title: 3D Dosimetric verification of volumetric-modulated arc therapy by portal dosimetry.
abstract: OBJECTIVE
  To demonstrate the feasibility of back-projection portal dosimetry for accurate 3D dosimetric verification of volumetric-modulated arc therapy (VMAT), pre-treatment as well as in vivo.
  
  
  METHODS
  Several modifications to our existing approach were implemented to make the method applicable to VMAT: (i) gantry angle-resolved data acquisition, (ii) calculation of the patient transmission, (iii) compensation for detector 'flex' and (iv) 3D dose reconstruction and evaluation.
  
  
  RESULTS
  Planned and EPID-(Electronic Portal Image Detector)-reconstructed dose distributions show good agreement for pre-treatment verification of two prostate, a stereotactic lung and a head-and-neck VMAT plan and for in vivo verification of VMAT treatments of prostate and lung cancer. Averaged over pre-treatment verifications, planned and measured isocentre dose ratios were -1.2% (range [-4.7%,1.8%]). 3D gamma analysis (3% maximum dose, 3mm) revealed mean gamma gamma(mean)=0.37 [0.34,0.39], maximum 1% gamma gamma(1%)=0.72 [0.66,0.81] and percentage of points with gamma1 P(gamma)(1)=99% [97%,100%]. For in vivo verification, the average isocentre dose ratio was -1.2% [-0.8%,-1.7%], gamma(mean)=0.52 [0.40,0.64], gamma(1%)=0.92 [0.76,1.08] and P(gamma)(1)=96% [93%,100%].
  
  
  CONCLUSIONS
  Our portal dosimetry method was successfully adapted for verification of VMAT treatments, pre-treatment as well as in vivo.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands. a.mans@nki.nl
authors.forename: Anton
authors.initials: A
authors.lastname: Mans
authors.forename: Peter
authors.initials: P
authors.lastname: Remeijer
authors.forename: Igor
authors.initials: I
authors.lastname: Olaciregui-Ruiz
authors.forename: Markus
authors.initials: M
authors.lastname: Wendling
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
authors.forename: Ben
authors.initials: B
authors.lastname: Mijnheer
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
authors.forename: Joep C
authors.initials: JC
authors.lastname: Stroom
mesh_terms: D005240:Feasibility Studies; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D021621:Imaging, Three-Dimensional; D008175:Lung Neoplasms / Q000532:radiotherapy; D008297:Male; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012984:Software