title: Relative profile and dose verification of intensity-modulated radiation therapy.
abstract: OBJECTIVE
  To develop a quality assurance (QA) procedure to assess the intensity profile and dosimetry for intensity-modulated (IM) treatment fields using electronic portal imaging devices (EPIDs).
  
  
  METHODS
  A series of rapidly acquired (approximately 1/sec) portal images are summed and converted to dose. For relative intensity QA, the intended profile is subtracted point-by-point from the measured profile forming a series of error values. The standard deviation, sigma, of the errors, a measure of the goodness of the match, is minimized by applying a normalization and uniform scatter subtraction from the measured profile. For dose verification (dose to isocenter), an empirically determined phantom-correction factor is added to incorporate the effect of patient presence on EPID readings. Seventy prostate treatment fields were used in a phantom study to verify these approaches. Sensitivity was studied by creating artificial mismatches.
  
  
  RESULTS
  The average sigma for relative profile verification is 3.3% (percentage of average intended intensity) whereas artificial mismatches resulted in sigma values from 5% to 27%. The average isocentric dose calculated from EPID readings is 1.001 relative to the planned dose with a standard deviation of 0.018.
  
  
  CONCLUSIONS
  An EPID can be used for profile verification and absolute isocentric dose measurement for IM fields.
authors.affiliation: Medical Physics Department, Memorial Sloan-Kettering Cancer Center, New York, NY, USA. changj@mskcc.org
authors.forename: J
authors.initials: J
authors.lastname: Chang
authors.forename: G S
authors.initials: GS
authors.lastname: Mageras
authors.forename: C S
authors.initials: CS
authors.lastname: Chui
authors.forename: C C
authors.initials: CC
authors.lastname: Ling
authors.forename: W
authors.initials: W
authors.lastname: Lutz
mesh_terms: D000465:Algorithms; D004582:Electronics, Medical / Q000295:instrumentation; D006801:Humans; D016014:Linear Models; D008297:Male; D019047:Phantoms, Imaging; D055585:Physical Phenomena; D010825:Physics; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D011786:Quality Control; D011879:Radiotherapy Dosage / Q000592:standards*; D011881:Radiotherapy, Computer-Assisted / Q000592:standards*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity