title: Effect of image-guidance frequency on geometric accuracy and setup margins in radiotherapy for locally advanced lung cancer.
abstract: OBJECTIVE
  To assess the relative effectiveness of five image-guidance (IG) frequencies on reducing patient positioning inaccuracies and setup margins for locally advanced lung cancer patients.
  
  
  METHODS
  Daily cone-beam computed tomography data for 100 patients (4,237 scans) were analyzed. Subsequently, four less-than-daily IG protocols were simulated using these data (no IG, first 5-day IG, weekly IG, and alternate-day IG). The frequency and magnitude of residual setup error were determined. The less-than-daily IG protocols were compared against the daily IG, the assumed reference standard. Finally, the population-based setup margins were calculated.
  
  
  RESULTS
  With the less-than-daily IG protocols, 20-43% of fractions incurred residual setup errors ≥ 5 mm; daily IG reduced this to 6%. With the exception of the first 5-day IG, reductions in systematic error (∑) occurred as the imaging frequency increased and only daily IG provided notable random error (σ) reductions (∑ = 1.5-2.2 mm, σ = 2.5-3.7 mm; ∑ = 1.8-2.6 mm, σ = 2.5-3.7 mm; and ∑ = 0.7-1.0 mm, σ = 1.7-2.0 mm for no IG, first 5-day IG, and daily IG, respectively. An overall significant difference in the mean setup error was present between the first 5-day IG and daily IG (p < .0001). The derived setup margins were 5-9 mm for less-than-daily IG and were 3-4 mm with daily IG.
  
  
  CONCLUSIONS
  Daily cone-beam computed tomography substantially reduced the setup error and could permit setup margin reduction and lead to a reduction in normal tissue toxicity for patients undergoing conventionally fractionated lung radiotherapy. Using first 5-day cone-beam computed tomography was suboptimal for lung patients, given the inability to reduce the random error and the potential for the systematic error to increase throughout the treatment course.
authors.affiliation: Radiation Medicine Program, Princess Margaret Hospital, Toronto, ON, Canada. Jane.Higgins@rmp.uhn.on.ca
authors.forename: Jane
authors.initials: J
authors.lastname: Higgins
authors.forename: Andrea
authors.initials: A
authors.lastname: Bezjak
authors.forename: Andrew
authors.initials: A
authors.lastname: Hope
authors.forename: Tony
authors.initials: T
authors.lastname: Panzarella
authors.forename: Winnie
authors.initials: W
authors.lastname: Li
authors.forename: John B C
authors.initials: JB
authors.lastname: Cho
authors.forename: Tim
authors.initials: T
authors.lastname: Craig
authors.forename: Anthony
authors.initials: A
authors.lastname: Brade
authors.forename: Alexander
authors.initials: A
authors.lastname: Sun
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Bissonnette
mesh_terms: D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy; D002985:Clinical Protocols / Q000592:standards; D054893:Cone-Beam Computed Tomography / Q000379:methods* / Q000592:standards; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*; D058958:Organs at Risk / Q000528:radiation effects; D056888:Patient Positioning / Q000379:methods*; D011832:Radiation Injuries / Q000517:prevention & control; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012015:Reference Standards; D055752:Small Cell Lung Carcinoma / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy