Isocenter characteristics of an external ring proton gantry

Purpose: Determine the shape, size, geometric center, and virtual center of the isocenter for a proton gantry and compare to electron/X-ray accelerator gantries. Methods and materials: The majority of commercial electron/X-ray accelerator gantries consist of a rotating treatment head mounted to a stationary stand through a slewing ring bearing. The world''s first proton gantry uses two rotating external rings, to which is mounted a fixed treatment nozzle with a movable snout that extends close to the center of rotation. The radial aspect of the isocenter for two similar proton gantries and two different electron/X-ray gantries were measured in the gantry frame of reference with a front pointer and a theodolite. These results were then transformed into room coordinates. The axial aspect of the isocenter was measured with a dial indicator. Results: The radial aspect of the isocenter for slewing ring gantries has the shape of two concentric circles. The radial aspect of the isocenter for external ring gantries is shaped like a butterfly. The size of the mechanical isocenter is independent of the gantry style. Conclusions: The locations of the geometric and virtual centers can be determined to within 0.2 mm. Multiple gantry angle treatments can be delivered with a single setup allowing 2 mm for gantry and nozzle deflections. Precision treatments can be delivered allowing only 0.5 mm if the measured isocenter path is applied. [Copyright &y& Elsevier]