discovered_01_2016

WWW.HZDR.DE 24 25 RESEARCH // THE HZDR RESEARCH MAGAZINE Protons making waves Richter’s team has now taken a huge step towards solving this problem. Just as a stone thrown into a pond causes waves to ripple across the surface, protons also generate gamma waves, called prompt gamma radiation, around the spot where they "land" in the patient’s body and deposit their destructive energy. Researchers capture this radiation with a slit camera – a gamma radiation detector. "The gamma rays tell us where the protons currently are in the human body," says Guntram Pausch, head of the research group "In-vivo Dosimetry" at OncoRay. The gamma rays that are generated by the proton beam are visualized in a detector plane. "This provides information about the edge, i.e. the spot where the gamma radiation suddenly drops because that’s where the proton beam stops." While there are other methods of measuring this "edge", the depth of the proton beam penetration, they are either too slow to yield any results while irradiation is in progress or not developed far enough to be used on human subjects. The slit camera has already passed its first test. In mid-August of 2015, the first measurements were taken during radiation treatment on a patient with a head and neck tumor. Prior to that, researchers had conducted innumerable experiments on plexiglas and tissue phantoms to prepare the camera for use on human subjects. "Plexiglas consists of carbon, hydrogen and oxygen, which means it’s very similar to humans," says Marlen Priegnitz from the Institute of Radiation Physics at HZDR. "You start with cubes, blocks and cylinders, then you go on to tissue-like materials that simulate fat, lung, muscle or brain tissue." By irradiating the phantoms with protons and making targeted modifications, the researchers were able to experimentally prove that the camera can indeed capture the expected variations in range. Only then they did venture to use the detector at the hospital. Margin of error halved "The patient knew that the measurement with the slit camera wouldn’t harm him, but wouldn’t benefit him either, but might help the next generation," Richter emphasizes. The treatment took a little longer, simply because it takes a few minutes to position the slit camera prior to radiotherapy. With the help of the slit camera, we were able to ascertain on different treatment days that the measured range of the proton beam did not vary by more than two millimeters in one area," Richter reports. A significant improvement over the margin of error that radiation therapists currently have to work with. "In the case of this specific patient, the estimated margin of error in the proton range used to be seven millimeters, which means we had to irradiate an area seven millimeters larger than the tumor to make sure we really hit it," Pausch explains. "The camera would allow us to limit this margin to three to four millimeters. Patients would benefit from this." Even if a few millimeters’ difference does not sound as though it justifies that much research effort – in the brain, irradiating an area as thin as orange peel can mean the difference between a brain tumor patient being able to speak after therapy – or not. "The deeper I penetrate into the patient, the greater the uncertainty in range," Richter adds. "When irradiating a prostate, for instance, the proton beam has a range of 25 centimeters, which means a margin of error of more than 10 millimeters." This makes it even more important to continuously measure the range of the proton beam during such multi-week treatments. And the prompt gamma camera, the prototype of which was developed by IBA and tested jointly with the team from Dresden, makes this possible. The OncoRay Center, which is operated jointly be the University Hospital, TU Dresden and HZDR, provides the perfect setting for testing such research projects – where clinical trials meet nuclear physics. Pausch explains that the goal is to integrate the slit camera into the proton beam equipment. Right now, researchers have to manually position the detector at a right angle to the direction of the proton beam. In the future, the slit camera will automatically transmit its measurement data and interrupt the treatment if the beam penetrates too deeply. "We have a long way to go before that," Pausch says. A realistic short- term success would be to use the slit camera to find out to what extent the theoretical planning of penetration depth matches up with the actual treatment on the patient. "If we could just learn that our calculations match up with the delivered treatment 99.9 per cent of the time, or how we have to modify it, it would help us to avoid having to irradiate so much healthy tissue around the tumor." And the patients would benefit. PUBLICATION: C. Richter, G. Pausch, S. Barczyk, M. Priegnitz et al.: "First clinical application of a prompt gamma based in vivo proton range verification system", in Radiotherapy and Oncology 2016 (DOI: 10.1016/j.radonc.2016.01.004) CONTACT _National Center for Radiation Research in Oncology – OncoRay High-Precision Radiotherapy Dr. Christian Richter christian.richter@oncoray.de www.oncoray.de _Institute of Radiooncology at HZDR In-vivo Dosimetry Dr. Guntram Pausch guntram.pausch@oncoray.de _Institute of Radiation Physics at HZDR Dr. Marlen Priegnitz m.priegnitz@hzdr.de

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