title: Comparison of patient megavoltage cone beam CT images acquired with an unflattened beam from a carbon target and a flattened treatment beam.
abstract: OBJECTIVE
  To use an imaging beam line (IBL) to obtain the first megavoltage cone-beam computed tomography (MV CBCT) images of patients with a low atomic number (Z) target, and to compare these images to those taken of the same patients with the 6 MV flattened beam from the treatment beam line (TBL).
  
  
  METHODS
  The IBL, which produces a 4.2 MV unflattened beam from a carbon target, was installed on a linear accelerator in use for radiotherapy. Provision was made for switching between the IBL and TBL for imaging the same patient with beams from the low-Z and high-Z targets. Dose was quoted as monitor units times the dose per monitor unit for the standard calibration geometry. Images were acquired with institutional approval and patient consent with both the IBL and TBL on a series of 23 patients undergoing radiotherapy. Patients were imaged daily to weekly and aligned to the planning CT using the images. Doses were reduced over the course of treatment to determine the minimum doses required for alignment. Images were assessed offline.
  
  
  RESULTS
  IBL MV CBCT images of prostate, head and neck, lung, and abdomen showed improvement in soft tissue contrast for the same dose as the TBL images. Bony anatomy, air cavities, and fiducial markers were sharper. CBCT with a dose of 1 cGy was sufficient for alignment of prostate and head and neck patients based on bony anatomy or implanted gold seeds, 2-4 cGy for lung, abdomen, and pelvis. Photon scatter in the patient had minimal effect on image quality. The metallic hip prosthesis in one patient showed reduced artifacts compared to diagnostic CT.
  
  
  CONCLUSIONS
  The IBL has the advantage of improved image quality at the same dose, or reduced dose for the same image quality, over the TBL.
authors.affiliation: Department of Radiation Oncology, University of California San Francisco, San Francisco, California 94143, USA. bfaddegon@radonc.ucsf.edu
authors.forename: Bruce A
authors.initials: BA
authors.lastname: Faddegon
authors.forename: Michele
authors.initials: M
authors.lastname: Aubin
authors.forename: Ali
authors.initials: A
authors.lastname: Bani-Hashemi
authors.forename: Bijumon
authors.initials: B
authors.lastname: Gangadharan
authors.forename: Alexander R
authors.initials: AR
authors.lastname: Gottschalk
authors.forename: Olivier
authors.initials: O
authors.lastname: Morin
authors.forename: Daren
authors.initials: D
authors.lastname: Sawkey
authors.forename: Vincent
authors.initials: V
authors.lastname: Wu
authors.forename: Sue S
authors.initials: SS
authors.lastname: Yom
mesh_terms: D001842:Bone and Bones / Q000473:pathology; D002138:Calibration; D002244:Carbon / Q000737:chemistry; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D004867:Equipment Design; D006258:Head and Neck Neoplasms / Q000473:pathology; D006801:Humans; D008175:Lung Neoplasms / Q000473:pathology; D008297:Male; D010386:Pelvic Neoplasms / Q000473:pathology; D017785:Photons; D011471:Prostatic Neoplasms / Q000473:pathology; D011878:Radiotherapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D012542:Scattering, Radiation
chemical_list: D002244:Carbon