title: Comparison of two immobilization techniques using portal film and digitally reconstructed radiographs for pediatric patients with brain tumors.
abstract: OBJECTIVE
  To compare the accuracy of two immobilization techniques for pediatric brain tumor patients.
  
  
  METHODS
  We analyzed data from 128 treatments involving 22 patients. Patients were immobilized with either a relocatable head frame (12 patients) or a vacuum bag (10 patients). Orthogonal portal films were used as verification images. Errors in patient positioning were measured by comparing verification images with digitally reconstructed radiographs generated by a three-dimensional treatment-planning system.
  
  
  RESULTS
  With the head frame, systematic errors ranged from 1.4 mm to 2.1 mm; random errors, from 1.7 mm to 2.1 mm. With the vacuum bag, systematic errors ranged from 2.1 mm to 2.5 mm; random errors, from 2.0 mm to 2.6 mm. For the head frame, the mean length of the radial displacement was 4.4 mm; 90% of the total three-dimensional deviation was less than 6.8 mm. The corresponding values for the vacuum bag were 5.0 and 6.6 mm, respectively.
  
  
  CONCLUSIONS
  The head frame and vacuum bag techniques limit the random and systematic errors in each of the three directions to within +/- 5 mm. We have used these results to determine the margin used to create the planning target volume for conformal radiation therapy.
authors.affiliation: Department of Radiation Oncology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA. yunping.zhu@stjude.org
authors.forename: Y
authors.initials: Y
authors.lastname: Zhu
authors.forename: J
authors.initials: J
authors.lastname: Stovall
authors.forename: L
authors.initials: L
authors.lastname: Butler
authors.forename: Q
authors.initials: Q
authors.lastname: Ji
authors.forename: M W
authors.initials: MW
authors.lastname: Gaber
authors.forename: S
authors.initials: S
authors.lastname: Samant
authors.forename: M R
authors.initials: MR
authors.lastname: Sontag
authors.forename: A J
authors.initials: AJ
authors.lastname: de Armendi
authors.forename: T E
authors.initials: TE
authors.lastname: Merchant
mesh_terms: D001932:Brain Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D002648:Child; D006801:Humans; D007091:Image Processing, Computer-Assisted; D055585:Physical Phenomena; D010825:Physics; D011859:Radiography; D012149:Restraint, Physical / Q000295:instrumentation / Q000379:methods*