Our novel gantry-independent robotic CBCT system provides high-accuracy volumetric image guidance for proton therapy. Advances in knowledge: Ceiling-mounted robotic CBCT provides a viable option than CT on-rails for partial gantry and fixed-beam proton systems with the added advantage of acquiring images at the treatment isocentre.

Purpose: To realize real‐time‐image gated proton beam therapy (RGPT) for treating mobile tumors. Methods: The rotating gantry of spot scanning proton beam therapy has been designed to equip two x‐ray fluoroscopy devices that enable real‐time imaging of the internal fiducial markers during respiration. Three‐dimensional position of the fiducial marker located near the tumor can be calculated from the fluoroscopic images obtained from orthogonal directions and therapeutic beam is gated only when the fiducial marker is within the predefined gating window. Image acquisition rate can be selected from discrete value ranging from 0.1 Hz to 30 Hz. In order to confirm the effectiveness of RGPT and apply it clinically, clinical commissioning was conducted. Commissioning tests were categorized to main three parts including geometric accuracy, temporal accuracy and dosimetric evaluation. Results: Developed real‐time imaging function has been installed and its basic performances have been confirmed. In the evaluation of geometric accuracy, coincidence of three‐dimensional treatment room coordinate system and imaging coordinate system was confirmed to be less than 1 mm. Fiducial markers (gold sphere and coil) were able to be tracked in simulated clinical condition using an anthropomorphic chest phantom. In the evaluation of temporal accuracy, latency from image acquisition to gate on/off signal was about 60 msec in typical case. In dosimetric evaluation, treatment beam characteristics including beam irradiation position and dose output were stable in gated irradiation. Homogeneity indices to the mobile target were 0.99 (static), 0.89 (w/o gating, motion is parallel to direction of scan), 0.75 (w/o gating, perpendicular), 0.98 (w/ gating, parallel) and 0.93 (w/ gating, perpendicular). Dose homogeneity to the mobile target can be maintained in RGPT. Conclusion: Real‐time imaging function utilizing x‐ray fluoroscopy has been developed and commissioned successfully in order to realize RGPT. Funding Support: This research was partially supported by Japan Society for the Promotion of Science (JSPS) through the FIRST Program. Conflict of Interest: Prof. Shirato has research fund from Hitachi Ltd, Mitsubishi Heavy Industries Ltd and Shimadzu Corporation.

Purpose: To describe the design and performance of a ceiling‐mounted robotic C‐arm CBCT system for image‐guided proton therapy. Methods: Uniquely different from traditional C‐arm CBCT used in interventional radiology, the imaging system was designed to provide volumetric image guidance for patients treated on a 190‐degree proton gantry system and a 6 degree‐of‐freedom (DOF) robotic patient positioner. The mounting of robotic arms to the ceiling rails, rather than gantry or nozzle, provides the flexibility in imaging locations (isocenter, iso+27cm in X, iso+100cm in Y) in the room and easier upgrade as technology advances. A kV X‐ray tube and a 43×43cm flat panel imager were mounted to a rotating C‐ring (87cm diameter), which is coupled to the C‐arm concentrically. Both C‐arm and the robotic arm remain stationary during imaging to maintain high position accuracy. Source‐to‐axis distance and source‐to‐imager distance are 100 and 150cm, respectively. A 14:1 focused anti‐scatter grid and a bowtie filer are used for image acquisition. A unique automatic collimator device of 4 independent blades for adjusting field of view and reducing patient dose has also been developed. Results: Sub‐millimeter position accuracy and repeatability of the robotic C‐arm were measured with a laser tracker. High quality CBCT images for positioning can be acquired with a weighted CTDI of 3.6mGy (head in 200° full fan mode: 100kV, 20mA, 20ms, 10fps)‐8.7 mGy (pelvis in 360° half fan mode: 125kV, 42mA, 20ms, 10fps). Image guidance accuracy achieved <1mm (3D vector) with automatic 3D‐3D registration for anthropomorphic head and pelvis phantoms. Since November 2015, 22 proton therapy patients have undergone daily CBCT imaging for 6 DOF positioning. Conclusion: Decoupled from gantry and nozzle, this CBCT system provides a unique solution for volumetric image guidance with half/partial proton gantry systems. We demonstrated that daily CBCT can be integrated into proton therapy for pre‐treatment position verification.