A robotic C-arm cone beam CT system for image-guided proton therapy: design and performance

Objective: A ceiling-mounted robotic C-arm cone beam CT (CBCT) system was developed for use with a 190 degrees proton gantry system and a 6-degree-of-freedom robotic patient positioner. We report on the mechanical design, system accuracy, image quality, image guidance accuracy, imaging dose, workflow, safety and collision-avoidance. Methods: The robotic CBCT system couples a rotating C-ring to the C-arm concentrically with a kV X-ray tube and a flat-panel imager mounted to the C-ring. CBCT images are acquired with flex correction and maximally 360 degrees rotation for a 53 cm field of view. The system was designed for clinical use with three imaging locations. Anthropomorphic phantoms were imaged to evaluate the image guidance accuracy. Results: The position accuracy and repeatability of the robotic C-arm was high (< 0.5 mm), as measured with a high-accuracy laser tracker. The isocentric accuracy of the C-ring rotation was within 0.7 mm. The coincidence of CBCT imaging and radiation isocentre was better than 1 mm. The average image guidance accuracy was within 1 mm and 1 degrees for the anthropomorphic phantoms tested. Daily volumetric imaging for proton patient positioning was specified for routine clinical practice. Conclusion: Our novel gantry-independent robotic CBCT system provides high-accuracy volumetric image guidance for proton therapy.