title: Set-up errors and planning target volume margins in head and neck cancer radiotherapy: a clinical study of image guidance with on-line cone-beam computed tomography.
abstract: BACKGROUND
  Set-up errors represent a source of uncertainty in head and neck (H&N) cancer radiotherapy. The present study evaluated set-up accuracy with the use of cone-beam computed tomography (CBCT) in order to establish the proper clinical target volume (CTV) to planning target volume (PTV) margins to be adopted.
  
  
  METHODS
  Local set-up accuracy was analysed for 44 H&N cancer patients since the implementation of CBCT. An on-line correction protocol was adopted, with the first 3 scans used to correct systematic errors with a 3-mm action level. The overall mean displacement (M), the population systematic (Σ) and random (σ) errors and the 3D vector length were calculated. PTV margins were calculated according to the van Herk formula (2.5Σ + 0.7σ).
  
  
  RESULTS
  A total of 420 CBCT scans were analysed. A systematic correction was needed in 43% of patients. The value of M was <1 mm in all directions; the values of Σ and σ ranged over 1-1.2 and 1.4-1.9 mm, respectively. Pre-correction PTV margins were 3.48, 4.08 and 4.33 mm along the 3 axes. The PTV margins calculated after online correction were <2.5 mm in all directions.
  
  
  CONCLUSIONS
  Kilovoltage CBCT is effective in evaluating set-up accuracy in H&N patients. CTV-PTV margins of 5 mm are safe and are currently adopted at our centre; however, some special situations, such as re-irradiation or the close proximity of organs at risk and high-dose regions, could benefit from daily image registration and lower (i.e., 3 mm) margins.
authors.affiliation: Unit of Radiotherapy, Niguarda Ca' Granda Hospital, P. Ospedale Maggiore 3, 20162, Milan, Italy. francesco.dionisi@ospedaleniguarda.it
authors.forename: Francesco
authors.initials: F
authors.lastname: Dionisi
authors.forename: Mauro Filippo
authors.initials: MF
authors.lastname: Palazzi
authors.forename: Francesco
authors.initials: F
authors.lastname: Bracco
authors.forename: Maria Grazia
authors.initials: MG
authors.lastname: Brambilla
authors.forename: Claudia
authors.initials: C
authors.lastname: Carbonini
authors.forename: Diego Dario
authors.initials: DD
authors.lastname: Asnaghi
authors.forename: Angelo Filippo
authors.initials: AF
authors.lastname: Monti
authors.forename: Alberto
authors.initials: A
authors.lastname: Torresin
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D005260:Female; D006258:Head and Neck Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009862:Online Systems; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D061089:Radiotherapy, Image-Guided