title: Technical Note: Millimeter precision in ultrasound based patient positioning: experimental quantification of inherent technical limitations.
abstract: OBJECTIVE
  To identify the relevant technical sources of error of a system based on three-dimensional ultrasound (3D US) for patient positioning in external beam radiotherapy. To quantify these sources of error in a controlled laboratory setting. To estimate the resulting end-to-end geometric precision of the intramodality protocol.
  
  
  METHODS
  Two identical free-hand 3D US systems at both the planning-CT and the treatment room were calibrated to the laboratory frame of reference. Every step of the calibration chain was repeated multiple times to estimate its contribution to overall systematic and random error. Optimal margins were computed given the identified and quantified systematic and random errors.
  
  
  RESULTS
  In descending order of magnitude, the identified and quantified sources of error were: alignment of calibration phantom to laser marks 0.78 mm, alignment of lasers in treatment vs planning room 0.51 mm, calibration and tracking of 3D US probe 0.49 mm, alignment of stereoscopic infrared camera to calibration phantom 0.03 mm. Under ideal laboratory conditions, these errors are expected to limit ultrasound-based positioning to an accuracy of 1.05 mm radially.
  
  
  CONCLUSIONS
  The investigated 3D ultrasound system achieves an intramodal accuracy of about 1 mm radially in a controlled laboratory setting. The identified systematic and random errors require an optimal clinical tumor volume to planning target volume margin of about 3 mm. These inherent technical limitations do not prevent clinical use, including hypofractionation or stereotactic body radiation therapy.
authors.affiliation: Department of Radiotherapy and Radiation Oncology, University Hospital of the Ludwig-Maximilians-University, Marchioninistraße 15, 81377 Munich, Germany.
authors.forename: Hendrik
authors.initials: H
authors.lastname: Ballhausen
authors.affiliation: Department of Radiotherapy and Radiation Oncology, University Hospital of the Ludwig-Maximilians-University, Marchioninistraße 15, 81377 Munich, Germany.
authors.forename: Sheila
authors.initials: S
authors.lastname: Hieber
authors.affiliation: Department of Radiotherapy and Radiation Oncology, University Hospital of the Ludwig-Maximilians-University, Marchioninistraße 15, 81377 Munich, Germany.
authors.forename: Minglun
authors.initials: M
authors.lastname: Li
authors.affiliation: Department of Radiotherapy and Radiation Oncology, University Hospital of the Ludwig-Maximilians-University, Marchioninistraße 15, 81377 Munich, Germany.
authors.forename: Claus
authors.initials: C
authors.lastname: Belka
authors.affiliation: Department of Radiotherapy and Radiation Oncology, University Hospital of the Ludwig-Maximilians-University, Marchioninistraße 15, 81377 Munich, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Reiner
mesh_terms: D002138:Calibration; D021621:Imaging, Three-Dimensional / Q000295:instrumentation / Q000379:methods*; D007259:Infrared Rays; D007834:Lasers; D056888:Patient Positioning / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D011878:Radiotherapy / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation; D015203:Reproducibility of Results; D014463:Ultrasonography / Q000295:instrumentation / Q000379:methods*