title: Verifying 4D gated radiotherapy using time-integrated electronic portal imaging: a phantom and clinical study.
abstract: BACKGROUND
  Respiration-gated radiotherapy (RGRT) can decrease treatment toxicity by allowing for smaller treatment volumes for mobile tumors. RGRT is commonly performed using external surrogates of tumor motion. We describe the use of time-integrated electronic portal imaging (TI-EPI) to verify the position of internal structures during RGRT delivery
  
  
  METHODS
  TI-EPI portals were generated by continuously collecting exit dose data (aSi500 EPID, Portal vision, Varian Medical Systems) when a respiratory motion phantom was irradiated during expiration, inspiration and free breathing phases. RGRT was delivered using the Varian RPM system, and grey value profile plots over a fixed trajectory were used to study object positions. Time-related positional information was derived by subtracting grey values from TI-EPI portals sharing the pixel matrix. TI-EPI portals were also collected in 2 patients undergoing RPM-triggered RGRT for a lung and hepatic tumor (with fiducial markers), and corresponding planning 4-dimensional CT (4DCT) scans were analyzed for motion amplitude.
  
  
  RESULTS
  Integral grey values of phantom TI-EPI portals correlated well with mean object position in all respiratory phases. Cranio-caudal motion of internal structures ranged from 17.5-20.0 mm on planning 4DCT scans. TI-EPI of bronchial images reproduced with a mean value of 5.3 mm (1 SD 3.0 mm) located cranial to planned position. Mean hepatic fiducial markers reproduced with 3.2 mm (SD 2.2 mm) caudal to planned position. After bony alignment to exclude set-up errors, mean displacement in the two structures was 2.8 mm and 1.4 mm, respectively, and corresponding reproducibility in anatomy improved to 1.6 mm (1 SD).
  
  
  CONCLUSIONS
  TI-EPI appears to be a promising method for verifying delivery of RGRT. The RPM system was a good indirect surrogate of internal anatomy, but use of TI-EPI allowed for a direct link between anatomy and breathing patterns.
authors.affiliation: Department of Radiation Oncology, VU University medical center, Amsterdam, The Netherlands. j.vansornsendekoste@vumc.nl
authors.forename: John R
authors.initials: JR
authors.lastname: van Sörnsen de Koste
authors.forename: Johan P
authors.initials: JP
authors.lastname: Cuijpers
authors.forename: Frank G M
authors.initials: FG
authors.lastname: de Geest
authors.forename: Frank J
authors.initials: FJ
authors.lastname: Lagerwaard
authors.forename: Ben J
authors.initials: BJ
authors.lastname: Slotman
authors.forename: Suresh
authors.initials: S
authors.lastname: Senan
mesh_terms: D004581:Electronics; D006801:Humans; D008175:Lung Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D009068:Movement; D009367:Neoplasm Staging; D019047:Phantoms, Imaging; D011856:Radiographic Image Enhancement / Q000379:methods*; D011878:Radiotherapy / Q000379:methods*; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012143:Respiratory Physiological Phenomena