Antiprotons Four Times More Effective than Protons for Cell Irradiation
Geneva, 31 October 2006. A pioneering experiment at CERN with potential future application in cancer therapy has produced its first results. Started in 2003, ACE (Antiproton Cell Experiment) is the first investigation of the biological effects of antiprotons.
“We have taken the first step towards a novel treatment for cancer. The results show that antiprotons are four times more effective than protons at terminating live cells. Although it still has to be compared with other existing methods, it is a breakthrough in this area of investigation.” says Michael Doser at CERN, one of the scientists collaborating on the experiment. ACE brings together a team of experts in the fields of physics, biology, and medicine from 10 institutes around the world.
Current particle beam therapy commonly uses protons to destroy tumour cells inside a patient. The ACE experiment directly compared the effectiveness of cell irradiation using protons and antiprotons. To simulate a cross-section of tissue inside a body, tubes were filled with hamster cells suspended in gelatine. Researchers sent a beam of protons or antiprotons with a range of 2 cm depth into one end of the tube, and evaluated the fraction of surviving cells after irradiation along the path of the beam.
The results showed that antiprotons were four times more effective than protons. When comparing a beam of antiprotons with a beam of protons that cause identical damage at the entrance to the target, the experiment found the damage to cells inflicted at the end of the beam path to be four times higher for antiprotons than for protons. Michael Holzscheiter, spokesperson of the ACE experiment, summarises: “To achieve the same level of damage to cells at the target area one needs four times fewer antiprotons than protons. This significantly reduces the damage to the cells along the entrance channel of the beam for antiprotons compared to protons. Due to the antiproton's unsurpassed ability to preserve healthy tissue while causing damage to a specific area, this type of beam could be highly valuable in treating cases of recurring cancer, where this property is vital.”
Antiprotons are antimatter; they have to be produced in small amounts in a laboratory with the help of a particle accelerator. When matter and antimatter particles meet, they annihilate, or destroy each other, transforming their mass into energy. The experiment makes use of this property as the antiproton would annihilate with a part of the nucleus of an atom in a tumour cell. The fragments produced from the energy released by the annihilation would be projected into adjacent tumour cells, which are in turn destroyed.
“CERN is a unique facility for this work. It is the only place in the world where an antiproton beam of sufficiently low energy and high quality is available. This is crucial for our research. Without access to the antiproton decelerator facility, these experiments would simply not have been possible.” says Niels Bassler, co-spokesperson of ACE. “This experiment is a fantastic example of how research in particle physics can generate innovative solutions with potential medical benefits.”
Researchers are currently conducting more tests to irradiate cells at a greater depth (about 15cm below the surface). Experiments to compare the effectiveness of antiprotons with another form of treatment using carbon ions will begin next month at GSI (Gesellschaft für Schwerionenforschung) in Germany. Further tests are planned to fully assess the effectiveness and suitability of antiprotons for cancer therapy, and to assure that less damage is caused to healthy tissues compared to other methods.
If all goes well, the first clinical application would still be a decade or more into the future.
Full results of the experiment are published today in Radiotherapy and Oncology – Journal of the European Society for Therapeutic Radiology and Oncology.
Holzscheiter MH et al., The Biological Effectiveness of Antiproton Irradiation, Radiother Oncol (2006), doi:10.1016/j.radonc.2006.09.012
The institutes collaborating on the experiment are:
Aarhus University Hospital, Denmark;
British Columbia Cancer Research Centre, Vancouver, Canada;
Centre Medicale Universitaire, Geneva, Switzerland;
CERN, Geneva, Switzerland;
TRIUMF, Vancouver, Canada;
University of Aarhus, Denmark;
University of California at Los Angeles (UCLA), California, USA;
University of Maastricht, The Netherlands.
University of Montenegro, Podgorica, Montenegro,
Vinca Institute of Nuclear Sciences, Belgrade, Serbia;
The field of particle beam cancer therapy was started in 1946 by R. R. Wilson’s seminal paper on “Radiological Use of Fast Protons” (Robert R. Wilson; Radiology 47 (1946)). He noted that protons and other heavy charged particles have a unique property of an “inverse dose profile”. Upon entering a human body, they deposit most of their energy at a depth given by the initial energy of the particles. This leads to a vast reduction of the energy deposited to healthy tissue situated in front of the targeted tumour. And as the particles stop at a defined depth, no energy is deposited beyond the target. In contrast, the energy deposited by X-rays peaks close to the surface and then exponentially decreases up to the exit of the body on the opposite side. This paper and the subsequent experimental work at Lawrence Berkeley Laboratory has led to the development of proton, and in more recent years carbon ion, cancer therapy and the development of 40+ centres worldwide having treated around 50,000 patients to date. Scientists continue to search for possible enhancements of the ratio of damage between the target area and the surrounding healthy tissue in order to reduce side effects and possible late effects. The unique properties of antiprotons, which behave like protons when entering the body but generate more energy at the target area through the annihilation and also deposit this extra energy in a biological more effective way is now the topic of experimental research of the ACE experiment at the Antiproton Decelerator (AD) at CERN.
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 CERN, the European Organization for Nuclear Research, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, the Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland and the United Kingdom. India, Israel, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and UNESCO have observer status.