title: [Influence of patient setup error on planning risk volumes for nasopharyngeal carcinoma patients receiving intensity-modulated radiation therapy].
abstract: OBJECTIVE
  Determination of planning risk volumes (PRVs) for an organ at risk greatly affects dose optimization in designing the intensity-modulated radiation therapy (IMRT) regimen. Patient setup errors have been found to closely correlate to the definition of PRVs. This study was to investigate the safety margin for the organ at risk during IMRT planning for nasopharyngeal carcinoma (NPC) patients.
  
  
  METHODS
  Nineteen NPC patients (stage T1-2N0M0) who received IMRT for the first time were studied. Repeated computed tomography (CT) scans were performed for the patients once a week during the whole treatment course. A total of 85 CT scan reports were obtained. Differences between patient positioning of each time and first treatment setup were caluculated by comparing the anatomical landmarks (that is, optical nerve, pituitary, spine, and parotid) on each CT scan image using Osiris software.
  
  
  RESULTS
  The displacement of optical nerve and pituitary in X, Y, and Z directions were, in absolute values, (0.86+/-0.53) mm, (0.84+/-0.68) mm, and (0.93+/-1.02)mm, respectively. The standard deviations (SDs) of systematic errors for the axial vector displacement were 0.83 mm, 1.08 mm, and 1.21 mm, while the SDs of random errors were 0.85 mm, 0.83 mm and 1.14 mm. The displacement of spine and parotid in X, Y, and Z directions were, in absolute values, (0.98+/-0.74) mm, (1.25+/-0.88) mm, and (1.43+/-1.02) mm, respectively. The SDs of systematic errors for axial vector displacement were 0.98 mm, 1.35 mm, and 1.87 mm, while the SDs of random errors were 1.02 mm, 1.46 mm, and 1.54 mm.
  
  
  CONCLUSIONS
  It is feasible to determine the size of a safety margin of IMRT for organs at risk using repeated CT scans for NPC patients.
authors.affiliation: State Key Laboratory of Oncology in South China,Guangzhou, Guangdong, 510060, P. R. China. lchengg@mail.sysu.edu.cn
authors.forename: Cheng-Guang
authors.initials: CG
authors.lastname: Lin
authors.forename: Guo-Wen
authors.initials: GW
authors.lastname: Li
authors.forename: Xiao-Zhen
authors.initials: XZ
authors.lastname: Liu
authors.forename: Liu-Wen
authors.initials: LW
authors.lastname: Lin
authors.forename: Chong
authors.initials: C
authors.lastname: Zhao
authors.forename: Shao-Peng
authors.initials: SP
authors.lastname: Liu
authors.forename: Lan-Feng
authors.initials: LF
authors.lastname: Lin
authors.forename: Li-Ping
authors.initials: LP
authors.lastname: Xiao
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009303:Nasopharyngeal Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D014057:Tomography, X-Ray Computed