Tracking Prostate Motion with Single kV Imager During Arc Radiotherapy

In research partially supported by Varian Medical Systems, Inc. (Palo Alto, CA, USA), physicists at Stanford University (Palo Alto, CA, USA) and Aarhus University Hospital (Aarhus, Denmark) have devised a technique for real-time tracking of the prostate motion using Varian's On-Board Imager that shows promise in paving the way for advanced clinical treatments.

Arc therapy techniques such as RapidArc technology are fast and efficient radiotherapy treatments delivered in a continuous rotation of the treatment machine around the patient. The position of the target must be updated in real-time in order for the dynamic multi-leaf collimator (DMLC) to track tumor motion. To date, a combination of MV (megavoltage) portal images and kV orthogonal images have been evaluated to achieve this.

"Acquiring megavoltage images is not ideal during RapidArc because the DMLC can block the view of the target during the treatment,” said Per Rugaard Poulsen, lead author of the research. "The kV image beam is not obscured by the treatment DMLC making the markers visible from all treatment angles, unlike with the MV beam.”

To test the system, arc radiotherapy was delivered to a "motion phantom” implanted with fiducial markers while continuously imaging with the kV beam. The target position was determined from these images, acquired from different projections during the arc, enabling the researchers to determine information about its three-dimensional (3D) motion. This single imager based DMLC tracking system was shown to have submillimeter accuracy for most types of prostate motion. Precise alignment of the treatment beam with the tumor position is crucial in radiotherapy treatments, as it can reduce the probability of complications due to the treatment.

"The results confirm what has been seen in simulation studies and show that the method is very robust to experimental uncertainties,” added Per Rugaard Poulsen. "We've demonstrated that image-based tracking during arc radiotherapy can be done completely without MV images, which is an important step along the way to image-based tracking during arc treatment delivery.”

Corey Zankowski, Varian's senior director of product management, said, "This research paves the way for real-time tracking of the DMLC with tumor motion during advanced arc therapy treatments, which should allow clinicians to reduce treatment margins around the tumor. Varian is committed to working closely with industry experts and researchers to enhance treatment accuracy with the goal of improving treatment outcomes.”

Varian Medical Systems is a manufacturer of medical devices and software for treating cancer and other medical conditions with radiotherapy, radiosurgery, proton therapy, and brachytherapy.

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Aarhus University Hospital