title: Intra-fractional bladder motion and margins in adaptive radiotherapy for urinary bladder cancer.
abstract: BACKGROUND
  The bladder is a tumour site well suited for adaptive radiotherapy (ART) due to large inter-fractional changes, but it also displays considerable intra-fractional motion. The aim of this study was to assess target coverage with a clinically applied method for plan selection ART and to estimate population-based and patient-specific intra-fractional margins, also relevant for a future re-optimisation strategy.
  
  
  METHODS
  Nine patients treated in a clinical phase II ART trial of daily plan selection for bladder cancer were included. In the library plans, 5 mm isotropic margins were added to account for intra-fractional changes. Pre-treatment and weekly repeat magnetic resonance imaging (MRI) series were acquired in which a full three-dimensional (3D) volume was scanned every second min for 10 min (a total of 366 scans in 61 series). Initially, the bladder clinical target volume (CTV) was delineated in all scans. The t = 0 min scan was then rigidly registered to the planning computed tomography (CT) and plan selections were simulated using the CTV_0 (at t = 0 min). To assess intra-fractional motion, coverage of the CTV_10 (at t = 10 min) was quantified using the applied PTV. Population-based margins were calculated using the van Herk margin recipe while patient-specific margins were calculated using a linear model.
  
  
  RESULTS
  For 49% of the cases, the CTV_10 extended more than 5 mm outside the CTV_0. However, in 58 of the 61 cases (97%) CTV_10 was covered by the selected PTV. Population-based margins of 14 mm Sup/Ant, 9 mm Post and 5 mm Inf/Lat were sufficient to cover the bladder. Using patient-specific margins, the overlap between PTV and bowel-cavity was reduced from 137 cm(3) with the plan selection strategy to 24 cm(3).
  
  
  CONCLUSIONS
  In this phase II ART trial, 5 mm isotropic margin for intra-fractional motion was sufficient even though considerable intra-fractional motion was observed. In online re-optimised ART, population-based margin can be applied although patient-specific margins are preferable.
authors: a Department of Medical Physics , Aarhus University/Aarhus University Hospital , Aarhus , Denmark.
authors: Caroline
authors: C
authors: Grønborg
authors: a Department of Medical Physics , Aarhus University/Aarhus University Hospital , Aarhus , Denmark.
authors: Anne
authors: A
authors: Vestergaard
authors: b Department of Oncology , Aarhus University/Aarhus University Hospital , Aarhus , Denmark.
authors: Morten
authors: M
authors: Høyer
authors: c Department of Radiation Oncology , University Hospital Grosshadern , LMU Munich, Munich , Germany.
authors: Matthias
authors: M
authors: Söhn
authors: d Department of Radiology , Aarhus University/Aarhus University Hospital , Aarhus , Denmark.
authors: Erik M
authors: EM
authors: Pedersen
authors: a Department of Medical Physics , Aarhus University/Aarhus University Hospital , Aarhus , Denmark.
authors: Jørgen B
authors: JB
authors: Petersen
authors: b Department of Oncology , Aarhus University/Aarhus University Hospital , Aarhus , Denmark.
authors: Mads
authors: M
authors: Agerbæk
authors: a Department of Medical Physics , Aarhus University/Aarhus University Hospital , Aarhus , Denmark.|b Department of Oncology , Aarhus University/Aarhus University Hospital , Aarhus , Denmark.
authors: Ludvig P
authors: LP
authors: Muren
mesh_terms: D006801:Humans; D021621:Imaging, Three-Dimensional; D008279:Magnetic Resonance Imaging; D009068:Movement; D061089:Radiotherapy, Image-Guided; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D001749:Urinary Bladder Neoplasms / Q000473:pathology* / Q000532:radiotherapy*