title: Feasibility study on inverse four-dimensional dose reconstruction using the continuous dose-image of EPID.
abstract: OBJECTIVE
  When an intensity-modulated radiation beam is delivered to a moving target, the interplay effect between dynamic beam delivery and the target motion due to miss-synchronization can cause unpredictable dose delivery. The portal dose image in electronic portal imaging device (EPID) represents radiation attenuated and scattered through target media. Thus, it may possess information about delivered radiation to the target. Using a continuous scan (cine) mode of EPID, which provides temporal dose images related to target and beam movements, the authors' goal is to perform four-dimensional (4D) dose reconstruction.
  
  
  METHODS
  To evaluate this hypothesis, first, the authors have derived and subsequently validated a fast method of dose reconstruction based on virtual beamlet calculations of dose responses using a test intensity-modulated beam. This method was necessary for processing a large number of EPID images pertinent for four-dimensional reconstruction. Second, cine mode acquisition after summation over all images was validated through comparison with integration mode acquisition on EPID (IAS3 and aS1000) for the test beam. This was to confirm the agreement of the cine mode with the integrated mode, specifically for the test beam, which is an accepted mode of image acquisition for dosimetry with EPID. Third, in-phantom film and exit EPID dosimetry was performed on a moving platform using the same beam. Heterogeneous as well as homogeneous phantoms were used. The cine images were temporally sorted at 10% interval. The authors have performed dose reconstruction to the in-phantom plane from the sorted cine images using the above validated method of dose reconstruction. The reconstructed dose from each cine image was summed to compose a total reconstructed dose from the test beam delivery, and was compared with film measurements.
  
  
  RESULTS
  The new method of dose reconstruction was validated showing greater than 95.3% pass rates of the gamma test with the criteria of dose difference of 3% and distance to agreement of 3 mm. The dose comparison of the reconstructed dose with the measured dose for the two phantoms showed pass rates higher than 96.4% given the same criteria.
  
  
  CONCLUSIONS
  Feasibility of 4D dose reconstruction was successfully demonstrated in this study. The 4D dose reconstruction demonstrated in this study can be a promising dose validation method for radiation delivery on moving organs.
authors: Radiation Medicine Program, Loma Linda University Medical Center, Loma Linda, California 92354, USA. medicphys@hotmail.com
authors: Inhwan Jason
authors: IJ
authors: Yeo
authors: Jae Won
authors: JW
authors: Jung
authors: Byong Yong
authors: BY
authors: Yi
authors: Jong Oh
authors: JO
authors: Kim
mesh_terms: D054893:Cone-Beam Computed Tomography; D055615:Electrical Equipment and Supplies; D005240:Feasibility Studies; D056973:Four-Dimensional Computed Tomography / Q000295:instrumentation*; D006801:Humans; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation*; D015203:Reproducibility of Results
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