title: Interfractional set-up errors evaluation by daily electronic portal imaging of IMRT in head and neck cancer patients.
abstract: BACKGROUND
  Interfractional set-up errors were assessed from daily portal images (PI) registration for head and neck cancer patients. We aimed to evaluate whether a daily PI is worthwhile and we derived the Planning Target Volume (PTV) margins from the estimation of systematic and random errors.
  
  
  METHODS
  Twenty patients were treated in supine position with a fixed 5-point mask immobilisation system and head-and-knee supports. DRRs (Digitally Reconstructed Radiograph) were obtained from the planning CT-scan and considered the reference images to be compared with two orthogonal PI by matching bone anatomy landmarks. A total of 567 PI were done. For the set-up errors analysis, we determined the systematic, random, and overall standard deviations (SD), as well as the overall means in three directions (cranio caudal CC, medio lateral ML and anterior posterior AP). PTV-margins were calculated according to three methods. Differences of SD regarding the overall displacements among portals performed every day and each 2, 3, or 4 days were tested.
  
  
  RESULTS
  The systematic set-up errors were less than 1 mm in the three directions whereas the random set-up errors were around 2 mm. PTV margins varied from 3 to 4 mm in the 3 directions. Corrections were significant in the CC direction only, in which the set-up error increased significantly when the scenario of one PI every 3 fractions was adopted.
  
  
  CONCLUSIONS
  It is of practical importance to apply on-line protocols with contouring of the bony landmarks on the PI in order to decrease the systematic mean error in this patient group. This study suggested that a PI in AP and ML directions once a week and every two days in the CC direction would be adequate to overcome the problem of set-up errors.
authors.affiliation: Department of Radiation Oncology, Institut Gustave-Roussy, Villejuif, France.
authors.forename: Berrin
authors.initials: B
authors.lastname: Pehlivan
authors.forename: Charlotte
authors.initials: C
authors.lastname: Pichenot
authors.forename: Marine
authors.initials: M
authors.lastname: Castaing
authors.forename: Anne
authors.initials: A
authors.lastname: Auperin
authors.forename: Dimitri
authors.initials: D
authors.lastname: Lefkopoulos
authors.forename: Rodrigo
authors.initials: R
authors.lastname: Arriagada
authors.forename: Jean
authors.initials: J
authors.lastname: Bourhis
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003952:Diagnostic Imaging; D019583:Dose Fractionation, Radiation; D004581:Electronics; D005260:Female; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007103:Immobilization / Q000295:instrumentation* / Q000379:methods; D008198:Lymph Nodes / Q000528:radiation effects*; D008297:Male; D008875:Middle Aged; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*