Scientists from The Institute of Cancer Research, London, found that the technique – known as 4D ultrasound speckle tracking – could track prostate movement to the sub-millimetre accuracy of CyberKnife’s X-ray tracking system.
Ultrasound produced a high imaging rate, and could in future be used to deliver cheaper, more accurate radiotherapy, the researchers said. The ultrasound system was completely non-invasive, requiring no surgically implanted markers, and no additional radiation.
The Royal Marsden NHS Foundation Trust is one of only three sites in the UK with Cyberknife, which uses a moving robotic arm to deliver focused radiation beams to tumours.
But the new findings show that use of ultrasound could be another way of improving radiotherapy for patients with prostate cancer, and could even help Cyberknife to track tumours even more closely.
The study, published today in Physics in Medicine and Biology, was funded in part by Cancer Research UK, with support from the EPSRC.
Prostate cancer radiotherapy uses precisely shaped radiation doses to limit damage to the bowel and bladder by radiation. But organs move naturally in the body – the prostate can shift by as much as 10mm during radiotherapy – so radiation plans add margins around the prostate to compensate.
Modern systems like CyberKnife track the prostate with X-rays to reduce these margins, but metal markers must be surgically placed into the prostate first for tracking to work. Ultrasound can track the prostate during radiotherapy without needing invasive surgery, but until now it hadn’t been clear whether it could estimate prostate movement as accurately.
Scientists at the ICR combined imaging data from five patients to realistically mimic prostate movement using a ‘phantom’ prostate object. They recorded the phantom prostate’s motion in three dimensions with both ultrasound and X-ray tracking and compared their results to the phantom prostate’s real movements.
The researchers saw that ultrasound estimated motion of the phantom prostate’s movement to within 1 millimetre for the most clinically relevant types of prostate movement, which was similar to X-ray tracking. The mechanically sweeping ultrasound probe was less able to follow side-to-side prostate motion, but could track movement every 0.6 seconds, and future ultrasound hardware developments are expected to allow even faster 3D scanning.
Study leader Dr Tuathan O’Shea, Postdoctoral Training Fellow in Radiotherapy and Imaging at the ICR, said: “Tracking tumours with ultrasound is still in its infancy but we wanted to know if ultrasound could produce similar results to CyberKnife’s tracking system. We found that ultrasound compared favourably, estimating prostate motion to sub-millimetre accuracy while maintaining a high imaging rate.
“If you want to take full advantage of image guidance systems like CyberKnife’s, you should be using the highest imaging rates possible, which would also help reduce margins for radiotherapy plans. Our study shows that ultrasound can estimate prostate motion and could help to reduce margins for prostate radiotherapy.”