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Design and Evaluation of a Robotic C-Arm CBCT System for Image-Guided Proton Therapy

C Hua

C Hua1*, W Yao1 , T Kidani2 , J Farr1 , K Tomida2 , S Ozawa2 , T Nishimura2 , T Fujusawa2 , R Shinagawa2 , T Merchant1 , (1) St. Jude Children's Research Hospital, Memphis, TN, (2) Hitachi, Ltd., Hitachi-shi, Ibaraki-ken, Japan


TU-FG-BRB-11 (Tuesday, August 2, 2016) 1:45 PM - 3:45 PM Room: Ballroom B

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 43x43cm 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.

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