title: Mechanical characterization of the varian Exact-arm and R-arm support systems for eight aS500 electronic portal imaging devices.
abstract: OBJECTIVE
  The aim of this study is to compare the positioning accuracy at different gantry angles of two electronic portal imaging devices (EPIDs) support arm systems by using EPID difference images as a measure for displacement. This work presents a comparison of the mechanical performance of eight Varian aS500 (Varian Medical Systems, Palo Alto, CA) EPIDs, mounted using either the Varian Exact-arm or R-arm.
  
  
  METHODS
  The mechanical performance of the two arm systems was compared by investigating the variation in sensitivity with gantry angle, both before and after the EPID position was adjusted after gantry rotation. Positional errors were investigated by subtracting images from a reference image taken at gantry 0 degrees, and the amplitude of the peaks and troughs at the field edges for longitudinal (radial) and lateral (transverse) profiles across the resulting image was related to the distance of displacement. Calibration curves based on a pixel-by-pixel shift were generated for each EPID and the Varian hand pendant accuracy was compared to the calibration data.
  
  
  RESULTS
  The response of the EPIDs was found to change with gantry rotation, with the largest difference at 180 degrees. The Exact-arm was found to correct well for any displacement, while the R-arm tended to overcorrect following repositioning using the hand pendant. The calibration curves were consistent within each set of matched linacs, and the hand pendant accuracy was similar for both arm systems, although generally in different directions. With respect to gantry rotation effects, the mechanical performance of the Exact-arm systems was found to be much better than that of the R-arm systems. At gantry positions 90 degrees, 270 degrees, and 180 degrees the average misalignment in the longitudinal direction was +4.2 +/- 0.2, +1.8 +/- 1.6, and +7.4 +/- 0.5 mm for the R-arms, and +2.9 +/- 0.2, +2.1 +/- 0.8, and +4.9 +/- 0.7 mm for the Exact-arms. In the lateral direction the average positional errors were +2.1 +/- 0.4, -4.7 +/- 0.4, and -2.5 +/- 0.5 mm for the R-arms, and -0.3 +/- 0.3, -0.5 +/- 0.3, and -0.4 +/- 0.2 mm for the Exact-arms. The hand pendant correction had minimal impact in the lateral direction for both arm systems. However in the longitudinal direction the mean errors for the R-arms were +3.4 +/- 0.7, +1.5 +/- 0.6, and +4.6 +/- 0.7 mm at gantry angles 90 degrees, 270 degrees, and 180 degrees, and the equivalent Exact-arm errors were +0.9 +/- 0.3, +1.2 +/- 0.3, and +1.9 +/- 0.9 mm, respectively.
  
  
  CONCLUSIONS
  The performance of the EPIDs demonstrate that the Exact-arm system provides a more reproducible position and better agreement with the EPID position as indicated on the EPID pendant at all gantry angles than the R-arm.
authors.affiliation: Radiotherapy Physics, Belfast Health and Social Care Trust, Northern Ireland Cancer Centre, Belfast City Hospital Site, Lisburn Road, Belfast BT9 7AB, Northern Ireland, United Kingdom. mark.grattan@belfasttrust.hscni.net
authors.forename: Mark W D
authors.initials: MW
authors.lastname: Grattan
authors.forename: Conor K
authors.initials: CK
authors.lastname: McGarry
mesh_terms: D002138:Calibration; D003198:Computer Simulation; D004582:Electronics, Medical; D004867:Equipment Design; D006801:Humans; D010315:Particle Accelerators; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D011878:Radiotherapy / Q000295:instrumentation* / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results