title: A technique of using gated-CT images to determine internal target volume (ITV) for fractionated stereotactic lung radiotherapy.
abstract: OBJECTIVE
  To develop and evaluate a technique and procedure of using gated-CT images in combination with PET image to determine the internal target volume (ITV), which could reduce the planning target volume (PTV) with adequate target coverage.
  
  
  METHODS
  A skin marker-based gating system connected to a regular single slice CT scanner was used for this study. A motion phantom with adjustable motion amplitude was used to evaluate the CT gating system. Specifically, objects of various sizes/shapes, considered as virtual tumors, were placed on the phantom to evaluate the number of phases of gated images required to determine the ITV while taking into account tumor size, shape and motion. A procedure of using gated-CT and PET images to define ITV for patients was developed and was tested in patients enrolled in an IRB approved protocol.
  
  
  RESULTS
  The CT gating system was capable of removing motion artifacts for target motion as large as 3-cm when it was gated at optimal phases. A phantom study showed that two gated-CT scans at the end of expiration and the end of inspiration would be sufficient to determine the ITV for tumor motion less than 1-cm, and another mid-phase scan would be required for tumors with 2-cm motion, especially for small tumors. For patients, the ITV encompassing visible tumors in all sets of gated-CT and regular spiral CT images seemed to be consistent with the target volume determined from PET images. PTV expanded from the ITV with a setup uncertainty margin had less volume than PTVs from spiral CT images with a 10-mm generalized margin or an individualized margin determined at fluoroscopy.
  
  
  CONCLUSIONS
  A technique of determining the ITV using gated-CT images was developed and was clinically implemented successfully for fractionated stereotactic lung radiotherapy.
authors.affiliation: Department of Radiation Oncology, Henry Ford Hospital, Detroit, MI, USA. jjin1@hfhs.org
authors.forename: Jian-Yue
authors.initials: JY
authors.lastname: Jin
authors.forename: Munther
authors.initials: M
authors.lastname: Ajlouni
authors.forename: Qing
authors.initials: Q
authors.lastname: Chen
authors.forename: Fang-Fang
authors.initials: FF
authors.lastname: Yin
authors.forename: Benjamin
authors.initials: B
authors.lastname: Movsas
mesh_terms: D000465:Algorithms; D016477:Artifacts; D019583:Dose Fractionation, Radiation; D005471:Fluoroscopy; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008168:Lung / Q000528:radiation effects*; D008175:Lung Neoplasms / Q000532:radiotherapy*; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal; D050397:Radiotherapy, Intensity-Modulated; D013238:Stereotaxic Techniques; D015898:Tomography Scanners, X-Ray Computed; D036542:Tomography, Spiral Computed; D014057:Tomography, X-Ray Computed / Q000379:methods*