title: Vector analysis of prostate patient setup with image-guided radiation therapy via kV cone beam computed tomography.
abstract: OBJECTIVE
  To analyze the daily setup variations in a cohort of intensity-modulated radiation therapy (IMRT) prostate cancer patients who had received daily image-guided RT without the use of fiducial markers to determine if daily image guidance is necessary.
  
  
  METHODS
  2134 Kilovoltage (kV) cone beam computed tomography (CBCT) images were analyzed, with three shifts recorded for each image. The number of times that the vector of the combined shifts would have exceeded the planning tumor volume (PTV) margin was tallied. Then, the average scalar shift of the first five images was removed from all subsequent images for a given patient, and the number of days for which the shift vector was greater than the three-dimensional clinical tumor volume-PTV (3D CTV-PTV) margin (8 mm, created with rolling ball technique) was recorded. Additionally, the scalar shifts from every other fraction were studied to determine if the individual patient's shift vector would be adequately sampled if CBCT was not performed daily, thus reducing patient imaging dose without compromising treatment quality.
  
  
  RESULTS
  There were 297 cases where the vector shift was initially greater than the PTV margin. By correcting each patient's data set by the average shift of their first five images the total was 248 cases. By considering only every other image of each patient data set (after correction for the first 5 days), only 137 days in which the CTV was outside the PTV would have been seen.
  
  
  CONCLUSIONS
  Daily imaging is recommended for prostate cancer IMRT patients in order to know the 3D (vector) position of the CTV and to ensure that it is always within the PTV margin. Correcting the data set by the average shift from the first 5 days reduces the overall number of outlier days but does not eliminate them completely.
authors.affiliation: Department of Radiation Oncology, University of California, Davis, Sacramento, California 95817, USA. julian.perks@ucdmc.ucdavis.edu
authors.forename: Julian
authors.initials: J
authors.lastname: Perks
authors.forename: Helen
authors.initials: H
authors.lastname: Turnbull
authors.forename: Tianxiao
authors.initials: T
authors.lastname: Liu
authors.forename: James
authors.initials: J
authors.lastname: Purdy
authors.forename: Richard
authors.initials: R
authors.lastname: Valicenti
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D054893:Cone-Beam Computed Tomography / Q000379:methods* / Q000706:statistics & numerical data; D057918:Fiducial Markers; D006801:Humans; D008297:Male; D008875:Middle Aged; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D013997:Time Factors; D047368:Tumor Burden