title: Assessment of a customised immobilisation system for head and neck IMRT using electronic portal imaging.
abstract: OBJECTIVE
  To evaluate set-up reproducibility of a cabulite shell and determine CTV-PTV margins for head and neck intensity-modulated-radiotherapy.
  
  
  METHODS
  Twenty patients were entered into the study. A total of 354 anterior and lateral isocentric electronic portal images (EPIs) were compared to simulator reference images.
  
  
  RESULTS
  About 94% of all translational displacements were < or =3 mm, and 99% < or =5 mm. The overall systematic error was 0.9 mm (+/-1.0SD) in the Right-Left, 0.7 mm (+/-0.9SD) in the Superior-Inferior and -0.02 mm (+/-1.1SD) in the Anterior-Posterior directions. The corresponding SDs of the random errors were +/-0.4, +/-0.6 and +/-0.7 mm. The estimated margins required from CTV-PTV were calculated according to the Van Herk formula was 2.9, 2.6 and 3.3 mm, respectively.
  
  
  CONCLUSIONS
  This head and neck immobilisation system is of sufficient accuracy for its use with IMRT treatments and a 3 mm CTV-PTV margin has been adopted.
authors.affiliation: Radiotherapy Department, Royal Marsden NHS Trust, London, UK.
authors.forename: Mandy
authors.initials: M
authors.lastname: Humphreys
authors.forename: M Teressa
authors.initials: MT
authors.lastname: Guerrero Urbano
authors.forename: Cefas
authors.initials: C
authors.lastname: Mubata
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Miles
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Harrington
authors.forename: Margaret
authors.initials: M
authors.lastname: Bidmead
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Nutting
mesh_terms: D004307:Dose-Response Relationship, Radiation; D004581:Electronics; D004867:Equipment Design; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D007103:Immobilization / Q000295:instrumentation*; D011832:Radiation Injuries / Q000517:prevention & control; D011878:Radiotherapy / Q000379:methods; D015203:Reproducibility of Results