title: Feasibility of automated proton therapy plan adaptation for head and neck tumors using cone beam CT images.
abstract: BACKGROUND
  Intensity modulated proton therapy (IMPT) of head and neck (H&N) tumors may benefit from plan adaptation to correct for the dose perturbations caused by weight loss and tumor volume changes observed in these patients. As cone beam CT (CBCT) is increasingly considered in proton therapy, it may be possible to use available CBCT images following intensity correction for plan adaptation. This is the first study exploring IMPT plan adaptation on CBCT images corrected and delineated by deformable image registration of the planning CT (pCT) to the CBCT, yielding a virtual CT (vCT).
  
  
  METHODS
  A Morphons algorithm was used to deform the pCTs and corresponding delineations of 9 H&N cancer patients to a weekly CBCT acquired within ±3 days of a control replanning CT scan (rpCT). The IMPT treatment plans were adapted using the vCT and the adapted and original plans were recalculated on the rpCT for dose/volume parameter evaluation of the impact of adaptation.
  
  
  RESULTS
  On the rpCT, the adapted plans were equivalent to the original plans in terms of target volumes D 95 and V 95, but showed a significant reduction of D 2 in these volumes. OAR doses were mostly equivalent or reduced. In particular, the adapted plans did not reduce parotid gland D mean, but the dose to the optical system. For three patients the spinal cord or brain stem received higher, though well below tolerance, maximum dose. Subsequent tightening of the treatment planning constraints for these OARs on new vCT-adapted plans did not degrade target coverage and yielded pCT equivalent plans on the vCT.
  
  
  CONCLUSIONS
  An offline automated procedure to generate an adapted IMPT plan on CBCT images was developed and investigated. When evaluating the adapted plan on a control rpCT we observed reduced D 2 in target volumes as major improvement. OAR sparing was only partially improved by the procedure. Despite potential limitations in the accuracy of the vCT approach, an improved quality of the adapted plans could be achieved.
authors.affiliation: Department of Radiation Oncology, LMU Munich, Munich, Germany.|Department of Medical Physics, Faculty of Physics, Ludwig-Maximilians-Universität München, Munich, Germany.
authors.forename: Christopher
authors.initials: C
authors.lastname: Kurz
authors.affiliation: Department of Radiation Oncology, LMU Munich, Munich, Germany.
authors.forename: Reinoud
authors.initials: R
authors.lastname: Nijhuis
authors.affiliation: Department of Radiation Oncology, LMU Munich, Munich, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Reiner
authors.affiliation: Department of Radiation Oncology, LMU Munich, Munich, Germany.
authors.forename: Ute
authors.initials: U
authors.lastname: Ganswindt
authors.affiliation: Department of Radiation Oncology, LMU Munich, Munich, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Thieke
authors.affiliation: Department of Radiation Oncology, LMU Munich, Munich, Germany.
authors.forename: Claus
authors.initials: C
authors.lastname: Belka
authors.affiliation: Department of Medical Physics, Faculty of Physics, Ludwig-Maximilians-Universität München, Munich, Germany.
authors.forename: Katia
authors.initials: K
authors.lastname: Parodi
authors.affiliation: Department of Radiation Oncology, LMU Munich, Munich, Germany. G.landry@lmu.de.|Department of Medical Physics, Faculty of Physics, Ludwig-Maximilians-Universität München, Munich, Germany. G.landry@lmu.de.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Landry
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; D001331:Automation; D054893:Cone-Beam Computed Tomography; D005240:Feasibility Studies; D005260:Female; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D008297:Male; D008875:Middle Aged; D010363:Pattern Recognition, Automated; D061766:Proton Therapy / Q000379:methods*; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D057188:Workflow
keywords: Adaptive radiation therapy; Cone beam CT; Deformable image registration; Head and neck cancer; Intensity modulated proton therapy