title: Calibration of EBT2 film using a red-channel PDD method in combination with a modified three-channel technique.
abstract: OBJECTIVE
  Ashland Inc. EBT2 and EBT3 films are widely used in quality assurance for radiation therapy; however, there remains a relatively high degree of uncertainty [B. Hartmann, M. Martisikova, and O. Jakel, "Homogeneity of Gafchromic EBT2 film," Med. Phys. 37, 1753-1756 (2010)]. Micke et al. (2011) recently improved the spatial homogeneity using all color channels of a flatbed scanner; however, van Hoof et al. (2012) pointed out that the corrected nonuniformity still requires further investigation for larger fields. To reduce the calibration errors and the uncertainty, the authors propose a new red-channel percentage-depth-dose method in combination with a modified three-channel technique.
  
  
  METHODS
  For the ease of comparison, the EBT2 film image used in the authors' previous study (2012) was reanalyzed using different approaches. Photon beams of 6-MV were delivered to two different films at two different beam on times, resulting in the absorption doses of ranging from approximately 30 to 300 cGy at the vertical midline of the film, which was set to be coincident with the central axis of the beam. The film was tightly sandwiched in a 30(3)-cm(3) polystyrene phantom, and the pixel values for red, green, and blue channels were extracted from 234 points on the central axis of the beam and compared with the corresponding depth doses. The film was first calibrated using the multichannel method proposed by Micke et al. (2010), accounting for nonuniformities in the scanner. After eliminating the scanner and dose-independent nonuniformities, the film was recalibrated via the dose-dependent optical density of the red channel and fitted to a power function. This calibration was verified via comparisons of the dose profiles extracted from the films, where three were exposed to a 60° physical wedge field and three were exposed to composite fields, and all of which were measured in a water phantom. A correction for optical attenuation was implemented, and treatment plans of intensity modulated radiation therapy and volumetric modulated arc therapy were evaluated.
  
  
  RESULTS
  The method described here demonstrated improved accuracy with reduced uncertainty. The relative error compared with the measurements of a water phantom was less than 1%, and the overall calibration uncertainty was less than 2%. Verification tests revealed that the results were close to those of the authors' previous study, and all differences were within 3%, except those with a high-dose gradient. The gamma pass rates (2%/2 mm) of the treatment plan evaluated using the method described here were greater than 99%, and no obvious stripe patterns were observed in the dose-difference maps.
  
  
  CONCLUSIONS
  Spatial homogeneity was significantly improved via the calibration method described here. This technique is both convenient and time-efficient because it does not require cutting the film, and only two exposures are necessary.
authors.affiliation: Departments of Medical Imaging and Radiological Sciences, I-Shou University, Kaohsiung 82445, Taiwan.
authors.forename: Liyun
authors.initials: L
authors.lastname: Chang
authors.affiliation: Department of Nursing, Chang Jung Christian University, Tainan 73657, Taiwan and Department of Radiation Oncology, Chi Mei Medical Center, Liouying, Tainan 73657, Taiwan.
authors.forename: Sheng-Yow
authors.initials: SY
authors.lastname: Ho
authors.affiliation: Medical Physics and Informatics Laboratory, Department of Electronics Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan.
authors.forename: Tsair-Fwu
authors.initials: TF
authors.lastname: Lee
authors.affiliation: Department of Medical Imaging and Radiological Sciences, I-Shou University, Kaohsiung 82445, Taiwan and Department of Radiation Oncology, E-Da Hospital, Kaohsiung 82445, Taiwan.
authors.forename: Shyh-An
authors.initials: SA
authors.lastname: Yeh
authors.affiliation: Department of Medical Imaging and Radiological Sciences, I-Shou University, Kaohsiung 82445, Taiwan.
authors.forename: Hueisch-Jy
authors.initials: HJ
authors.lastname: Ding
authors.affiliation: Department of Radiation Oncology, Sinlau Christian Hospital, Tainan 70142, Taiwan.
authors.forename: Pang-Yu
authors.initials: PY
authors.lastname: Chen
mesh_terms: D002138:Calibration; D005373:Film Dosimetry / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated; D035501:Uncertainty