title: Examining margin reduction and its impact on dose distribution for prostate cancer patients undergoing daily cone-beam computed tomography.
abstract: OBJECTIVE
  To examine the dosimetric impact of margin reduction and quantify residual error after three-dimensional (3D) image registration using daily cone-beam computed tomography (CBCT) for prostate cancer patients.
  
  
  METHODS
  One hundred forty CBCTs from 5 prostate cancer patients were examined. Two intensity-modulated radiotherapy plans were generated on CT simulation on the basis of two planning target volume (PTV) margins: 10 mm all around the prostate and seminal vesicles except 6 mm posteriorly (10/6) and 5 mm all around except 3 mm posteriorly (5/3). Daily CBCT using the Varian On-Board Imaging System was acquired. The 10/6 and 5/3 simulation plans were overlaid onto each CBCT, and each CBCT plan was calculated. To examine residual error, PlanCT/CBCT intensity-based 3D image registration was performed for prostate localization using center of mass and maximal border displacement.
  
  
  RESULTS
  Prostate coverage was within 2% between the 10/6 and 5/3 plans. Seminal vesicle coverage was reduced with the 5/3 plan compared with the 10/6 plan, with coverage difference within 7%. The 5/3 plan allowed 30-50% sparing of bladder and rectal high-dose regions. For residual error quantification, center of mass data show that 99%, 93%, and 96% of observations fall within 3 mm in the left-right, anterior-posterior, and superior-inferior directions, respectively. Maximal border displacement observations range from 79% to 99%, within 5 mm for all directions.
  
  
  CONCLUSIONS
  Cone-beam CT dosimetrically validated a 10/6 margin when soft-tissue localization is not used. Intensity-based 3D image registration has the potential to improve target localization and to provide guidelines for margin definition.
authors.affiliation: Josephine Ford Downriver Center for Oncology, Henry Ford Health System, Brownstown, MI, USA. rhammou1@hfhs.org
authors.forename: Rabih
authors.initials: R
authors.lastname: Hammoud
authors.forename: Samir H
authors.initials: SH
authors.lastname: Patel
authors.forename: Deepak
authors.initials: D
authors.lastname: Pradhan
authors.forename: Jinkoo
authors.initials: J
authors.lastname: Kim
authors.forename: Harrison
authors.initials: H
authors.lastname: Guan
authors.forename: Shidong
authors.initials: S
authors.lastname: Li
authors.forename: Benjamin
authors.initials: B
authors.lastname: Movsas
mesh_terms: D002138:Calibration; D054893:Cone-Beam Computed Tomography; D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy; D011832:Radiation Injuries / Q000517:prevention & control; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated; D012007:Rectum / Q000000981:diagnostic imaging; D012015:Reference Standards; D012189:Retrospective Studies; D012669:Seminal Vesicles / Q000000981:diagnostic imaging; D047368:Tumor Burden; D001743:Urinary Bladder / Q000000981:diagnostic imaging