Robotic intrafractional US guidance for liver SABR: System design, beam avoidance, and clinical imaging.
OBJECTIVE
To present a system for robotic 4D ultrasound (US) imaging concurrent with radiotherapy beam delivery and estimate the proportion of liver stereotactic ablative body radiotherapy (SABR) cases in which robotic US image guidance can be deployed without interfering with clinically used VMAT beam configurations.


METHODS
The image guidance hardware comprises a 4D US machine, an optical tracking system for measuring US probe pose, and a custom-designed robot for acquiring hands-free US volumes. In software, a simulation environment incorporating the LINAC, couch, planning CT, and robotic US guidance hardware was developed. Placement of the robotic US hardware was guided by a target visibility map rendered on the CT surface by using the planning CT to simulate US propagation. The visibility map was validated in a prostate phantom and evaluated in patients by capturing live US from imaging positions suggested by the visibility map. In 20 liver SABR patients treated with VMAT, the simulation environment was used to virtually place the robotic hardware and US probe. Imaging targets were either planning target volumes (PTVs, range 5.9-679.5 ml) or gross tumor volumes (GTVs, range 0.9-343.4 ml). Presence or absence of mechanical interference with LINAC, couch, and patient body as well as interferences with treated beams was recorded.


RESULTS
For PTV targets, robotic US guidance without mechanical interference was possible in 80% of the cases and guidance without beam interference was possible in 60% of the cases. For the smaller GTV targets, these proportions were 95% and 85%, respectively. GTV size (1/20), elongated shape (1/20), and depth (1/20) were the main factors limiting the availability of noninterfering imaging positions. The robotic US imaging system was deployed in two liver SABR patients during CT simulation with successful acquisition of 4D US sequences in different imaging positions.


CONCLUSIONS
This study indicates that for VMAT liver SABR, robotic US imaging of a relevant internal target may be possible in 85% of the cases while using treatment plans currently deployed in the clinic. With beam replanning to account for the presence of robotic US guidance, intrafractional US may be an option for 95% of the liver SABR cases.
SoniTrack Systems Inc., Palo Alto, California 94304.
Jeffrey
J
Schlosser
Department of Radiation Oncology, School of Medicine, Stanford University, Stanford, California 94305.
Ren Hui
RH
Gong
Institute for Robotics and Cognitive Systems, University of Luebeck, Luebeck 23538, Germany.
Ralf
R
Bruder
Institute for Robotics and Cognitive Systems, University of Luebeck, Luebeck 23538, Germany.
Achim
A
Schweikard
Biorobotics Lab, Department of Mechanical Engineering, Stanford University, Stanford, California 94305.
Sungjune
S
Jang
Biorobotics Lab, Department of Mechanical Engineering, Stanford University, Stanford, California 94305.
John
J
Henrie
Department of Radiology, School of Medicine, Stanford University, Stanford, California 94305.
Aya
A
Kamaya
Department of Radiation Oncology, School of Medicine, Stanford University, Stanford, California 94305.
Albert
A
Koong
Department of Radiation Oncology, School of Medicine, Stanford University, Stanford, California 94305.
Daniel T
DT
Chang
Department of Radiation Oncology, School of Medicine, Stanford University, Stanford, California 94305.
Dimitre
D
Hristov
D019583:Dose Fractionation, Radiation; D004868:Equipment Failure; D006801:Humans; D008113:Liver Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D016634:Radiosurgery / Q000295:instrumentation*; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation*; D050397:Radiotherapy, Intensity-Modulated; D012371:Robotics; D014463:Ultrasonography