title: An EPID-based method for comprehensive verification of gantry, EPID and the MLC carriage positional accuracy in Varian linacs during arc treatments.
abstract: BACKGROUND
  In modern radiotherapy, it is crucial to monitor the performance of all linac components including gantry, collimation system and electronic portal imaging device (EPID) during arc deliveries. In this study, a simple EPID-based measurement method has been introduced in conjunction with an algorithm to investigate the stability of these systems during arc treatments with the aim of ensuring the accuracy of linac mechanical performance.
  
  
  METHODS
  The Varian EPID sag, gantry sag, changes in source-to-detector distance (SDD), EPID and collimator skewness, EPID tilt, and the sag in MLC carriages as a result of linac rotation were separately investigated by acquisition of EPID images of a simple phantom comprised of 5 ball-bearings during arc delivery. A fast and robust software package was developed for automated analysis of image data. Twelve Varian linacs of different models were investigated.
  
  
  RESULTS
  The average EPID sag was within 1 mm for all tested linacs. All machines showed less than 1 mm gantry sag. Changes in SDD values were within 1.7 mm except for three linacs of one centre which were within 9 mm. Values of EPID skewness and tilt were negligible in all tested linacs. The maximum sag in MLC leaf bank assemblies was around 1 mm. The EPID sag showed a considerable improvement in TrueBeam linacs.
  
  
  CONCLUSIONS
  The methodology and software developed in this study provide a simple tool for effective investigation of the behaviour of linac components with gantry rotation. It is reproducible and accurate and can be easily performed as a routine test in clinics.
authors.affiliation: School of Physics, University of Western Australia, Crawley, WA, 6009, Australia. pejman.rowshanfarzad@uwa.edu.au.
authors.forename: Pejman
authors.initials: P
authors.lastname: Rowshanfarzad
authors.affiliation: Radiotherapy Physics, Northern Ireland Cancer Centre, Belfast Health and Social Care Trust, Belfast, UK. conor.mcgarry@belfasttrust.hscni.net.|Centre for Cancer Research and Cell Biology, Queen's University Belfast, Belfast, UK. conor.mcgarry@belfasttrust.hscni.net.
authors.forename: Conor K
authors.initials: CK
authors.lastname: McGarry
authors.affiliation: Department of Radiation Oncology, Calvary Mater Newcastle Hospital, Newcastle, NSW, 2310, Australia. Michael.Barnes@calvarymater.org.au.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Barnes
authors.affiliation: Department of Radiation Oncology, Sir Charles Gairdner Hospital, Nedlands, WA, 6009, Australia. Mahsheed.Sabet@health.wa.gov.au.
authors.forename: Mahsheed
authors.initials: M
authors.lastname: Sabet
authors.affiliation: School of Physics, University of Western Australia, Crawley, WA, 6009, Australia. Martin.Ebert@health.wa.gov.au.|Department of Radiation Oncology, Sir Charles Gairdner Hospital, Nedlands, WA, 6009, Australia. Martin.Ebert@health.wa.gov.au.
authors.forename: Martin A
authors.initials: MA
authors.lastname: Ebert
mesh_terms: D019047:Phantoms, Imaging; D011834:Radiation Monitoring / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation* / Q000379:methods*