title: Robotic ultrasound-guided SBRT of the prostate: feasibility with respect to plan quality.
abstract: OBJECTIVE
  Advances in radiation therapy delivery systems have enabled motion compensated SBRT of the prostate. A remaining challenge is the integration of fast, non-ionizing volumetric imaging. Recently, robotic ultrasound has been proposed as an intra-fraction image modality. We study the impact of integrating a light-weight robotic arm carrying an ultrasound probe with the CyberKnife system. Particularly, we analyze the effect of different robot poses on the plan quality.
  
  
  METHODS
  A method to detect the collision of beams with the robot or the transducer was developed and integrated into our treatment planning system. A safety margin accounts for beam motion and uncertainties. Using strict dose bounds and the objective to maximize target coverage, we generated a total of 7650 treatment plans for five different prostate cases. For each case, ten different poses of the ultrasound robot and transducer were considered. The effect of different sets of beam source positions and different motion margins ranging from 5 to 50 mm was analyzed.
  
  
  RESULTS
  Compared to reference plans without the ultrasound robot, the coverage typically drops for all poses. Depending on the patient, the robot pose, and the motion margin, the reduction in coverage may be up to 50 % points. However, for all patient cases, there exist poses for which the loss in coverage was below 1 % point for motion margins of up to 20 mm. In general, there is a positive correlation between the number of treatment beams and the coverage.
  
  
  CONCLUSIONS
  While the blocking of beam directions has a negative effect on the plan quality, the results indicate that a careful choice of the ultrasound robot's pose and a large solid angle covered by beam starting positions can offset this effect. Identifying robot poses that yield acceptable plan quality and allow for intra-fraction ultrasound image guidance, therefore, appears feasible.
authors.affiliation: Institute of Medical Technology, Hamburg University of Technology, Hamburg, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Gerlach
authors.affiliation: Institute for Robotics and Cognitive Systems, Universität zu Lübeck, Lübeck, Germany.
authors.forename: Ivo
authors.initials: I
authors.lastname: Kuhlemann
authors.affiliation: Institute for Robotics and Cognitive Systems, Universität zu Lübeck, Lübeck, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Jauer
authors.affiliation: Institute for Robotics and Cognitive Systems, Universität zu Lübeck, Lübeck, Germany.
authors.forename: Ralf
authors.initials: R
authors.lastname: Bruder
authors.affiliation: Institute for Robotics and Cognitive Systems, Universität zu Lübeck, Lübeck, Germany.
authors.forename: Floris
authors.initials: F
authors.lastname: Ernst
authors.affiliation: Europäisches Cyberknife Zentrum München-Großhadern, Munich, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Fürweger
authors.affiliation: Institute of Medical Technology, Hamburg University of Technology, Hamburg, Germany. schlaefer@tuhh.de.
authors.forename: Alexander
authors.initials: A
authors.lastname: Schlaefer
mesh_terms: D005240:Feasibility Studies; D006801:Humans; D008297:Male; D009038:Motion; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided; D012371:Robotics / Q000379:methods*; D014463:Ultrasonography / Q000379:methods*
keywords: CyberKnife; Image-guided radiation therapy; Robotics; SBRT; Treatment planning; Ultrasound