title: Linking log files with dosimetric accuracy--A multi-institutional study on quality assurance of volumetric modulated arc therapy.
abstract: OBJECTIVE
  To systematically evaluate machine specific quality assurance (QA) for volumetric modulated arc therapy (VMAT) based on log files by applying a dynamic benchmark plan.
  
  
  METHODS
  A VMAT benchmark plan was created and tested on 18 Elekta linacs (13 MLCi or MLCi2, 5 Agility) at 4 different institutions. Linac log files were analyzed and a delivery robustness index was introduced. For dosimetric measurements an ionization chamber array was used. Relative dose deviations were assessed by mean gamma for each control point and compared to the log file evaluation.
  
  
  RESULTS
  Fourteen linacs delivered the VMAT benchmark plan, while 4 linacs failed by consistently terminating the delivery. The mean leaf error (±1SD) was 0.3±0.2 mm for all linacs. Large MLC maximum errors up to 6.5 mm were observed at reversal positions. Delivery robustness index accounting for MLC position correction (0.8-1.0) correlated with delivery time (80-128 s) and depended on dose rate performance. Dosimetric evaluation indicated in general accurate plan reproducibility with γ(mean)(±1 SD)=0.4±0.2 for 1 mm/1%. However single control point analysis revealed larger deviations and attributed well to log file analysis.
  
  
  CONCLUSIONS
  The designed benchmark plan helped identify linac related malfunctions in dynamic mode for VMAT. Log files serve as an important additional QA measure to understand and visualize dynamic linac parameters.
authors.affiliation: Lake Constance Radiation Oncology Center Singen-Friedrichshafen, Germany. Electronic address: pasler@strahlentherapie-fn.de.
authors.forename: Marlies
authors.initials: M
authors.lastname: Pasler
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Jochem
authors.initials: J
authors.lastname: Kaas
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Thijs
authors.initials: T
authors.lastname: Perik
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Job
authors.initials: J
authors.lastname: Geuze
authors.affiliation: EBG MedAustron GmbH, Wiener Neustadt and Department of Radiotherapy, Medical University of Vienna, Austria.
authors.forename: Ralf
authors.initials: R
authors.lastname: Dreindl
authors.affiliation: Institute of Medical Physics, Landeskrankenhaus Feldkirch, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Künzler
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Frits
authors.initials: F
authors.lastname: Wittkamper
authors.affiliation: Div. Medical Radiation Physics, Department of Radiation Oncology, Medical University Vienna/AKH Vienna, Austria; Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University Vienna, Austria.
authors.forename: Dietmar
authors.initials: D
authors.lastname: Georg
mesh_terms: D006801:Humans; D010315:Particle Accelerators; D011785:Quality Assurance, Health Care; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods* / Q000592:standards; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods* / Q000592:standards; D015203:Reproducibility of Results
keywords: Dosimetry; Leaf position errors; Log files; QA; VMAT