title: Treatment simulations with a statistical deformable motion model to evaluate margins for multiple targets in radiotherapy for high-risk prostate cancer.
abstract: OBJECTIVE
  Deformation and correlated target motion remain challenges for margin recipes in radiotherapy (RT). This study presents a statistical deformable motion model for multiple targets and applies it to margin evaluations for locally advanced prostate cancer i.e. RT of the prostate (CTV-p), seminal vesicles (CTV-sv) and pelvic lymph nodes (CTV-ln).
  
  
  METHODS
  The 19 patients included in this study, all had 7-10 repeat CT-scans available that were rigidly aligned with the planning CT-scan using intra-prostatic implanted markers, followed by deformable registrations. The displacement vectors from the deformable registrations were used to create patient-specific statistical motion models. The models were applied in treatment simulations to determine probabilities for adequate target coverage, e.g. by establishing distributions of the accumulated dose to 99% of the target volumes (D99) for various CTV-PTV expansions in the planning-CTs.
  
  
  RESULTS
  The method allowed for estimation of the expected accumulated dose and its variance of different DVH parameters for each patient. Simulations of inter-fractional motion resulted in 7, 10, and 18 patients with an average D99 >95% of the prescribed dose for CTV-p expansions of 3mm, 4mm and 5mm, respectively. For CTV-sv and CTV-ln, expansions of 3mm, 5mm and 7 mm resulted in 1, 11 and 15 vs. 8, 18 and 18 patients respectively with an average D99 >95% of the prescription.
  
  
  CONCLUSIONS
  Treatment simulations of target motion revealed large individual differences in accumulated dose mainly for CTV-sv, demanding the largest margins whereas those required for CTV-p and CTV-ln were comparable.
authors.affiliation: Department of Medical Physics, Aarhus University Hospital, Denmark; Department of Oncology, Aarhus University Hospital, Denmark. Electronic address: sarathoe@rm.dk.
authors.forename: Sara
authors.initials: S
authors.lastname: Thörnqvist
authors.forename: Liv B
authors.initials: LB
authors.lastname: Hysing
authors.forename: Andras G
authors.initials: AG
authors.lastname: Zolnay
authors.forename: Matthias
authors.initials: M
authors.lastname: Söhn
authors.forename: Mischa S
authors.initials: MS
authors.lastname: Hoogeman
authors.forename: Ludvig P
authors.initials: LP
authors.lastname: Muren
authors.forename: Lise
authors.initials: L
authors.lastname: Bentzen
authors.forename: Ben J M
authors.initials: BJ
authors.lastname: Heijmen
mesh_terms: D006801:Humans; D008297:Male; D015233:Models, Statistical; D009038:Motion; D010388:Pelvis / Q000000981:diagnostic imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D011835:Radiation Protection; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012669:Seminal Vesicles / Q000000981:diagnostic imaging; D014057:Tomography, X-Ray Computed / Q000379:methods
keywords: Margin expansion; PCA; Pelvic lymph nodes; Seminal vesicles; Statistical deformable motion model