A robotic treatment delivery system to facilitate dynamic conformal synchrotron radiotherapy

In clinical radiotherapy, the patient remains static during treatment and only the source is dynamically manipulated. In synchrotron radiotherapy, the beam is fixed, and is horizontally wide and vertically small, requiring the patient to be moved through the beam to ensure full target coverage, while shaping the field to conform to the target. No clinical system exists that performs both dynamic motion of the patient and dynamic shaping of the beam.

We developed and tested a new dynamic treatment delivery system capable of delivering conformal fields with a robotic patient positioning system for use on the Imaging and Medical Beamline (IMBL) at the Australian Nuclear Science and Technology Organisation, Australian Synchrotron.

An industrial robotic manipulator was modified to enable dynamic radiotherapy treatments on IMBL. The robot, combined with a carbon-fiber treatment couch-top and a recently developed dynamic collimator, formed the basis of the new treatment delivery system. To synchronize the motions of the robot and collimator, a real-time, hardware-based event-handling system was utilized. To test the system, a ball bearing in a medical physics phantom was treated with circular fields ranging from 5 to 40 mm in diameter and at treatment speeds from 2 to 50 mm  $s^{-1}$ . The position of the ball bearing was compared to the center of the circular fields and the positional and temporal accuracy of the treatment delivery system was assessed, and appropriate treatment margins for the system were determined.

The vertical position of the ball bearing varied with treatment delivery speed ( $-1.06\mathrm{to}0.93\mathrm{mm}$ ) while the horizontal position remained consistent ( $-0.05\mathrm{to}0.09\mathrm{mm}$ ). The time-delay between the robot and the collimator remained consistent ( $-35.5\mathrm{ms}\mathrm{to}18.5\mathrm{ms}$ ) at treatment speeds above $2{\mathrm{mms}}^{-1}$ . Data at $2\mathrm{mm}{s}^{-1}$ was right at the edge of both the robot capabilities and the analysis technique, and had larger variations in timing ( $0.0\mathrm{ms}\mathrm{to}57.9\mathrm{ms}$ ). Horizontal margins of $0.51\mathrm{mm}$ and vertical margins of up to $2.3\mathrm{mm}$ were calculated for the treatment delivery system.

We have implemented the first robotic treatment delivery system for synchrotron radiotherapy treatments. The largest errors were observed in the direction of motion of the patient through the beam and with future improvements, can be reduced. The system was both accurate and repeatable and is ready to support future treatments on IMBL.