title: Inter- and intrafraction uncertainty in prostate bed image-guided radiotherapy.
abstract: OBJECTIVE
  The goals of this study were to measure inter- and intrafraction setup error and prostate bed motion (PBM) in patients undergoing post-prostatectomy image-guided radiotherapy (IGRT) and to propose appropriate population-based three-dimensional clinical target volume to planning target volume (CTV-PTV) margins in both non-IGRT and IGRT scenarios.
  
  
  METHODS
  In this prospective study, 14 patients underwent adjuvant or salvage radiotherapy to the prostate bed under image guidance using linac-based kilovoltage cone-beam CT (kV-CBCT). Inter- and intrafraction uncertainty/motion was assessed by offline analysis of three consecutive daily kV-CBCT images of each patient: (1) after initial setup to skin marks, (2) after correction for positional error/immediately before radiation treatment, and (3) immediately after treatment.
  
  
  RESULTS
  The magnitude of interfraction PBM was 2.1 mm, and intrafraction PBM was 0.4 mm. The maximum inter- and intrafraction prostate bed motion was primarily in the anterior-posterior direction. Margins of at least 3-5 mm with IGRT and 4-7 mm without IGRT (aligning to skin marks) will ensure 95% of the prescribed dose to the clinical target volume in 90% of patients.
  
  
  CONCLUSIONS
  PBM is a predominant source of intrafraction error compared with setup error and has implications for appropriate PTV margins. Based on inter- and estimated intrafraction motion of the prostate bed using pre- and post-kV-CBCT images, CBCT IGRT to correct for day-to-day variances can potentially reduce CTV-PTV margins by 1-2 mm. CTV-PTV margins for prostate bed treatment in the IGRT and non-IGRT scenarios are proposed; however, in cases with more uncertainty of target delineation and image guidance accuracy, larger margins are recommended.
authors.affiliation: Department of Radiation Oncology, London Health Sciences Centre, London, Canada.
authors.forename: Kitty
authors.initials: K
authors.lastname: Huang
authors.forename: David A
authors.initials: DA
authors.lastname: Palma
authors.forename: Danielle
authors.initials: D
authors.lastname: Scott
authors.forename: Danielle
authors.initials: D
authors.lastname: McGregor
authors.forename: Stewart
authors.initials: S
authors.lastname: Gaede
authors.forename: Slav
authors.initials: S
authors.lastname: Yartsev
authors.forename: Glenn
authors.initials: G
authors.lastname: Bauman
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Louie
authors.forename: George
authors.initials: G
authors.lastname: Rodrigues
mesh_terms: D000368:Aged; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D057918:Fiducial Markers; D006801:Humans; D008297:Male; D008875:Middle Aged; D009068:Movement; D011446:Prospective Studies; D011467:Prostate / Q000000981:diagnostic imaging*; D017430:Prostate-Specific Antigen / Q000097:blood; D011471:Prostatic Neoplasms / Q000097:blood / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy* / Q000601:surgery; D061106:Radiotherapy Setup Errors; D018714:Radiotherapy, Adjuvant / Q000379:methods; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D016879:Salvage Therapy / Q000379:methods; D035501:Uncertainty
chemical_list: D017430:Prostate-Specific Antigen