title: Comparison of different strategies to use four-dimensional computed tomography in treatment planning for lung cancer patients.
abstract: OBJECTIVE
  To discuss planning target volumes (PTVs) based on internal target volume (PTVITV), exhale-gated radiotherapy (PTVGating), and a new proposed midposition (PTVMidP; time-weighted mean tumor position) and compare them with the conventional free-breathing CT scan PTV (PTVConv).
  
  
  METHODS
  Respiratory motion induces systematic and random geometric uncertainties. Their contribution to the clinical target volume (CTV)-to-PTV margins differs for each PTV approach. The uncertainty margins were calculated using a dose-probability-based margin recipe (based on patient statistics). Tumor motion in four-dimensional CT scans was determined using a local rigid registration of the tumor. Geometric uncertainties for interfractional setup errors and tumor baseline variation were included. For PTVGating, the residual motion within a 30% gating (time) window was determined. The concepts were evaluated in terms of required CTV-to-PTV margin and PTV volume for 45 patients.
  
  
  RESULTS
  Over the patient group, the PTVITV was on average larger (+6%) and the PTVGating and PTVMidP smaller (-10%) than the PTVConv using an off-line (bony anatomy) setup correction protocol. With an on-line (soft tissue) protocol the differences in PTV compared with PTVConv were +33%, -4%, and 0, respectively.
  
  
  CONCLUSIONS
  The internal target volume method resulted in a significantly larger PTV than conventional CT scanning. The exhale-gated and mid-position approaches were comparable in terms of PTV. However, mid-position (or mid-ventilation) is easier to use in the clinic because it only affects the planning part of treatment and not the delivery.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands.
authors.forename: Jochem W H
authors.initials: JW
authors.lastname: Wolthaus
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
authors.forename: José S A
authors.initials: JS
authors.lastname: Belderbos
authors.forename: Maddalena M G
authors.initials: MM
authors.lastname: Rossi
authors.forename: Joos V
authors.initials: JV
authors.lastname: Lebesque
authors.forename: Eugène M F
authors.initials: EM
authors.lastname: Damen
mesh_terms: D002138:Calibration; D005260:Female; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy; D008297:Male; D009068:Movement; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012119:Respiration; D012189:Retrospective Studies; D013997:Time Factors; D014057:Tomography, X-Ray Computed / Q000379:methods*; D047368:Tumor Burden