Robotic ultrasound-guided SBRT of the prostate: feasibility with respect to plan quality

  • Stefan Gerlach
  • Ivo Kuhlemann
  • Philipp Jauer
  • Ralf Bruder
  • Floris Ernst
  • Christoph Fürweger
  • Alexander SchlaeferEmail author
Original Article



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.


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.


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.


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.


SBRT Image-guided radiation therapy CyberKnife Treatment planning Ultrasound Robotics 


Compliance with ethical standards


This study was partially funded by Deutsche Forschungsgemeinschaft (Grants ER 817/1-1 and SCHL 1844/3-1).

Conflict of interest

Ralf Bruder is co-inventor of a patent pending method for positioning an ultrasound transducer. Floris Ernst has received Grants from Varian Medical Systems, Inc. The other authors declare no conflict of interest.

Ethical approval

This article is based on fully anonymized treatment planning data and does not contain any studies with human participants or animals performed by any of the authors.

Informed consent

For this type of study formal consent is not required.


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Copyright information

© CARS 2016

Authors and Affiliations

  • Stefan Gerlach
    • 1
  • Ivo Kuhlemann
    • 2
  • Philipp Jauer
    • 2
  • Ralf Bruder
    • 2
  • Floris Ernst
    • 2
  • Christoph Fürweger
    • 3
  • Alexander Schlaefer
    • 1
    Email author
  1. 1.Institute of Medical TechnologyHamburg University of TechnologyHamburgGermany
  2. 2.Institute for Robotics and Cognitive SystemsUniversität zu LübeckLübeckGermany
  3. 3.Europäisches Cyberknife Zentrum München-GroßhadernMunichGermany

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