title: An optimised IGRT correction vector determined from a displacement vector field: a proof of principle of a decision-making aid for re-planning.
abstract: BACKGROUND
  To present a new method that determines an optimised IGRT couch correction vector from a displacement vector field (DVF). The DVF is computed by a deformable image registration (DIR) method. The proposed method can improve the quality of volume-of-interest (VOI) alignment in image guided radiation therapy (IGRT), and can serve as a decision-making aid for re-planning.
  
  
  METHODS
  The proposed method was demonstrated using the CT data sets of 11 head-and-neck cancer patients with daily kilovoltage control-CTs. A DVF was computed for each control-CT using a DIR method. The DVF was used for voxel tracking and re-contouring of the VOIs in the control-CTs. Then a rigid body transformation, which could be used as couch correction vector, was optimised. The aim of the optimisation process was to find a vector and rotations that map the deformed VOIs into a specified territory. This territory was defined by a margin extension of the VOIs at the time of the planning process. Within this extension, VOI motion and deformation was tolerated. The objective function in the optimisation process was the sum of all volume fractions outside the defined territories.
  
  
  RESULTS
  The proposed method was able to find a correction vector, which resulted in a coverage of the target volumes of at least 98% in 52.3% of all fractions. In contrast, a standard IGRT correction using a rigid registration method only fulfilled this criterion in 22.6% of all fractions. The optimisation process took an average of 1.5 minutes per fraction.
  
  
  CONCLUSIONS
  The knowledge of the deformation of the anatomy allows the determination of an optimised rigid correction vector using our method. The method ensures controlled mapping of the VOIs despite small deformations. If no optimised vector can be determined, re-planning should be considered. Thus, our method can also serve as a decision-making aid for re-planning.
authors.affiliation: Department of Medical Physics in Radiation Oncology, DKFZ , Heidelberg , Germany.
authors.forename: Eva Maria
authors.initials: EM
authors.lastname: Stoiber
authors.forename: Michael
authors.initials: M
authors.lastname: Schwarz
authors.forename: Jürgen
authors.initials: J
authors.lastname: Debus
authors.forename: Rolf
authors.initials: R
authors.lastname: Bendl
authors.forename: Kristina
authors.initials: K
authors.lastname: Giske
mesh_terms: D003657:Decision Making; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D061089:Radiotherapy, Image-Guided; D014057:Tomography, X-Ray Computed