title: The potential of MRI-guided online adaptive re-optimisation in radiotherapy of urinary bladder cancer.
abstract: OBJECTIVE
  Adaptive radiotherapy (ART) using plan selection is being introduced clinically for bladder cancer, but the challenge of how to compensate for intra-fractional motion remains. The purpose of this study was to assess target coverage with respect to intra-fractional motion and the potential for normal tissue sparing in MRI-guided ART (MRIGART) using isotropic (MRIGARTiso), an-isotropic (MRIGARTanIso) and population-based margins (MRIGARTpop).
  
  
  METHODS
  Nine bladder cancer patients treated in a phase II trial of plan selection underwent 6-7 weekly repeat MRI series, each with volumetric scans acquired over a 10 min period. Adaptive re-planning on the 0 min MRI scans was performed using density override, simulating a hypo-fractionated schedule. Target coverage was evaluated on the 10 min scan to quantify the impact of intra-fractional motion.
  
  
  RESULTS
  MRIGARTanIso reduced the course-averaged PTV by median 304 cc compared to plan selection. Bladder shifts affected target coverage in individual fractions for all strategies. Two patients had a v95% of the bladder below 98% for MRIGARTiso. MRIGARTiso decreased the bowel V25 with 15-46 cc compared to MRIGARTpop.
  
  
  CONCLUSIONS
  Online re-optimised ART has a considerable normal tissue sparing potential. MRIGART with online corrections for target shift during a treatment fraction should be considered in ART for bladder cancer.
authors.affiliation: Department of Medical Physics, Aarhus University/Aarhus University Hospital, Denmark; Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom. Electronic address: annveste@rm.dk.
authors.forename: Anne
authors.initials: A
authors.lastname: Vestergaard
authors.affiliation: Academic Urology Unit, The Institute of Cancer Research, The Royal Marsden NHS Foundation Trust, Sutton, United Kingdom.
authors.forename: Shaista
authors.initials: S
authors.lastname: Hafeez
authors.affiliation: Department of Medical Physics, Aarhus University/Aarhus University Hospital, Denmark.
authors.forename: Ludvig P
authors.initials: LP
authors.lastname: Muren
authors.affiliation: Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom.
authors.forename: Simeon
authors.initials: S
authors.lastname: Nill
authors.affiliation: Department of Oncology, AarhusUniversity/Aarhus University Hospital, Denmark.
authors.forename: Morten
authors.initials: M
authors.lastname: Høyer
authors.affiliation: Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom.
authors.forename: Vibeke N
authors.initials: VN
authors.lastname: Hansen
authors.affiliation: Department of Medical Physics, Aarhus University/Aarhus University Hospital, Denmark.
authors.forename: Caroline
authors.initials: C
authors.lastname: Grønborg
authors.affiliation: Department of Radiology, Aarhus University/Aarhus University Hospital, Denmark.
authors.forename: Erik M
authors.initials: EM
authors.lastname: Pedersen
authors.affiliation: Department of Medical Physics, Aarhus University/Aarhus University Hospital, Denmark.
authors.forename: Jørgen B
authors.initials: JB
authors.lastname: Petersen
authors.affiliation: Academic Urology Unit, The Institute of Cancer Research, The Royal Marsden NHS Foundation Trust, Sutton, United Kingdom.
authors.forename: Robert
authors.initials: R
authors.lastname: Huddart
authors.affiliation: Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom.
authors.forename: Uwe
authors.initials: U
authors.lastname: Oelfke
mesh_terms: D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061089:Radiotherapy, Image-Guided; D001749:Urinary Bladder Neoplasms / Q000532:radiotherapy*
keywords: Adaptive radiotherapy; Bladder cancer; Dose accumulation; MRI guided; Re-optimisation