title: Real-time Image-guided Adaptive-predictive Prostate Radiotherapy using Rectal Diameter as a Predictor of Motion.
abstract: OBJECTIVE
  To investigate a relationship between maximum rectal diameter (MRD) on pre-treatment cone beam computed tomography (CBCT) and intra-fraction prostate motion, in the context of an adaptive image-guided radiotherapy (IGRT) method.
  
  
  METHODS
  The MRD was measured on 2125 CBCTs from 55 retrospective patient datasets and related to prostate displacement from intra-fraction imaging. A linear regression model was developed to determine a threshold MRD associated with a high probability of small prostate displacement. Standard and reduced adaptive margin plans were created to compare rectum and bladder normal tissue complication probability (NTCP) with each method.
  
  
  RESULTS
  A per-protocol analysis carried out on 1910 fractions from 51 patients showed with 90% confidence that for a MRD≤3 cm, prostate displacement will be ≤5 mm and that for a MRD≤3.5 cm, prostate displacement will be ≤5.5 mm. In the first scenario, if adaptive therapy was used instead of standard therapy, median reductions in NTCP for rectum and bladder were 0.5% (from 9.5% to 9%) and 1.3% (from 6.6% to 5.3%), respectively. In the second scenario, the NTCP for rectum and bladder would have median reductions of 1.1% and 2.6%, respectively.
  
  
  CONCLUSIONS
  We have identified a potential method for adaptive prostate IGRT based upon predicting small prostate intra-fraction motion by measuring MRD on pre-treatment CBCT.
authors: Radiation Therapy Services, Peter MacCallum Cancer Centre, Bendigo, Victoria, Australia; Medical Imaging & Radiation Sciences, Monash University, Melbourne, Victoria, Australia. Electronic address: richard.oates@petermac.org.
authors: R
authors: R
authors: Oates
authors: Radiation Oncology, Townsville Cancer Centre, Townsville, Queensland, Australia.
authors: A
authors: A
authors: Brown
authors: Radiation Oncology, Townsville Cancer Centre, Townsville, Queensland, Australia; College of Medicine and Dentistry, James Cook University, Queensland, Australia.
authors: A
authors: A
authors: Tan
authors: Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia.
authors: F
authors: F
authors: Foroudi
authors: Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia.
authors: M
authors: M
authors: Lim Joon
authors: Medical Imaging & Radiation Sciences, Monash University, Melbourne, Victoria, Australia.
authors: M
authors: M
authors: Schneider
authors: Biostatistics and Clinical Trials, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia; Sir Peter MacCallum Department of Oncology, The University of Melbourne, Parkville, Victoria, Australia.
authors: A
authors: A
authors: Herschtal
authors: Sir Peter MacCallum Department of Oncology, The University of Melbourne, Parkville, Victoria, Australia; Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia.
authors: T
authors: T
authors: Kron
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods; D006801:Humans; D008297:Male; D009038:Motion; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D012007:Rectum; D012189:Retrospective Studies; D001743:Urinary Bladder
keywords: Adaptive; IGRT; NTCP; prostate cancer; radiotherapy