title: Measuring radiotherapy setup errors at multiple neck levels in nasopharyngeal cancer (NPC): A case for differential PTV expansion.
abstract: OBJECTIVE
  We aim to quantify the magnitude of the systematic and random setup errors at three different anatomical levels of the neck in Nasopharyngeal Carcinoma (NPC) when clivus matching is used, and recommend appropriate PTV margins for each level.
  
  
  METHODS
  Thirty-six patients undergoing image-guided radiotherapy (IGRT) each with 9 scheduled CBCTs were reviewed. The magnitude of setup errors were measured at the level of the clivus, C4 and C7 vertebrae, before and after CBCT correction. The 3D displacements, systematic and random errors were calculated for each level. The appropriate PTV expansion was determined using Van Herk's formula.
  
  
  RESULTS
  Mean 3D displacement was 1.88, 2.66 and 3.35 mm at the clivus, C4 and C7 before correction. The differences were statistically significant (p<0.05). The PTV margin required without correction was 2.33, 4.33 and 6.52 mm respectively. These were reduced to 1.20, 3.72 and 6.08 mm after CBCT corrections.
  
  
  CONCLUSIONS
  Variability is seen in setup errors at the clivus, C4 and C7 vertebral levels. A variable planning margin approach with reduced margin at the clivus is recommended. Use of daily CBCT allows the PTV expansion to be reduced to 1.2 mm.
authors.affiliation: Department of Radiation Oncology, National University Cancer Institute, Singapore (NCIS), National University Health System (NUHS), Singapore. Electronic address: timothy_cheo@nuhs.edu.sg.
authors.forename: Timothy
authors.initials: T
authors.lastname: Cheo
authors.affiliation: Department of Radiation Oncology, National University Cancer Institute, Singapore (NCIS), National University Health System (NUHS), Singapore.
authors.forename: Yvonne
authors.initials: Y
authors.lastname: Loh
authors.affiliation: Department of Radiation Oncology, National University Cancer Institute, Singapore (NCIS), National University Health System (NUHS), Singapore.
authors.forename: Desiree
authors.initials: D
authors.lastname: Chen
authors.affiliation: Department of Radiation Oncology, National University Cancer Institute, Singapore (NCIS), National University Health System (NUHS), Singapore; Department of Radiation Oncology, Farrer Park Hospital, Singapore.
authors.forename: Khai Mun
authors.initials: KM
authors.lastname: Lee
authors.affiliation: Department of Radiation Oncology, National University Cancer Institute, Singapore (NCIS), National University Health System (NUHS), Singapore.
authors.forename: Ivan
authors.initials: I
authors.lastname: Tham
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002277:Carcinoma; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D000077274:Nasopharyngeal Carcinoma; D009303:Nasopharyngeal Neoplasms / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061106:Radiotherapy Setup Errors / Q000517:prevention & control*; D061089:Radiotherapy, Image-Guided / Q000379:methods; D012189:Retrospective Studies; D055815:Young Adult
keywords: Nasopharyngeal Carcinoma; PTV margin; Setup errors