title: Influence of bladder and rectal volume on spatial variability of a bladder tumor during radical radiotherapy.
abstract: OBJECTIVE
  To assess the spatial variability of a bladder tumor relative to the planning target volume boundaries during radical radiotherapy, and furthermore to develop strategies to reduce spatial variability.
  
  
  METHODS
  Seventeen patients with solitary T2-T4N0M0 bladder cancer were treated with a technique delivering 40 Gy/2 Gy in 20 fractions to the whole bladder with a concomitant boost to the bladder tumor of 20 Gy in 1 Gy fractions in an overall time of 4 weeks. CT scans were made weekly, immediately after treatment, and matched with the planning CT scan. Spatial variability of the tumor, as well as bladder volume and rectal diameter, were scored for each patient each week.
  
  
  RESULTS
  In 65% of patients, a part of the tumor appeared outside the planning target volume boundaries at least one time during the course of radiotherapy. No consistent relation of this variability with time was found. Bladder volumes and rectal diameters showed marked variability during the course of treatment. A large initial bladder volume and rectal diameter predicted a large volume variation and a large tumor spatial variability.
  
  
  CONCLUSIONS
  In this study, a margin of 1.5 to 2 cm seemed to be inadequate in 65% of the patients with respect to spatial variability. Bladder volume and rectal diameter were found to be predictive for spatial variability of a bladder tumor during concomitant boost radiotherapy.
authors.affiliation: Department of Radiotherapy, Academic Medical Center, Amsterdam, The Netherlands.
authors.forename: Floris J
authors.initials: FJ
authors.lastname: Pos
authors.forename: Kees
authors.initials: K
authors.lastname: Koedooder
authors.forename: Maarten C C M
authors.initials: MC
authors.lastname: Hulshof
authors.forename: Geertjan
authors.initials: G
authors.lastname: van Tienhoven
authors.forename: Dionisio
authors.initials: D
authors.lastname: González González
mesh_terms: D006801:Humans; D009068:Movement; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000379:methods*; D012007:Rectum / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D013997:Time Factors; D014057:Tomography, X-Ray Computed; D001743:Urinary Bladder / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D001749:Urinary Bladder Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*