title: Strategy for online correction of rotational organ motion for intensity-modulated radiotherapy of prostate cancer.
abstract: OBJECTIVE
  To develop and evaluate a correction strategy for prostate rotation using gantry and collimator angle adjustments.
  
  
  METHODS
  Gantry and collimator angle adjustments were used to correct for prostate rotation without rotating the table. A formula to partially correct for left-right (LR) rotations was derived through geometric analysis of rotation-induced clinical target volume (CTV) beam's-eye-view shape changes. For 10 prostate patients, intensity-modulated radiotherapy (IMRT) plans with different margins were created. Simulating CTV LR rotation and correcting each beam by a collimator rotation, the corrected CTV dose was compared with the original and uncorrected dose. Effects of residual geometric uncertainties were assessed using a Monte Carlo technique. A large number of treatments representative for prostate patients were simulated. Dose probability histograms of the minimum CTV dose (D min) were derived, with and without online correction, resulting in a more realistic margin estimate.
  
  
  RESULTS
  Dosimetric analysis of all IMRT plans showed that, with rotational correction and a 2-mm margin, D min was constant to within 3% for LR rotations up to +/-15 degrees . The Monte Carlo dose probability histograms showed that, with correction, a margin of 4 mm ensured that 90% of patients received a D min >or=95% of the prescribed dose. Without correction a margin of 6 mm was required.
  
  
  CONCLUSIONS
  We developed and tested a practical method for (online) correction of prostate rotation, allowing safe and straightforward implementation of margin reduction and dose escalation.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands.
authors.forename: Erik-Jan
authors.initials: EJ
authors.lastname: Rijkhorst
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
authors.forename: Joos V
authors.initials: JV
authors.lastname: Lebesque
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
mesh_terms: D000465:Algorithms; D006801:Humans; D008297:Male; D009010:Monte Carlo Method; D009068:Movement; D011467:Prostate / Q000000981:diagnostic imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012007:Rectum / Q000000981:diagnostic imaging; D012399:Rotation; D014057:Tomography, X-Ray Computed