title: Statistical modeling of CTV motion and deformation for IMRT of early-stage rectal cancer.
abstract: OBJECTIVE
  To derive and validate a statistical model of motion and deformation for the clinical target volume (CTV) of early-stage rectal cancer patients.
  
  
  METHODS
  For 16 patients, 4 to 5 magnetic resonance images (MRI) were acquired before each fraction was administered. The CTV was delineated on each MRI. Using a leave-one-out methodology, we constructed a population-based principal component analysis (PCA) model of the CTV motion and deformation of 15 patients, and we tested the model on the left-out patient. The modeling error was calculated as the amount of the CTV motion-deformation of the left-out-patient that could not be explained by the PCA model. Next, the PCA model was used to construct a PCA target volume (PCA-TV) by accumulating motion-deformations simulated by the model. A PCA planning target volume (PTV) was generated by expanding the PCA-TV by uniform margins. The PCA-PTV was compared with uniform and nonuniform CTV-to-PTV margins. To allow comparison, geometric margins were determined to ensure adequate coverage, and the volume difference between the PTV and the daily CTV (CTV-to-PTV volume) was calculated.
  
  
  RESULTS
  The modeling error ranged from 0.9 ± 0.5 to 2.9 ± 2.1 mm, corresponding to a reduction of the CTV motion-deformation between 6% and 60% (average, 23% ± 11%). The reduction correlated with the magnitude of the CTV motion-deformation (P<.001, R=0.66). The PCA-TV and the CTV required 2-mm and 7-mm uniform margins, respectively. The nonuniform CTV-to-PTV margins were 4 mm in the left, right, inferior, superior, and posterior directions and 8 mm in the anterior direction. Compared to uniform and nonuniform CTV-to-PTV margins, the PCA-based PTV significantly decreased (P<.001) the average CTV-to-PTV volume by 128 ± 20 mL (49% ± 4%) and by 35 ± 6 mL (20% ± 3.5%), respectively.
  
  
  CONCLUSIONS
  The CTV motion-deformation of a new patient can be explained by a population-based PCA model. A PCA model-generated PTV significantly improved sparing of organs at risk compared to uniform and nonuniform CTV-to-PTV margins.
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands. Electronic address: M.L.Bondar@umcutrecht.nl.
authors.forename: Luiza
authors.initials: L
authors.lastname: Bondar
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Martijn
authors.initials: M
authors.lastname: Intven
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: J P Maarten
authors.initials: JP
authors.lastname: Burbach
authors.affiliation: Delft Institute of Applied Mathematics, Delft University of Technology, Delft, The Netherlands.
authors.forename: Eka
authors.initials: E
authors.lastname: Budiarto
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Kleijnen
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Marielle
authors.initials: M
authors.lastname: Philippens
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Bram
authors.initials: B
authors.lastname: van Asselen
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Enrica
authors.initials: E
authors.lastname: Seravalli
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Onne
authors.initials: O
authors.lastname: Reerink
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Bas
authors.initials: B
authors.lastname: Raaymakers
mesh_terms: D059925:Anatomic Landmarks / Q000033:anatomy & histology; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods; D008297:Male; D015233:Models, Statistical; D009068:Movement; D009367:Neoplasm Staging; D059351:Organ Sparing Treatments / Q000379:methods / Q000706:statistics & numerical data; D058958:Organs at Risk / Q000033:anatomy & histology / Q000528:radiation effects; D025341:Principal Component Analysis / Q000379:methods*; D011832:Radiation Injuries / Q000517:prevention & control; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods / Q000706:statistics & numerical data*; D012004:Rectal Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D047368:Tumor Burden