title: Associations between volume changes and spatial dose metrics for the urinary bladder during local versus pelvic irradiation for prostate cancer.
abstract: BACKGROUND
  Inter-fractional variation in urinary bladder volumes during the course of radiotherapy (RT) for prostate cancer causes deviations between planned and delivered doses. This study compared planned versus daily cone-beam CT (CBCT)-based spatial bladder dose distributions, for prostate cancer patients receiving local prostate treatment (local treatment) versus prostate including pelvic lymph node irradiation (pelvic treatment).
  
  
  METHODS
  Twenty-seven patients (N = 15 local treatment; N = 12 pelvic treatment) were treated using daily image-guided RT (1.8 Gy@43-45 fx), adhering to a full bladder/empty rectum protocol. For each patient, 9-10 CBCTs were registered to the planning CT, using the clinically applied translations. The urinary bladder was manually segmented on each CBCT, 3 mm inner shells were generated, and semi and quadrant sectors were created using axial/coronal cuts. Planned and delivered DVH metrics were compared across patients and between the two groups of treatment (t-test, p < .05; Holm-Bonferroni correction). Associations between bladder volume variations and the dose-volume histograms (DVH) of the bladder and its sectors were evaluated (Spearman's rank correlation coefficient, rs).
  
  
  RESULTS
  Bladder volumes varied considerably during RT (coefficient of variation: 16-58%). The population-averaged planned and delivered DVH metrics were not significantly different at any dose level. Larger treatment bladder volumes resulted in increased absolute volume of the posterior/inferior bladder sector receiving intermediate-high doses, in both groups. The superior bladder sector received less dose with larger bladder volumes for local treatments (rs± SD: -0.47 ± 0.32), but larger doses for pelvic treatments (rs± SD: 0.74 ± 0.24).
  
  
  CONCLUSIONS
  Substantial bladder volume changes during the treatment course occurred even though patients were treated under a full bladder/daily image-guided protocol. Larger bladder volumes resulted in less bladder wall spared at the posterior-inferior sector, regardless the treatment received. Contrary, larger bladder volumes meant larger delivered doses to the superior bladder sector for pelvic RT but smaller doses for local treatments.
authors.affiliation: a Department of Medical Physics , Aarhus University Hospital , Aarhus , Denmark.
authors.forename: Oscar
authors.initials: O
authors.lastname: Casares-Magaz
authors.affiliation: b Department of Radiation Medicine and Applied Sciences , University of California San Diego , La Jolla , CA , USA.
authors.forename: Vitali
authors.initials: V
authors.lastname: Moiseenko
authors.affiliation: b Department of Radiation Medicine and Applied Sciences , University of California San Diego , La Jolla , CA , USA.
authors.forename: Austin
authors.initials: A
authors.lastname: Hopper
authors.affiliation: b Department of Radiation Medicine and Applied Sciences , University of California San Diego , La Jolla , CA , USA.
authors.forename: Niclas Johan
authors.initials: NJ
authors.lastname: Pettersson
authors.affiliation: c Department of Medical Physics , Memorial Sloan Kettering Cancer Center , New York City , NY , USA.
authors.forename: Maria
authors.initials: M
authors.lastname: Thor
authors.affiliation: b Department of Radiation Medicine and Applied Sciences , University of California San Diego , La Jolla , CA , USA.
authors.forename: Rick
authors.initials: R
authors.lastname: Knopp
authors.affiliation: c Department of Medical Physics , Memorial Sloan Kettering Cancer Center , New York City , NY , USA.
authors.forename: Joseph O
authors.initials: JO
authors.lastname: Deasy
authors.affiliation: a Department of Medical Physics , Aarhus University Hospital , Aarhus , Denmark.
authors.forename: Ludvig Paul
authors.initials: LP
authors.lastname: Muren
authors.affiliation: b Department of Radiation Medicine and Applied Sciences , University of California San Diego , La Jolla , CA , USA.
authors.forename: John
authors.initials: J
authors.lastname: Einck
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008297:Male; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000473:pathology / Q000528:radiation effects; D010388:Pelvis / Q000000981:diagnostic imaging / Q000473:pathology* / Q000528:radiation effects; D011467:Prostate / Q000000981:diagnostic imaging / Q000473:pathology* / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology* / Q000532:radiotherapy; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012007:Rectum / Q000000981:diagnostic imaging / Q000473:pathology* / Q000528:radiation effects; D001743:Urinary Bladder / Q000000981:diagnostic imaging / Q000473:pathology* / Q000528:radiation effects