title: Investigating deformable image registration and scatter correction for CBCT-based dose calculation in adaptive IMPT.
abstract: OBJECTIVE
  This work aims at investigating intensity corrected cone-beam x-ray computed tomography (CBCT) images for accurate dose calculation in adaptive intensity modulated proton therapy (IMPT) for prostate and head and neck (H&N) cancer. A deformable image registration (DIR)-based method and a scatter correction approach using the image data obtained from DIR as prior are characterized and compared on the basis of the same clinical patient cohort for the first time.
  
  
  METHODS
  Planning CT (pCT) and daily CBCT data (reconstructed images and measured projections) of four H&N and four prostate cancer patients have been considered in this study. A previously validated Morphons algorithm was used for DIR of the planning CT to the current CBCT image, yielding a so-called virtual CT (vCT). For the first time, this approach was translated from H&N to prostate cancer cases in the scope of proton therapy. The warped pCT images were also used as prior for scatter correction of the CBCT projections for both tumor sites. Single field uniform dose and IMPT (only for H&N cases) treatment plans have been generated with a research version of a commercial planning system. Dose calculations on vCT and scatter corrected CBCT (CBCTcor) were compared by means of the proton range and a gamma-index analysis. For the H&N cases, an additional diagnostic replanning CT (rpCT) acquired within three days of the CBCT served as additional reference. For the prostate patients, a comprehensive contour comparison of CBCT and vCT, using a trained physician's delineation, was performed.
  
  
  RESULTS
  A high agreement of vCT and CBCTcorwas found in terms of the proton range and gamma-index analysis. For all patients and indications between 95% and 100% of the proton dose profiles in beam's eye view showed a range agreement of better than 3 mm. The pass rate in a (2%,2 mm) gamma-comparison was between 96% and 100%. For H&N patients, an equivalent agreement of vCT and CBCTcorto the reference rpCT was observed. However, for the prostate cases, an insufficient accuracy of the vCT contours retrieved from DIR was found, while the CBCTcorcontours showed very high agreement to the contours delineated on the raw CBCT.
  
  
  CONCLUSIONS
  For H&N patients, no considerable differences of vCT and CBCTcorwere found. For prostate cases, despite the high dosimetric agreement, the DIR yields incorrect contours, probably due to the more pronounced anatomical changes in the abdomen and the reduced soft-tissue contrast in the CBCT. Using the vCT as prior, these inaccuracies can be overcome and images suitable for accurate delineation and dose calculation in CBCT-based adaptive IMPT can be retrieved from scatter correction of the CBCT projections.
authors: Department of Radiation Oncology, LMU Munich, Munich 81377, Germany and Department of Medical Physics, Ludwig-Maximilians-Universität München, Garching bei München 85748, Germany.
authors: Christopher
authors: C
authors: Kurz
authors: Department of Radiation Oncology, LMU Munich, Munich 81377, Germany.
authors: Florian
authors: F
authors: Kamp
authors: Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114.
authors: Yang-Kyun
authors: YK
authors: Park
authors: Department of Medical Physics, Ludwig-Maximilians-Universität München, Garching bei München 85748, Germany.
authors: Christoph
authors: C
authors: Zöllner
authors: Université de Lyon, CREATIS, CNRS UMR5220m Inserm U1044, INSA-Lyon, Université Lyon 1, Lyon F69373, France.
authors: Simon
authors: S
authors: Rit
authors: Department of Oncology, Aarhus University Hospital, Aarhus 8000, Denmark.
authors: David
authors: D
authors: Hansen
authors: Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Centre, Maastricht 6229 ET, The Netherlands.
authors: Mark
authors: M
authors: Podesta
authors: Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114.
authors: Gregory C
authors: GC
authors: Sharp
authors: Department of Radiation Oncology, LMU Munich, Munich 81377, Germany.
authors: Minglun
authors: M
authors: Li
authors: Department of Radiation Oncology, LMU Munich, Munich 81377, Germany.
authors: Michael
authors: M
authors: Reiner
authors: Department of Radiation Oncology, LMU Munich, Munich 81377, Germany.
authors: Jan
authors: J
authors: Hofmaier
authors: Department of Radiation Oncology, LMU Munich, Munich 81377, Germany.
authors: Sebastian
authors: S
authors: Neppl
authors: Department of Radiation Oncology, LMU Munich, Munich 81377, Germany.
authors: Christian
authors: C
authors: Thieke
authors: Department of Radiation Oncology, LMU Munich, Munich 81377, Germany.
authors: Reinoud
authors: R
authors: Nijhuis
authors: Department of Radiation Oncology, LMU Munich, Munich 81377, Germany.
authors: Ute
authors: U
authors: Ganswindt
authors: Department of Radiation Oncology, LMU Munich, Munich 81377, Germany.
authors: Claus
authors: C
authors: Belka
authors: Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114.
authors: Brian A
authors: BA
authors: Winey
authors: Department of Medical Physics, Ludwig-Maximilians-Universität München, Garching bei München 85748, Germany.
authors: Katia
authors: K
authors: Parodi
authors: Department of Medical Physics, Ludwig-Maximilians-Universität München, Garching bei München 85748, Germany.
authors: Guillaume
authors: G
authors: Landry
mesh_terms: D054893:Cone-Beam Computed Tomography; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided; D050397:Radiotherapy, Intensity-Modulated; D012542:Scattering, Radiation