title: Delineating the inner bladder surface using uniform contractions from the outer surface under variable bladder filling conditions.
abstract: OBJECTIVE
  To evaluate the methods to delineate the inner bladder (IB) surface using a uniform contraction from the outer bladder (OB) surface, assuming the bladder wall (BW) is either of constant thickness, constant volume or variable volume.
  
  
  METHODS
  14 prostate intensity-modulated radiotherapy patients with 2 planning CTs were identified. For both CTs, OB was delineated using model-based segmentation. IB was delineated manually. Then, using uniform contractions from OB, the position of IB was approximated using a: 2.5-mm contraction, patient-specific contraction, patient-specific constant wall volume method and variable wall volume method. The structures created using those strategies were compared against the manual IB contours using geometric and dosimetric indices.
  
  
  RESULTS
  In the presence of variable bladder filling, use of a generic or patient-specific constant contraction resulted in a significant overestimation of IB volume (+12 and +13 cm(3), respectively; p < 0.001) that was inversely correlated with the difference in urine volume between the scans (R(2) > 0.86). Mean differences across 95% of IB surfaces were ≤2 mm for methods using either constant or variable wall volume. Mean dose-volume histogram (DVH) differences were <1 cm(3) across the whole BW DVH when using the method that assumed a variable wall volume.
  
  
  CONCLUSIONS
  The variable volume BW model provided the best approximation of the IB surface position under varying filling conditions, based on geometric and dosimetric indices.
  
  
  CONCLUSIONS
  Use of the equation derived in this research provides a quick and accurate method to delineate the hollow BW on serial imaging for the purposes of dose reconstruction.
authors.affiliation: 1 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, ON, Canada.|2 Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada.|3 Faculty of Science, Charles Sturt University, NSW, Australia.
authors.forename: Tara
authors.initials: T
authors.lastname: Rosewall
authors.affiliation: 1 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, ON, Canada.|2 Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada.
authors.forename: Andrew
authors.initials: A
authors.lastname: Bayley
authors.affiliation: 1 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, ON, Canada.|2 Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada.
authors.forename: Charles
authors.initials: C
authors.lastname: Catton
authors.affiliation: 1 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, ON, Canada.|2 Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada.
authors.forename: Peter
authors.initials: P
authors.lastname: Chung
authors.affiliation: 3 Faculty of Science, Charles Sturt University, NSW, Australia.|4 Australian School of Advanced Medicine, Macquarie University, NSW, Australia.
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Currie
authors.affiliation: 1 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, ON, Canada.|2 Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada.
authors.forename: Robert
authors.initials: R
authors.lastname: Heaton
authors.affiliation: 3 Faculty of Science, Charles Sturt University, NSW, Australia.|4 Australian School of Advanced Medicine, Macquarie University, NSW, Australia.
authors.forename: Janelle
authors.initials: J
authors.lastname: Wheat
authors.affiliation: 1 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, ON, Canada.|2 Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada.
authors.forename: Michael
authors.initials: M
authors.lastname: Milosevic
mesh_terms: D000465:Algorithms; D006801:Humans; D008297:Male; D008962:Models, Theoretical; D009929:Organ Size; D001743:Urinary Bladder / Q000033:anatomy & histology*