title: Deriving patient-specific planning target volume for partial bladder image guided radiation therapy.
abstract: OBJECTIVE
  Image guided radiation therapy (IGRT) using bony anatomy for bladder cancer requires the use of large population-based planning target volume (PTV) margins to compensate for geometric uncertainties. This may result in a large volume of normal tissue being irradiated unnecessarily. Identification of the clinical target volume (CTV) is also a challenge during target delineation and treatment position verification. This study describes the use of lipiodol (Guerbet, US) and cone beam computed tomography (CBCT) in deriving patient-specific PTV (PS-PTV) for partial bladder IGRT.
  
  
  METHODS
  Twelve patients underwent lipiodol injection into the bladder wall prior to radiation treatment. A PS-PTV was generated by the following: (1) Delineating partial bladder CTV (CTVpb) on 15 CBCTs; (2) registering the CBCTs with the planning CT image using lipiodol; (3) combining the 15 CTVpb to create an occupancy volume (OV); and (4) expanding the OV by 3 mm. Its efficacy in reducing irradiated volume and in providing coverage was assessed by comparing it with a 20-mm population-based PTV (popPTV) and using phase 2 CBCTs.
  
  
  RESULTS
  The median PS-PTV and popPTV (cm(3)) were 102 (range, 37-336) and 325 (range, 211-631), respectively. Median distance between the CTVpb and the PS-PTV edge (mm) were 6 superior, 6 right, 7 left, 7 anterior, 8 posterior, and 11 inferior. The absolute median reduction in the overlapping volume of rectum, small bowel, and large bowel were 0.3 cm(3), 5.3 cm(3), and 13.0 cm(3), respectively. Despite large reductions in volume and margin compared with popPTV, PS-PTV achieved 100% target coverage.
  
  
  CONCLUSIONS
  Using lipiodol and CBCT to derive PS-PTV facilitated large reductions in the irradiated normal tissue volume without compromising target coverage.
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Center, Toronto, Canada. Electronic address: Vickie.kong@rmp.uhn.on.ca.
authors.forename: Vickie
authors.initials: V
authors.lastname: Kong
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Center, Toronto, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Canada.
authors.forename: Tara
authors.initials: T
authors.lastname: Rosewall
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Center, Toronto, Canada.
authors.forename: Susan
authors.initials: S
authors.lastname: Chen
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Center, Toronto, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Canada.
authors.forename: Tim
authors.initials: T
authors.lastname: Craig
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Center, Toronto, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Canada.
authors.forename: Robert
authors.initials: R
authors.lastname: Bristow
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Center, Toronto, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Canada.
authors.forename: Michael
authors.initials: M
authors.lastname: Milosevic
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Center, Toronto, Canada.
authors.forename: Maisie
authors.initials: M
authors.lastname: Kwan
authors.affiliation: Radiation Medicine Program, Princess Margaret Cancer Center, Toronto, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Canada.
authors.forename: Peter
authors.initials: P
authors.lastname: Chung
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D005500:Follow-Up Studies; D006801:Humans; D058958:Organs at Risk; D011379:Prognosis; D011446:Prospective Studies; D011880:Radiotherapy Planning, Computer-Assisted; D061089:Radiotherapy, Image-Guided; D012189:Retrospective Studies; D001743:Urinary Bladder / Q000000981:diagnostic imaging / Q000528:radiation effects*; D001749:Urinary Bladder Neoplasms / Q000532:radiotherapy*