title: Suitability of markerless EPID tracking for tumor position verification in gated radiotherapy.
abstract: OBJECTIVE
  To maximize the benefits of respiratory gated radiotherapy (RGRT) of lung tumors real-time verification of the tumor position is required. This work investigates the feasibility of markerless tracking of lung tumors during beam-on time in electronic portal imaging device (EPID) images of the MV therapeutic beam.
  
  
  METHODS
  EPID movies were acquired at ∼2 fps for seven lung cancer patients with tumor peak-to-peak motion ranges between 7.8 and 17.9 mm (mean: 13.7 mm) undergoing stereotactic body radiotherapy. The external breathing motion of the abdomen was synchronously measured. Both datasets were retrospectively analyzed in PortalTrack, an in-house developed tracking software. The authors define a three-step procedure to run the simulations: (1) gating window definition, (2) gated-beam delivery simulation, and (3) tumor tracking. First, an amplitude threshold level was set on the external signal, defining the onset of beam-on/-off signals. This information was then mapped onto a sequence of EPID images to generate stamps of beam-on/-hold periods throughout the EPID movies in PortalTrack, by obscuring the frames corresponding to beam-off times. Last, tumor motion in the superior-inferior direction was determined on portal images by the tracking algorithm during beam-on time. The residual motion inside the gating window as well as target coverage (TC) and the marginal target displacement (MTD) were used as measures to quantify tumor position variability.
  
  
  RESULTS
  Tumor position monitoring and estimation from beam's-eye-view images during RGRT was possible in 67% of the analyzed beams. For a reference gating window of 5 mm, deviations ranging from 2% to 86% (35% on average) were recorded between the reference and measured residual motion. TC (range: 62%-93%; mean: 77%) losses were correlated with false positives incidence rates resulting mostly from intra-/inter-beam baseline drifts, as well as sudden cycle-to-cycle fluctuations in exhale positions. Both phenomena can lead to considerable deviations (with MTD values up to a maximum of 7.8 mm) from the intended tumor position, and in turn may result in a marginal miss. The difference between tumor traces determined within the gating window against ground truth trajectory maps was 1.1 ± 0.7 mm on average (range: 0.4-2.3 mm).
  
  
  CONCLUSIONS
  In this retrospective analysis of motion data, it is demonstrated that the system is capable of determining tumor positions in the plane perpendicular to the beam direction without the aid of fiducial markers, and may hence be suitable as an online verification tool in RGRT. It may be possible to use the tracking information to enable on-the-fly corrections to intra-/inter-beam variations by adapting the gating window by means of a robotic couch.
authors.affiliation: Institute for Research and Development on Advanced Radiation Technologies (radART), Paracelsus Medical University, 5020 Salzburg, Austria; University Clinic for Radiotherapy and Radio-Oncology, Landeskrankenhaus Salzburg, Paracelsus Medical University Clinics, 5020 Salzburg, Austria; and Department of Physics and Astronomy, University of Canterbury, Christchurch 8140, New Zealand.
authors.forename: Marco
authors.initials: M
authors.lastname: Serpa
authors.affiliation: Department of Radiation Oncology, University of Wuerzburg, D-97080 Wuerzburg, Germany.
authors.forename: Kurt
authors.initials: K
authors.lastname: Baier
authors.affiliation: Department of Radiation Oncology, University Medical Center Hamburg Eppendorf, D-20246 Hamburg, Germany.
authors.forename: Florian
authors.initials: F
authors.lastname: Cremers
authors.affiliation: Department of Radiation Oncology, University of Wuerzburg, D-97080 Wuerzburg, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Guckenberger
authors.affiliation: Department of Radiation Oncology, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Juergen
authors.initials: J
authors.lastname: Meyer
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D005189:False Positive Reactions; D006801:Humans; D015994:Incidence; D008175:Lung Neoplasms / Q000532:radiotherapy*; D009068:Movement; D016634:Radiosurgery / Q000379:methods*; D011878:Radiotherapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D011881:Radiotherapy, Computer-Assisted / Q000379:methods; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012984:Software