title: Stochastic triangulation for prostate positioning during radiotherapy using short CBCT arcs.
abstract: OBJECTIVE
  Fast and reliable tumor localization is an important part of today's radiotherapy utilizing new delivery techniques. This proof-of-principle study demonstrates the use of a method called herein 'stochastic triangulation' for this purpose. Stochastic triangulation uses very short imaging arcs and a few projections.
  
  
  METHODS
  A stochastic Maximum A Posteriori (MAP) estimator is proposed based on an uncertainty-driven model of the acquisition geometry and inter-/intra-fractional deformable anatomy. The application of this method was designed to use the available linac-mounted cone-beam computed tomography (CBCT) and/or electronic portal imaging devices (EPID) for the patient setup based on short imaging arcs. For the proof-of-principle clinical demonstration, the MAP estimator was applied to 5 CBCT scans of a prostate cancer patient with 2 implanted gold markers. Estimation was performed for several (18) very short imaging arcs of 5° with 10 projections resulting in 90 estimations.
  
  
  RESULTS
  Short-arc stochastic triangulation led to residual radial errors compared to manual inspection with a mean value of 1.4mm and a standard deviation of 0.9 mm (median 1.2mm, maximum 3.8mm) averaged over imaging directions all around the patient. Furthermore, abrupt intra-fractional motion of up to 10mm resulted in radial errors with a mean value of 1.8mm and a standard deviation of 1.1mm (median 1.5mm, maximum 5.6mm). Slow periodic intra-fractional motions in the range of 12 mm resulted in radial errors with a mean value of 1.8mm and a standard deviation of 1.1mm (median 1.6mm, maximum 4.7 mm).
  
  
  CONCLUSIONS
  Based on this study, the proposed stochastic method is fast, robust and can be used for inter- as well as intra-fractional target localization using current CBCT units.
authors: Regensburg University Medical Center, Department of Radiation Oncology, Regensburg, Germany. wolfgang@hoegele.de
authors: Wolfgang
authors: W
authors: Hoegele
authors: Rainer
authors: R
authors: Loeschel
authors: Barbara
authors: B
authors: Dobler
authors: Oliver
authors: O
authors: Koelbl
authors: Clair
authors: C
authors: Beard
authors: Piotr
authors: P
authors: Zygmanski
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D056888:Patient Positioning; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D013269:Stochastic Processes