title: A comparison between four immobilization systems for pelvic radiation therapy using CBCT and paired kilovoltage portals based image-guided radiotherapy.
abstract: BACKGROUND
  We commonly use 6- and 4-clamped thermoplastic molds (TMs) for rigid immobilization during pelvic radiotherapy (RT), sometimes a vacuum cushion (VC) is also used as leg support with TM. Our objective was to report the setup margins (SMs) associated with the different systems, to analyze whether any of these systems is superior, and to analyze whether any of them showed better reproducibility in any particular direction.
  
  
  METHODS
  Retrospective analysis was done by dividing the patients into four groups: 6-clamp with VC (6CVC), 6-clamp without VC (6CNC), 4-clamp with VC (4CVC), and 4-clamp without VC (4CNC). A repeat offline review was done for all patients and errors were tabulated. Statistical methods were then applied.
  
  
  RESULTS
  Total 24 patients had 413 image-guided RT (IGRT) sessions, 312 were cone beam computed tomography scan (CBCT) scans and 101 were paired kilovoltage portals (kVp). There was no statistically significant difference between 6CVC and 6CNC. However, while comparing 4CVC and 4CNC, a statistically significant difference was seen in all directions. VC improved precision in vertical and lateral direction mainly, while the 6-clamped TM improved reproducibility in longitudinal direction.
  
  
  CONCLUSIONS
  SM was low for all the four immobilization systems studied. There is no added benefit of using a VC with 6-clamped TM for pelvic RT. Use of a VC is recommended with 4-clamped TM to improve overall reproducibility. 6-clamped TM helps keep the errors low.
authors.affiliation: Department of Radiation Oncology, Fortis International Oncology Center, Noida, India.
authors.forename: Gagan
authors.initials: G
authors.lastname: Saini
authors.forename: Anchal
authors.initials: A
authors.lastname: Aggarwal
authors.forename: Syed Ashraf
authors.initials: SA
authors.lastname: Jafri
authors.forename: Vineeta
authors.initials: V
authors.lastname: Goel
authors.forename: Thomas
authors.initials: T
authors.lastname: Ranjitsingh
authors.forename: Ram
authors.initials: R
authors.lastname: Munjal
authors.forename: Anil Kumar
authors.initials: AK
authors.lastname: Anand
mesh_terms: D054893:Cone-Beam Computed Tomography; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D008297:Male; D015233:Models, Statistical; D010315:Particle Accelerators; D056888:Patient Positioning; D010388:Pelvis / Q000528:radiation effects*; D011878:Radiotherapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D015203:Reproducibility of Results; D012149:Restraint, Physical; D012189:Retrospective Studies