title: 2D EPID dose calibration for pretreatment quality control of conformal and IMRT fields: A simple and fast convolution approach.
abstract: OBJECTIVE
  This work presents an original algorithm that converts the signal of an electronic portal imaging device (EPID) into absorbed dose in water at the depth of maximum.
  
  
  METHODS
  The model includes a first image pre-processing step that accounts for the non-uniformity of the detector response but also for the perturbation of the signal due to backscatter radiation. Secondly, the image is converted into absorbed dose to water through a linear conversion function associated with a dose redistribution kernel. These two computation parameters were modelled by correlating the on-axis EPID signal with absorbed dose measurements obtained on square fields by using an ionization chamber placed in water at the depth of maximum dose. The accuracy of the algorithm was assessed by comparing the dose determined from the EPID signal with the dose derived by the treatment planning system (TPS) using the ϒ-index. These comparisons were performed on 8 conformal radiotherapy treatment fields (3DCRT) and 18 modulated fields (IMRT).
  
  
  RESULTS
  For a dose difference and a distance-to-agreement set to 3% of the maximum dose and 2 mm respectively, the mean percentage of points with a ϒ-value less than or equal to 1 was 99.8% ± 0.1% for 3DCRT fields and 96.8% ± 2.7% for IMRT fields. Moreover, the mean gamma values were always less than 0.5 whatever the treatment technique.
  
  
  CONCLUSIONS
  These results confirm that our algorithm is an accurate and suitable tool for clinical use in a context of IMRT quality assurance programmes.
authors.affiliation: Groupe Oncorad-Garonne, Service de Radiothérapie, Clinique Pasteur, L'Atrium, 1, rue de la Petite-vitesse, 31300 Toulouse, France; INSERM, Imagerie Cérébrale et Handicaps Neurologiques, UMR 825, F-31059 Toulouse, France. Electronic address: camilleri.jeremy@gmail.com.
authors.forename: Jérémy
authors.initials: J
authors.lastname: Camilleri
authors.affiliation: Groupe Oncorad-Garonne, Service de Radiothérapie, Clinique Pasteur, L'Atrium, 1, rue de la Petite-vitesse, 31300 Toulouse, France.
authors.forename: Jocelyne
authors.initials: J
authors.lastname: Mazurier
authors.affiliation: Groupe Oncorad-Garonne, Service de Radiothérapie, Clinique Pasteur, L'Atrium, 1, rue de la Petite-vitesse, 31300 Toulouse, France.
authors.forename: Denis
authors.initials: D
authors.lastname: Franck
authors.affiliation: Groupe Oncorad-Garonne, Service de Radiothérapie, Clinique du Pont-de-Chaume, 330 avenue Marcel-Unal, 82000 Montauban, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Dudouet
authors.affiliation: Groupe Oncorad-Garonne, Service de Radiothérapie, Clinique Pasteur, L'Atrium, 1, rue de la Petite-vitesse, 31300 Toulouse, France.
authors.forename: Igor
authors.initials: I
authors.lastname: Latorzeff
authors.affiliation: INSERM UMR 1037, Team 15: Multi-resolution dosimetry for radiotherapy optimisation, Centre de recherche en Cancérologie de Toulouse, F-31062 Toulouse, France; Université Toulouse III-Paul Sabatier, UMR1037 CRCT, F-31000 Toulouse, France.
authors.forename: Xavier
authors.initials: X
authors.lastname: Franceries
mesh_terms: D000465:Algorithms; D002138:Calibration; D004867:Equipment Design; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D007477:Ions; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011785:Quality Assurance, Health Care; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D020266:Radiotherapy, Conformal / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012542:Scattering, Radiation; D014867:Water / Q000737:chemistry
chemical_list: D007477:Ions; D014867:Water
keywords: IMRT quality assurance; Patient-specific pretreatment verifications; Portal dosimetry