title: Comparison of CT number calibration techniques for CBCT-based dose calculation.
abstract: OBJECTIVE
  The aim of this work was to compare and validate various computed tomography (CT) number calibration techniques with respect to cone beam CT (CBCT) dose calculation accuracy.
  
  
  METHODS
  CBCT dose calculation accuracy was assessed for pelvic, lung, and head and neck (H&N) treatment sites for two approaches: (1) physics-based scatter correction methods (CBCTr); (2) density override approaches including assigning water density to the entire CBCT (W), assignment of either water or bone density (WB), and assignment of either water or lung density (WL). Methods for CBCT density assignment within a commercially available treatment planning system (RSauto), where CBCT voxels are binned into six density levels, were assessed and validated. Dose-difference maps and dose-volume statistics were used to compare the CBCT dose distributions with the ground truth of a planning CT acquired the same day as the CBCT.
  
  
  RESULTS
  For pelvic cases, all CTN calibration methods resulted in average dose-volume deviations below 1.5 %. RSauto provided larger than average errors for pelvic treatments for patients with large amounts of adipose tissue. For H&N cases, all CTN calibration methods resulted in average dose-volume differences below 1.0 % with CBCTr (0.5 %) and RSauto (0.6 %) performing best. For lung cases, WL and RSauto methods generated dose distributions most similar to the ground truth.
  
  
  CONCLUSIONS
  The RSauto density override approach is an attractive option for CTN adjustments for a variety of anatomical sites. RSauto methods were validated, resulting in dose calculations that were consistent with those calculated on diagnostic-quality CT images, for CBCT images acquired of the lung, for patients receiving pelvic RT in cases without excess adipose tissue, and for H&N cases.
authors.affiliation: Joint Department of Physics, Institute of Cancer Research, The Royal Marsden NHS Foundation Trust, London, SM2 5NG, UK. alex.dunlop@rmh.nhs.uk.|The Royal Marsden Hospital, Sutton, Surrey, SM2 5PT, Downs Road, UK. alex.dunlop@rmh.nhs.uk.
authors.forename: Alex
authors.initials: A
authors.lastname: Dunlop
authors.affiliation: Joint Department of Physics, Institute of Cancer Research, The Royal Marsden NHS Foundation Trust, London, SM2 5NG, UK.
authors.forename: Dualta
authors.initials: D
authors.lastname: McQuaid
authors.affiliation: Joint Department of Physics, Institute of Cancer Research, The Royal Marsden NHS Foundation Trust, London, SM2 5NG, UK.
authors.forename: Simeon
authors.initials: S
authors.lastname: Nill
authors.affiliation: The Royal Marsden Hospital, Sutton, Surrey, SM2 5PT, Downs Road, UK.|The Institute of Cancer Research, London, SM2 5NG, UK.
authors.forename: Julia
authors.initials: J
authors.lastname: Murray
authors.affiliation: Department of Medical Physics, Karolinska University Hospital, Stockholm, 171 76, Sweden.
authors.forename: Gavin
authors.initials: G
authors.lastname: Poludniowski
authors.affiliation: Joint Department of Physics, Institute of Cancer Research, The Royal Marsden NHS Foundation Trust, London, SM2 5NG, UK.
authors.forename: Vibeke N
authors.initials: VN
authors.lastname: Hansen
authors.affiliation: The Royal Marsden Hospital, Sutton, Surrey, SM2 5PT, Downs Road, UK.|The Institute of Cancer Research, London, SM2 5NG, UK.
authors.forename: Shreerang
authors.initials: S
authors.lastname: Bhide
authors.affiliation: The Institute of Cancer Research, London, SM2 5NG, UK.
authors.forename: Christopher
authors.initials: C
authors.lastname: Nutting
authors.affiliation: The Royal Marsden Hospital, Sutton, Surrey, SM2 5PT, Downs Road, UK.|The Institute of Cancer Research, London, SM2 5NG, UK.
authors.forename: Kevin
authors.initials: K
authors.lastname: Harrington
authors.affiliation: The Royal Marsden Hospital, Sutton, Surrey, SM2 5PT, Downs Road, UK.
authors.forename: Kate
authors.initials: K
authors.lastname: Newbold
authors.affiliation: Joint Department of Physics, Institute of Cancer Research, The Royal Marsden NHS Foundation Trust, London, SM2 5NG, UK.
authors.forename: Uwe
authors.initials: U
authors.lastname: Oelfke
mesh_terms: D000465:Algorithms; D002138:Calibration; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008175:Lung Neoplasms / Q000532:radiotherapy*; D008297:Male; D010039:Otorhinolaryngologic Neoplasms / Q000532:radiotherapy*; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012542:Scattering, Radiation; D001749:Urinary Bladder Neoplasms / Q000532:radiotherapy*
keywords: Adaptive radiation therapy, ART; Cone beam computed tomography; Density; Dose calculation; Hounsfield units