title: Fast, low-dose patient localization on TomoTherapy via topogram registration.
abstract: OBJECTIVE
  To investigate a protocol which efficiently localizes TomoTherapy patients with a scout imaging (topogram) mode that can be used with or instead of 3D megavoltage computed tomography (MVCT) imaging.
  
  
  METHODS
  The process presented here is twofold: (a) The acquisition of the topogram using the TomoTherapy MV imaging system and (b) the generation of a digitally reconstructed topogram (DRT) derived from a standard kV CT simulation data set. The unique geometric characteristics of the current TomoTherapy imaging system were explored both theoretically and by acquiring topograms of anthropomorphic phantoms and comparing these images to DRT images. The performance of the MV topogram imaging system in terms of image quality, dose incurred to the patient, and acquisition time was investigated using ionization chamber and radiographic film measurements.
  
  
  RESULTS
  The time required to acquire a clinically usable topogram, limited by the maximum couch speed of 4.0 cm s(-1), was 12.5 s for a 50 cm long field. The patient dose was less than 1% of that delivered by a helical MVCT scan. Further refinements within the current TomoTherapy system, most notably decreasing the imaging beam repetition rate during MV topogram acquisition, would further reduce the topogram dose to less than 25 microGy per scan without compromising image quality.
  
  
  CONCLUSIONS
  Topogram localization on TomoTherapy is a fast and low-dose alternative to 3D MVCT localization. A protocol designed that exclusively utilized MV topograms would result in a 30-fold reduction in imaging time and a 100-fold reduction in dose from localization scans using the current TomoTherapy workflow.
authors.affiliation: Department of Radiation Oncology, Washington University in St. Louis, Missouri 63110, USA. kmoore@radonc.wustl.edu
authors.forename: Kevin L
authors.initials: KL
authors.lastname: Moore
authors.forename: Geethpriya
authors.initials: G
authors.lastname: Palaniswaamy
authors.forename: Benjamin
authors.initials: B
authors.lastname: White
authors.forename: S Murty
authors.initials: SM
authors.lastname: Goddu
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Low
mesh_terms: D006801:Humans; D009369:Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011835:Radiation Protection / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D013382:Subtraction Technique; D014057:Tomography, X-Ray Computed / Q000379:methods*