title: CT verification of isocentre relocatability using stereotactic mask fixation system.
abstract: OBJECTIVE
  This study introduces a non-invasive method based on computed tomography (CT) verification to ensure patients are accurately positioned before fractionated stereotactic radiotherapy. It enables quality control of mask positioning with reference to the CT images of the treatment plan.
  
  
  METHODS
  A mask system, together with a dental impression moulded mouth bite, was used for patient immobilisation. In order to facilitate relevant image comparison, special alignment during CT localisation was discussed in the study. The accuracy of patient set-up was studied by assessing the isocentre position in relation to the patient's anatomical structure. The planning CT images were applied as a reference and the study was applied to 261 cranial applications.
  
  
  RESULTS
  The results show that the mean and the maximum overall displacements at the isocentre were 0.7 and 2.5 mm, respectively. The mean and the maximum rotational displacement in the axial plane were 0.56 degrees and 2 degrees, respectively. The mean translational displacement and rotational displacement were close to zero when considering the direction of movement.
  
  
  CONCLUSIONS
  The results indicate that the systematic error of the mask system and the verification method are minimal. Advantages of this technique include the simple set-up, three-dimensional quantification and short study time (10-15 min). It is therefore practical to implement on a routine basis. Investigation of the ability to relocate the mask is also recommended to justify the required safety margin between the clinical and planning target volumes.
authors.affiliation: Department of Clinical Oncology, Tuen Mun Hospital, Hong Kong, People's Republic of China. Victy_Wong@hotmail.com
authors.forename: V Y W
authors.initials: VY
authors.lastname: Wong
authors.forename: S Y
authors.initials: SY
authors.lastname: Tung
authors.forename: T W
authors.initials: TW
authors.lastname: Leung
authors.forename: K H S
authors.initials: KH
authors.lastname: Ho
mesh_terms: D019583:Dose Fractionation, Radiation; D006801:Humans; D008397:Masks; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*