title: Comparative analysis of image guidance in two institutions for prostate cancer patients.
abstract: OBJECTIVE
  The analysis of systematic and random errors obtained from the pooled data on inter-fraction prostate motion during radiation therapy in two institutions.
  
  
  METHODS
  Data of 6085 observations for 216 prostate cancer patients treated on tomotherapy units in two institutions of position correction shifts obtained by co-registration of planning and daily CT studies were investigated. Three independent variables: patient position (supine or prone), target (prostate or prostate bed), and imaging mode (normal or coarse) were analyzed. Systematic and random errors were evaluated and used to calculate the margins for different options of referencing based on the position corrections observed with one, three, or five imaging sessions.
  
  
  RESULTS
  Statistical analysis showed that only the difference between normal and coarse modes of imaging was significant, which allowed to merge the supine and prone position sub-groups as well as the prostate and prostate bed patients. In the normal and coarse imaging groups, the margins calculated using systematic and random errors in the medio-lateral and cranio-caudal directions (5.5 mm and 4.5 mm, respectively) were similar, but significantly different (5.3 mm for the normal mode and 7.1 mm for the coarse mode) in the anterio-posterior direction. The reference scheme based on the first three fractions (R3) was found to be the optimal one.
  
  
  CONCLUSIONS
  The R3 reference scheme effectively reduced systematic and random errors. Larger margins in the anterio-posterior direction should be used during prostate treatment on the tomotherapy unit, as coarse imaging mode is chosen in order to reduce imaging time and dose.
authors.affiliation: Department of Medical Physics, Greater Poland Cancer Centre, Poznan, Poland ; Department of Electroradiology, University of Medical Sciences, Poznan, Poland.
authors.forename: Tomasz
authors.initials: T
authors.lastname: Piotrowski
authors.affiliation: Department of Physics, London Regional Cancer Program, London Health Sciences Centre, London, ON, Canada ; Department of Biophysics, University of Western Ontario, London, ON, Canada ; Department of Oncology, University of Western Ontario, London, ON, Canada.
authors.forename: Slav
authors.initials: S
authors.lastname: Yartsev
authors.affiliation: Department of Radiation Oncology, London Regional Cancer Program, London Health Sciences Centre, London, ON, Canada ; Department of Oncology, University of Western Ontario, London, ON, Canada ; Department of Epidemiology and Biostatistics, University of Western Ontario, London, ON, Canada.
authors.forename: George
authors.initials: G
authors.lastname: Rodrigues
authors.affiliation: 2nd Radiotherapy Department, Greater Poland Cancer Centre, Poznan, Poland.
authors.forename: Tomasz
authors.initials: T
authors.lastname: Bajon
keywords: Helical tomotherapy; Image guidance; MVCT; Margins; Pooled database; Prostate cancer