title: Intrafraction bladder motion in radiation therapy estimated from pretreatment and posttreatment volumetric imaging.
abstract: OBJECTIVE
  The use of image guidance protocols using soft tissue anatomy identification before treatment can reduce interfractional variation. This makes intrafraction clinical target volume (CTV) to planning target volume (PTV) changes more important, including those resulting from intrafraction bladder filling and motion. The purpose of this study was to investigate the required intrafraction margins for soft tissue image guidance from pretreatment and posttreatment volumetric imaging.
  
  
  METHODS
  Fifty patients with muscle-invasive bladder cancer (T2-T4) underwent an adaptive radiation therapy protocol using daily pretreatment cone beam computed tomography (CBCT) with weekly posttreatment CBCT. A total of 235 pairs of pretreatment and posttreatment CBCT images were retrospectively contoured by a single radiation oncologist (CBCT-CTV). The maximum bladder displacement was measured according to the patient's bony pelvis movement during treatment, intrafraction bladder filling, and bladder centroid motion.
  
  
  RESULTS
  The mean time between pretreatment and posttreatment CBCT was 13 minutes, 52 seconds (range, 7 min 52 sec to 30 min 56 sec). Taking into account patient motion, bladder centroid motion, and bladder filling, the required margins to cover intrafraction changes from pretreatment to posttreatment in the superior, inferior, right, left, anterior, and posterior were 1.25 cm (range, 1.19-1.50 cm), 0.67 cm (range, 0.58-1.12 cm), 0.74 cm (range, 0.59-0.94 cm), 0.73 cm (range, 0.51-1.00 cm), 1.20 cm (range, 0.85-1.32 cm), and 0.86 cm (range, 0.73-0.99), respectively. Small bladders on pretreatment imaging had relatively the largest increase in pretreatment to posttreatment volume.
  
  
  CONCLUSIONS
  Intrafraction motion of the bladder based on pretreatment and posttreatment bladder imaging can be significant particularly in the anterior and superior directions. Patient motion, bladder centroid motion, and bladder filling all contribute to changes between pretreatment and posttreatment imaging. Asymmetric expansion of CTV to PTV should be considered. Care is required in using image-guided radiation therapy protocols that reduce CTV to PTV margins based only on daily pretreatment soft tissue position.
authors: Division of Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia. farshad.foroudi@petermac.org
authors: Farshad
authors: F
authors: Foroudi
authors: Daniel
authors: D
authors: Pham
authors: Mathias
authors: M
authors: Bressel
authors: Suki
authors: S
authors: Gill
authors: Tomas
authors: T
authors: Kron
mesh_terms: D059925:Anatomic Landmarks / Q000000981:diagnostic imaging; D054893:Cone-Beam Computed Tomography; D019583:Dose Fractionation, Radiation; D006801:Humans; D009068:Movement; D009929:Organ Size; D010384:Pelvic Bones / Q000000981:diagnostic imaging; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D012189:Retrospective Studies; D013997:Time Factors; D001743:Urinary Bladder / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D001749:Urinary Bladder Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*; D014556:Urine