title: An empirical calibration method for an a-Si portal imaging device: applications in pretreatment verification of IMRT.
abstract: OBJECTIVE
  A new calibration method for an amorphous silicon (a-Si) electronic portal imaging device (EPID) used for dose measurements in pretreatment verification (field-related) of intensity-modulated radiation therapy (IMRT) with sliding-window technique. The method is independent of data contained in the multileaf collimator (MLC) leaf-motion files and of any calculations made by the treatment planning system (TPS).
  
  
  METHODS
  Sensitivity of the EPID is dependent on radiation energy. For fluence-modulated fields, different dose/reading calibration factors are associated with each pixel of the image acquired by calculating equivalent areas representing the exact ratio between primary and scatter components. The dose measured in the detector plane was compared with that calculated with TPS by using gamma-analysis. Each calibration factor was compared with that calculated by considering the individual contributions of primary and secondary radiation obtained using the convolution method with analytical kernel for homogeneous media.
  
  
  RESULTS
  In 837/854 (98%) of the clinical fields analysed, the proportion of irradiated area in which the gamma-index was <1.0 exceeded 95%. The overall average gamma-index was 0.39. There was good agreement between the dose/reading calibration factors obtained with the empirical algorithm and with the convolution method.
  
  
  CONCLUSIONS
  The proposed calibration method is suitable for routine clinical pretreatment verification in IMRT.
authors.affiliation: Dipartimento di Medicina Sperimentale, Università degli Studi dell'Insubria, Varese, Italy. leopoldo.conte@uninsubria.it
authors.forename: L
authors.initials: L
authors.lastname: Conte
authors.forename: C
authors.initials: C
authors.lastname: Mordacchini
authors.forename: L
authors.initials: L
authors.lastname: Pozzi
authors.forename: C
authors.initials: C
authors.lastname: Vite
mesh_terms: D002138:Calibration; D004867:Equipment Design; D006801:Humans; D015233:Models, Statistical; D011874:Radiometry / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*; D050397:Radiotherapy, Intensity-Modulated; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity; D012825:Silicon
chemical_list: D012825:Silicon