title: Set-up errors analyses in IMRT treatments for nasopharyngeal carcinoma to evaluate time trends, PTV and PRV margins.
abstract: BACKGROUND
  the aims of this study were to analyze the systematic and random interfractional set-up errors during Intensity Modulated Radiation Therapy (IMRT) in 20 consecutive nasopharyngeal carcinoma (NPC) patients by means of Electronic Portal Images Device (EPID), to define appropriate Planning Target Volume (PTV) and Planning Risk Volume (PRV) margins, as well as to investigate set-up displacement trend as a function of time during fractionated RT course.
  
  
  METHODS
  before EPID clinical implementation, an anthropomorphic phantom was shifted intentionally 5 mm to all directions and the EPIs were compared with the digitally reconstructed radiographs (DRRs) to test the system's capability to recognize displacements observed in clinical studies. Then, 578 clinical images were analyzed with a mean of 29 images for each patient.
  
  
  RESULTS
  phantom data showed that the system was able to correct shifts with an accuracy of 1 mm. As regards clinical data, the estimated population systematic errors were 1.3 mm for left-right (L-R), 1 mm for superior-inferior (S-I) and 1.1 mm for anterior-posterior (A-P) directions, respectively. Population random errors were 1.3 mm, 1.5 mm and 1.3 mm for L-R, S-I and A-P directions, respectively. PTV margin was at least 3.4, 3 and 3.2 mm for L-R, S-I and A-P direction, respectively. PRV margins for brainstem and spinal cord were 2.3, 2 and 2.1 mm and 3.8, 3.5 and 3.2 mm for L-R, A-P and S-I directions, respectively. Set-up error displacements showed no significant changes as the therapy progressed (p>0.05), although displacements >3 mm were found more frequently when severe weight loss or tumor nodal shrinkage occurred.
  
  
  CONCLUSIONS
  these results enable us to choose margins that guarantee with sufficient accuracy the coverage of PTVs and organs at risk sparing. Collected data confirmed the need for a strict check of patient position reproducibility in case of anatomical changes.
authors.affiliation: Department of Medical Physics, Fondazione IRCCS Istituto Nazionale Tumori, Milan, Italy. valeria.mongioj@istitutotumori.mi.it
authors.forename: Valeria
authors.initials: V
authors.lastname: Mongioj
authors.forename: Ester
authors.initials: E
authors.lastname: Orlandi
authors.forename: Mauro
authors.initials: M
authors.lastname: Palazzi
authors.forename: Elena
authors.initials: E
authors.lastname: Deponti
authors.forename: Franceschini
authors.initials: F
authors.lastname: Marzia
authors.forename: Claudio
authors.initials: C
authors.lastname: Stucchi
authors.forename: Claudia
authors.initials: C
authors.lastname: Sangalli
authors.forename: Carlo
authors.initials: C
authors.lastname: Fallai
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Zonca
authors.forename: Patrizia
authors.initials: P
authors.lastname: Olmi
authors.forename: Emanuele
authors.initials: E
authors.lastname: Pignoli
mesh_terms: D000328:Adult; D000368:Aged; D002277:Carcinoma / Q000532:radiotherapy*; D005260:Female; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D008297:Male; D008875:Middle Aged; D009303:Nasopharyngeal Neoplasms / Q000532:radiotherapy*; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated; D013997:Time Factors