when compared to our prior constant motor velocity approach Conclusions: In the event of patient head deviations, a properly tuned PID controller can offer significant time sparing advantages. This would allow for quicker initial setup after coarse couch positioning and a higher duty cycle during treatment.