title: Prostate bed motion may cause geographic miss in post-prostatectomy image-guided intensity-modulated radiotherapy.
abstract: BACKGROUND
  There is little data to guide radiation oncologists on appropriate margin selection in the post-prostatectomy setting. The aim of this study was to quantify interfraction variation in motion of the prostate bed to determine these margins.
  
  
  METHODS
  The superior and inferior surgical clips in the prostate bed were tracked on pretreatment cone beam CT images (n = 377) for 40 patients who had received post-prostatectomy radiotherapy. Prostate bed motion was calculated for the upper and lower segments by measuring the position of surgical clips located close to midline relative to bony anatomy in the axial (translational) and sagittal (tilt) planes. The frequency of potential geographic misses was calculated for either 1 cm or 0.5 cm posterior planning target volume margins.
  
  
  RESULTS
  The mean magnitude of movement of the prostate bed in the anterior-posterior, superior-inferior and left-right planes, respectively, were as follows: upper portion, 0.50 cm, 0.28 cm, 0.10 cm; lower portion, 0.18 cm, 0.18 cm, 0.08 cm. The random and systematic errors, respectively, of the prostate bed motion in the anterior-posterior, superior-inferior and left-right planes, respectively, were as follows: upper portion, 0.47 cm and 0.50 cm, 0.28 cm and 0.27 cm, 0.11 cm and 0.11 cm; lower portion, 0.17 cm and 0.18 cm, 0.17 cm and 0.19 cm, 0.08 cm and 0.10 cm. Most geographic misses occurred in the upper prostate bed in the anterior-posterior plane. The median prostate bed tilt was 1.8° (range -23.4° to 42.3°).
  
  
  CONCLUSIONS
  Variability was seen in all planes for the movement of both surgical clips. The greatest movement occurred in the anterior-posterior plane in the upper prostate bed, which could cause geographic miss of treatment delivery. The variability in the movement of the superior and inferior clips indicates a prostate bed tilt that would be difficult to correct with standard online matching techniques. This creates a strong argument for using anisotropic planning target volume margins in post-prostatectomy radiotherapy.
authors.affiliation: Northern Sydney Cancer Centre, Radiation Oncology Department, Royal North Shore Hospital, Australia; Faculty of Health Sciences, University of Sydney, Sydney, New South Wales, Australia.
authors.forename: Linda J
authors.initials: LJ
authors.lastname: Bell
authors.forename: Jennifer
authors.initials: J
authors.lastname: Cox
authors.forename: Thomas
authors.initials: T
authors.lastname: Eade
authors.forename: Marianne
authors.initials: M
authors.lastname: Rinks
authors.forename: Andrew
authors.initials: A
authors.lastname: Kneebone
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D016477:Artifacts; D001513:Beds; D006801:Humans; D008297:Male; D008875:Middle Aged; D009038:Motion; D056888:Patient Positioning / Q000295:instrumentation / Q000379:methods*; D011182:Postoperative Care / Q000379:methods; D011468:Prostatectomy; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011859:Radiography; D018714:Radiotherapy, Adjuvant / Q000379:methods; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016896:Treatment Outcome
keywords: IGRT; cone beam CT; geographic miss; post-prostatectomy radiotherapy; prostate cancer