title: Comparison of set up accuracy among three common immobilisation systems for intensity modulated radiotherapy of nasopharyngeal carcinoma patients.
abstract: BACKGROUND
  In intensity modulated radiotherapy (IMRT) of nasopharyngeal carcinoma (NPC) patients, an effective immobilisation system is important to minimise set up deviation. This study evaluated the effectiveness of three immobilisation systems by assessing their set up deviations.
  
  
  METHODS
  Patients were randomly assigned to one of the three immobilisation systems: (1) supine on head rest and base plate (HB); (2) supine with alpha cradle supporting the head and shoulder (AC); (3) supine with vacuum bag supporting the head and shoulder (VB). CBCT was conducted weekly for each patient on the linear accelerator. Image registration was conducted at the nasopharynx (NP) and cervical regions. The translational displacements (latero-medial, antero-posterior and cranio-caudal), rotational displacements (pitch, yaw and roll) and 3D vectors obtained at the NP and cervical regions were recorded and compared among the three systems.
  
  
  RESULTS
  The mean translational and rotational deviations were within 3 mm and 2°, respectively, and the range of 3D vector was 1.53-3.47 mm. At the NP region, the AC system demonstrated the smallest translational and rotational deviations and 3D vector. The differences were significant except for the latero-medial, yaw and roll directions. Similarly, at the cervical region, the AC system showed smaller translational and rotational deviations and 3D vector, with only the cranio-caudal and yaw deviations that did not reach statistical significance.
  
  
  CONCLUSIONS
  Set up deviation was greater in the neck than the NP region. The set up accuracy of the AC system was better than the other two systems, and it is recommended for IMRT of NPC patients in our institution.
authors.affiliation: State Key Laboratory of Oncology in South China, Sun Yat-sen University Cancer Center, Guangzhou, China.
authors.forename: Cheng-Guang
authors.initials: CG
authors.lastname: Lin
authors.affiliation: State Key Laboratory of Oncology in South China, Sun Yat-sen University Cancer Center, Guangzhou, China.
authors.forename: Sen-Kui
authors.initials: SK
authors.lastname: Xu
authors.affiliation: State Key Laboratory of Oncology in South China, Sun Yat-sen University Cancer Center, Guangzhou, China.
authors.forename: Wen-Yan
authors.initials: WY
authors.lastname: Yao
authors.affiliation: State Key Laboratory of Oncology in South China, Sun Yat-sen University Cancer Center, Guangzhou, China.
authors.forename: Yu-Qi
authors.initials: YQ
authors.lastname: Wu
authors.affiliation: State Key Laboratory of Oncology in South China, Sun Yat-sen University Cancer Center, Guangzhou, China.
authors.forename: Jian-Lan
authors.initials: JL
authors.lastname: Fang
authors.affiliation: Department of Health Technology and Informatics, Hong Kong Polytechnic University, Hung Hom, Hong Kong.
authors.forename: Vincent W C
authors.initials: VWC
authors.lastname: Wu
mesh_terms: D000328:Adult; D000368:Aged; D002277:Carcinoma / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D054893:Cone-Beam Computed Tomography; D005260:Female; D006801:Humans; D007103:Immobilization / Q000295:instrumentation*; D008297:Male; D008875:Middle Aged; D000077274:Nasopharyngeal Carcinoma; D009303:Nasopharyngeal Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*; D061106:Radiotherapy Setup Errors; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation*; D016896:Treatment Outcome; D055815:Young Adult
keywords: Cone beam CT; immobilisation system; intensity modulated radiotherapy; nasopharyngeal carcinoma; set up accuracy