title: Simulation of pseudo-CT images based on deformable image registration of ultrasound images: A proof of concept for transabdominal ultrasound imaging of the prostate during radiotherapy.
abstract: OBJECTIVE
  Imaging of patient anatomy during treatment is a necessity for position verification and for adaptive radiotherapy based on daily dose recalculation. Ultrasound (US) image guided radiotherapy systems are currently available to collect US images at the simulation stage (USsim), coregistered with the simulation computed tomography (CT), and during all treatment fractions. The authors hypothesize that a deformation field derived from US-based deformable image registration can be used to create a daily pseudo-CT (CTps) image that is more representative of the patients' geometry during treatment than the CT acquired at simulation stage (CTsim).
  
  
  METHODS
  The three prostate patients, considered to evaluate this hypothesis, had coregistered CT and US scans on various days. In particular, two patients had two US-CT datasets each and the third one had five US-CT datasets. Deformation fields were computed between pairs of US images of the same patient and then applied to the corresponding USsim scan to yield a new deformed CTps scan. The original treatment plans were used to recalculate dose distributions in the simulation, deformed and ground truth CT (CTgt) images to compare dice similarity coefficients, maximum absolute distance, and mean absolute distance on CT delineations and gamma index (γ) evaluations on both the Hounsfield units (HUs) and the dose.
  
  
  RESULTS
  In the majority, deformation did improve the results for all three evaluation methods. The change in gamma failure for dose (γDose, 3%, 3 mm) ranged from an improvement of 11.2% in the prostate volume to a deterioration of 1.3% in the prostate and bladder. The change in gamma failure for the CT images (γCT, 50 HU, 3 mm) ranged from an improvement of 20.5% in the anus and rectum to a deterioration of 3.2% in the prostate.
  
  
  CONCLUSIONS
  This new technique may generate CTps images that are more representative of the actual patient anatomy than the CTsim scan.
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands.
authors.forename: Skadi
authors.initials: S
authors.lastname: van der Meer
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands; Oncology Solutions Department, Philips Research, High Tech Campus 34, Eindhoven 5656 AE, The Netherlands; and Department of Biomedical Engineering, University of Technology Eindhoven, Den Dolech 2, Eindhoven 5600 MB, The Netherlands.
authors.forename: Saskia M
authors.initials: SM
authors.lastname: Camps
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands.
authors.forename: Wouter J C
authors.initials: WJ
authors.lastname: van Elmpt
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands.
authors.forename: Mark
authors.initials: M
authors.lastname: Podesta
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands.
authors.forename: Pedro Gomes
authors.initials: PG
authors.lastname: Sanches
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands.
authors.forename: Ben G L
authors.initials: BG
authors.lastname: Vanneste
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands and Oncology Solutions Department, Philips Research, High Tech Campus 34, Eindhoven 5656 AE, The Netherlands.
authors.forename: Davide
authors.initials: D
authors.lastname: Fontanarosa
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands and Medical Physics Unit, Department of Oncology, McGill University, Montréal, Québec H4A 3J1, Canada.
authors.forename: Frank
authors.initials: F
authors.lastname: Verhaegen
mesh_terms: D000005:Abdomen; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D011467:Prostate / Q000000981:diagnostic imaging* / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D011879:Radiotherapy Dosage; D061089:Radiotherapy, Image-Guided; D014057:Tomography, X-Ray Computed; D014463:Ultrasonography