title: Automatic image guidance for prostate IMRT using low dose CBCT.
abstract: OBJECTIVE
  Varian's On-Board Imager is a linac-integrated cone-beam CT (CBCT) system used at the authors' institution to acquire images prior to delivering each fraction of prostate intensity modulated radiotherapy. The images are used to determine a couch shift that realigns the tumor with the position obtained in the planning CT. However, this manual image-guided radiotherapy (IGRT) technique is operator dependent, time consuming, offers limited degrees of freedom, and requires significant imaging dose over the course of treatment. To overcome these problems, the authors propose two fully automatic IGRT techniques that require significantly less imaging dose.
  
  
  METHODS
  Dose is reduced by lowering the x-ray tube mA s during CBCT acquisition at the cost of increasing image noise. In "forward" IGRT, the CBCT image is automatically registered to the planning CT to obtain the necessary couch shift. The "reverse" technique offers additional degrees of freedom as it involves nonrigid registration of the planning CT to the CBCT. Both techniques were evaluated using images of an anthropomorphic phantom with simulated motion and by retrospectively analyzing data from ten prostate cancer patients.
  
  
  RESULTS
  IGRT error for the phantom data at 100% relative imaging dose was 8.2 +/- 3.7, 3.5 +/- 1.2,, and 2.1 +/- 0.6 mm for setup only, forward, and reverse techniques, respectively. For patient images acquired at 100% relative imaging dose, the errors were 5.4 +/- 1.7, 5.0 +/- 1.6, 5.0 +/- 2.0, and 4.0 +/- 1.6 mm for setup only, manual forward (performed clinically), automatic forward, and reverse IGRT, respectively. Furthermore, imaging dose could be reduced to 20% without a significant loss in image guidance accuracy.
  
  
  CONCLUSIONS
  The presented image guidance methods are accurate while requiring only 20% of the standard imaging dose. The combination of low dose, automation, and accuracy enables frequent corrections during treatment, possibly leading to reduced margins and improved treatment outcomes.
authors.affiliation: Department of Medical Physics, Juravinski Cancer Centre, 699 Concession Street, Hamilton, Ontario L8V 5C2, Canada. marcin.wierzbicki@jcc.hhsc.ca
authors.forename: Marcin
authors.initials: M
authors.lastname: Wierzbicki
authors.forename: Bryan
authors.initials: B
authors.lastname: Schaly
authors.forename: Terry
authors.initials: T
authors.lastname: Peters
authors.forename: Rob
authors.initials: R
authors.lastname: Barnett
mesh_terms: D001331:Automation; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D008297:Male; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated