title: Isocenter characteristics of an external ring proton gantry.
abstract: OBJECTIVE
  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
  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.
authors.affiliation: Department of Radiation Medicine, Loma Linda University Medical Center, Loma Linda, CA 92354, USA. moyers@proton.llumc.edu
authors.forename: Michael F
authors.initials: MF
authors.lastname: Moyers
authors.forename: Waylan
authors.initials: W
authors.lastname: Lesyna
mesh_terms: D002138:Calibration; D004867:Equipment Design; D010315:Particle Accelerators / Q000295:instrumentation*; D055585:Physical Phenomena; D010825:Physics; D061766:Proton Therapy; D011878:Radiotherapy / Q000295:instrumentation*; D012399:Rotation; D013679:Technology, Radiologic / Q000295:instrumentation / Q000592:standards; D011522:Protons
chemical_list: D011522:Protons