title: Gamma index comparison of three VMAT QA systems and evaluation of their sensitivity to delivery errors.
abstract: OBJECTIVE
  To compare detectors for dosimetric verification before VMAT treatments and evaluate their sensitivity to errors.
  
  
  METHODS
  Measurements using three detectors (ArcCheck, 2d array 729 and EPID) were used to validate the dosimetric accuracy of the VMAT delivery. Firstly, performance of the three devices was studied. Secondly, to assess the reliability of the detectors, 59 VMAT treatment plans from a variety of clinical sites were considered. Thirdly, systematic variations in collimator, couch and gantry angle plus MLC positioning were applied to four clinical treatments (two prostate, two head and neck cases) in order to establish the detection sensitivity of the three devices. Measurements were compared with TPS computed doses via gamma analysis (3%/3 mm and 2%/2 mm) with an agreement of at least 95% and 90% respectively in all pixels. Effect of the errors on the dose distributions was analyzed.
  
  
  RESULTS
  Repeatability and reproducibility were excellent for the three devices. The average pass rate for the 59 cases was superior to 98% for all devices with 3%/3 mm criteria. It was found that for the plans delivered with errors, the sensitivity was quite similar for all devices. Devices were able to detect a 2 mm opened or closed MLC error with 3%/3 mm tolerance level. An error of 3° in collimator, gantry or couch rotation was detected by the three devices using 2%/2 mm criteria.
  
  
  CONCLUSIONS
  All three devices have the potential to detect errors with more or less the same threshold. Nevertheless, there is no guarantee that pretreatment QA will catch delivery errors.
authors.affiliation: Department of Medical Physics, Institut Claudius Regaud - Institut Universitaire du Cancer de Toulouse, 1 avenue Irène Joliot Curie, 31059 Toulouse Cedex 9, France. Electronic address: vieillevigne.laure@iuct-oncopole.fr.
authors.forename: Laure
authors.initials: L
authors.lastname: Vieillevigne
authors.affiliation: Department of Medical Physics, Institut Claudius Regaud - Institut Universitaire du Cancer de Toulouse, 1 avenue Irène Joliot Curie, 31059 Toulouse Cedex 9, France. Electronic address: jeremy.molinier@gmail.com.
authors.forename: Jeremy
authors.initials: J
authors.lastname: Molinier
authors.affiliation: Department of Medical Physics, Institut Claudius Regaud - Institut Universitaire du Cancer de Toulouse, 1 avenue Irène Joliot Curie, 31059 Toulouse Cedex 9, France. Electronic address: brun.thomas@iuct-oncopole.fr.
authors.forename: Thomas
authors.initials: T
authors.lastname: Brun
authors.affiliation: Department of Medical Physics, Institut Claudius Regaud - Institut Universitaire du Cancer de Toulouse, 1 avenue Irène Joliot Curie, 31059 Toulouse Cedex 9, France. Electronic address: ferrand.regis@iuct-oncopole.fr.
authors.forename: Regis
authors.initials: R
authors.lastname: Ferrand
mesh_terms: D006801:Humans; D019300:Medical Errors / Q000517:prevention & control*; D009369:Neoplasms / Q000532:radiotherapy; D011786:Quality Control; D011874:Radiometry; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000592:standards*; D012399:Rotation
keywords: Delivery error; Gamma index; Quality assurance; RapidArc