Breakthrough in real time tracking of prostate motion during arc
9 March 2010
Researchers in the US and Denmark have made a breakthrough in
image-guided targeting of prostate tumours during arc radiotherapy
In research partially supported by Varian Medical Systems,
physicists at Stanford University, California and Aarhus University
Hospital, Denmark have devised a method 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 Varian’s 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 tumour motion. To date, a
combination of MV portal images and kV orthogonal images have been
tested to achieve this.
“Acquiring mega-voltage images is not ideal during RapidArc
because the DMLC can block the view of the target during the
treatment,” says 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 3D motion.
This single imager based DMLC tracking system was shown to have
sub-millimeter accuracy for most types of prostate motion. Precise
alignment of the treatment beam with the tumor position is vital in
radiotherapy treatments, as it can reduce the likelihood 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,”
adds 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.”
An upcoming publication from these researchers will focus on the
application of this method to respiratory motion.