title: Residual seminal vesicle displacement in marker-based image-guided radiotherapy for prostate cancer and the impact on margin design.
abstract: OBJECTIVE
  The objectives of this study were to quantify residual interfraction displacement of seminal vesicles (SV) and investigate the efficacy of rotation correction on SV displacement in marker-based prostate image-guided radiotherapy (IGRT). We also determined the effect of marker registration on the measured SV displacement and its impact on margin design.
  
  
  METHODS
  SV displacement was determined relative to marker registration by using 296 cone beam computed tomography scans of 13 prostate cancer patients with implanted markers. SV were individually registered in the transverse plane, based on gray-value information. The target registration error (TRE) for the SV due to marker registration inaccuracies was estimated. Correlations between prostate gland rotations and SV displacement and between individual SV displacements were determined.
  
  
  RESULTS
  The SV registration success rate was 99%. Displacement amounts of both SVs were comparable. Systematic and random residual SV displacements were 1.6 mm and 2.0 mm in the left-right direction, respectively, and 2.8 mm and 3.1 mm in the anteroposterior (AP) direction, respectively. Rotation correction did not reduce residual SV displacement. Prostate gland rotation around the left-right axis correlated with SV AP displacement (R(2) = 42%); a correlation existed between both SVs for AP displacement (R(2) = 62%); considerable correlation existed between random errors of SV displacement and TRE (R(2) = 34%).
  
  
  CONCLUSIONS
  Considerable residual SV displacement exists in marker-based IGRT. Rotation correction barely reduced SV displacement, rather, a larger SV displacement was shown relative to the prostate gland that was not captured by the marker position. Marker registration error partly explains SV displacement when correcting for rotations. Correcting for rotations, therefore, is not advisable when SV are part of the target volume. Margin design for SVs should take these uncertainties into account.
authors.affiliation: Department of Radiation Oncology, Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands.
authors.forename: Monique H P
authors.initials: MH
authors.lastname: Smitsmans
authors.forename: Josien
authors.initials: J
authors.lastname: de Bois
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
authors.forename: Charles N
authors.initials: CN
authors.lastname: Catton
authors.forename: David A
authors.initials: DA
authors.lastname: Jaffray
authors.forename: Joos V
authors.initials: JV
authors.lastname: Lebesque
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D057918:Fiducial Markers; D006046:Gold; D006801:Humans; D008297:Male; D009068:Movement; D056888:Patient Positioning; D011446:Prospective Studies; D011467:Prostate / Q000000981:diagnostic imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012669:Seminal Vesicles / Q000000981:diagnostic imaging*
chemical_list: D006046:Gold