title: Clinical comparison of positional accuracy and stability between dedicated versus conventional masks for immobilization in cranial stereotactic radiotherapy using 6-degree-of-freedom image guidance system-integrated platform.
abstract: OBJECTIVE
  To compare the positioning accuracy and stability of two distinct noninvasive immobilization devices, a dedicated (D-) and conventional (C-) mask, and to evaluate the applicability of a 6-degrees-of-freedom (6D) correction, especially to the C-mask, based on our initial experience with cranial stereotactic radiotherapy (SRT) using ExacTrac (ET)/Robotics integrated into the Novalis Tx platform.
  
  
  METHODS
  The D- and C-masks were the BrainLAB frameless mask system and a general thermoplastic mask used for conventional radiotherapy such as whole brain irradiation, respectively. A total of 148 fractions in 71 patients and 125 fractions in 20 patients were analyzed for the D- and C-masks, respectively. For the C-mask, 3D correction was applied to the initial 10 patients, and thereafter, 6D correction was adopted. The 6D residual errors (REs) in the initial setup, after correction (pre-treatment), and during post-treatment were measured and compared.
  
  
  RESULTS
  The D-mask provided no significant benefit for initial setup. The post-treatment median 3D vector displacements (interquatile range) were 0.38 mm (0.22, 0.60) and 0.74 mm (0.49, 1.04) for the D- and C-masks, respectively (p<0.001). The post-treatment maximal translational REs were within 1 mm and 2 mm for the D- and C-masks, respectively, and notably within 1.5 mm for the C-mask with 6D correction. The pre-treatment 3D vector displacements were significantly correlated with those for post-treatment in both masks.
  
  
  CONCLUSIONS
  The D-mask confers positional stability acceptable for SRT. For the C-mask, 6D correction is also recommended, and an additional setup margin of 0.5 mm to that for the D-mask would be sufficient. The tolerance levels for the pre-treatment REs should similarly be set as small as possible for both systems.
authors.affiliation: Department of Radiology, Gifu University Graduate School of Medicine, Japan. ohtakara@gifu-u.ac.jp
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Ohtakara
authors.forename: Shinya
authors.initials: S
authors.lastname: Hayashi
authors.forename: Hidekazu
authors.initials: H
authors.lastname: Tanaka
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Hoshi
authors.forename: Masashi
authors.initials: M
authors.lastname: Kitahara
authors.forename: Katsuya
authors.initials: K
authors.lastname: Matsuyama
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Okada
mesh_terms: D016371:Cranial Irradiation / Q000295:instrumentation*; D019583:Dose Fractionation, Radiation; D005260:Female; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D007103:Immobilization / Q000295:instrumentation*; D008297:Male; D008397:Masks; D058958:Organs at Risk; D056888:Patient Positioning; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*; D012371:Robotics / Q000295:instrumentation*; D018709:Statistics, Nonparametric; D013238:Stereotaxic Techniques