title: Analysis of reproducibility of respiration-triggered gated radiotherapy for lung tumors.
abstract: OBJECTIVE
  Respiration-gated radiotherapy (RGRT) can decrease the toxicity of chemo-radiotherapy (CT-RT) by allowing use of smaller treatment fields. RGRT requires a predictable relationship between tumor position and external surrogate, which must be verified during treatment. Time-integrated electronic portal imaging (TI-EPI) identifies mean intra-fractional positions of moving structures, and was used to study reproducibility of anatomy during RGRT for lung tumors.
  
  
  METHODS
  TI-EPIs were acquired using an amorphous silicon-based electronic portal imaging system (EPID, aS500) in continuous image acquisition mode in 11 patients treated with audio-coached RGRT at end-inspiration. The Varian Real-time Position Management (RPM) system was used for 4DCT imaging and RGRT delivery. All TI-EPI portals were co-registered to corresponding digitally reconstructed radiographs (DRR) of the planning 4DCT using the spinal column. Displacements in tumor position or that of an adjacent bronchus during RGRT was measured relative to the reference structure on the DRR.
  
  
  RESULTS
  Vertebra-matched portals revealed systematic (Sigma) and random (sigma) errors of 1.8 and 1.3mm in medial-lateral direction and 1.7 and 1.7 mm in cranial-caudal direction, indicating a reproducible tumor/bronchus position during the RPM-triggered gates.
  
  
  CONCLUSIONS
  RGRT delivery at end-inspiration can achieve reproducible internal anatomy in 'gated' fields delivered with audio-coaching.
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands.
authors.forename: Femke O B
authors.initials: FO
authors.lastname: Spoelstra
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 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: D019583:Dose Fractionation, Radiation; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D009068:Movement; D009367:Neoplasm Staging; D011879:Radiotherapy Dosage; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012119:Respiration; D014057:Tomography, X-Ray Computed; D016896:Treatment Outcome