title: Three-dimensional patient setup errors at different treatment sites measured by the Tomotherapy megavoltage CT.
abstract: OBJECTIVE
  Reduction of interfraction setup uncertainty is vital for assuring the accuracy of conformal radiotherapy. We report a systematic study of setup error to assess patients' three-dimensional (3D) localization at various treatment sites.
  
  
  METHODS
  Tomotherapy megavoltage CT (MVCT) images were scanned daily in 259 patients from 2005-2008. We analyzed 6,465 MVCT images to measure setup error for head and neck (H&N), chest/thorax, abdomen, prostate, legs, and total marrow irradiation (TMI). Statistical comparisons of the absolute displacements across sites and time were performed in rotation (R), lateral (x), craniocaudal (y), and vertical (z) directions.
  
  
  RESULTS
  The global systematic errors were measured to be less than 3 mm in each direction with increasing order of errors for different sites: H&N, prostate, chest, pelvis, spine, legs, and TMI. The differences in displacements in the x, y, and z directions, and 3D average displacement between treatment sites were significant (p < 0.01). Overall improvement in patient localization with time (after 3-4 treatment fractions) was observed. Large displacement (> 5 mm) was observed in the 75(th) percentile of the patient groups for chest, pelvis, legs, and spine in the x and y direction in the second week of the treatment.
  
  
  CONCLUSIONS
  MVCT imaging is essential for determining 3D setup error and to reduce uncertainty in localization at all anatomical locations. Setup error evaluation should be performed daily for all treatment regions, preferably for all treatment fractions.
authors.affiliation: Department of Therapeutic Radiology - Radiation Oncology, University of Minnesota Medical School, 420 Delaware Street SE, Mayo Mail Code 494, 55455, Minneapolis, MN, USA. huixx019@umn.edu
authors.forename: S K
authors.initials: SK
authors.lastname: Hui
authors.forename: E
authors.initials: E
authors.lastname: Lusczek
authors.forename: T
authors.initials: T
authors.lastname: DeFor
authors.forename: K
authors.initials: K
authors.lastname: Dusenbery
authors.forename: S
authors.initials: S
authors.lastname: Levitt
mesh_terms: D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008297:Male; D009369:Neoplasms / Q000532:radiotherapy*; D058958:Organs at Risk; D056888:Patient Positioning; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061106:Radiotherapy Setup Errors / Q000009:adverse effects / Q000517:prevention & control*; D020266:Radiotherapy, Conformal / Q000379:methods*; D011882:Radiotherapy, High-Energy / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*; D014584:User-Computer Interface