Antimatter could fight cancer

A pioneering experiment at CERN with potential future applications in cancer therapy has produced its first results. Researchers found that antiprotons are four times more effective than protons for cell irradiation.

From left to right: Michael Holzscheiter with collaborators Niels Bassler and Helge Knudsen in front of the ACE experiment. The particle beam enters a tube of cells in the centre of a tank which contains a solution of glycerol and water.

ACE (Antiproton Cell Experiment, also known as AD-4) is a small experiment with a potentially big impact. With an apparatus that looks like a small fish tank you can fit in the boot of a car, you would never have guessed that they are investigating the use of antimatter to treat cancer. Started in 2003, this is the first experiment to study the biological effects of antiprotons. It brings together a team of experts in the fields of physics, biology and medicine from 10 institutes around the world. The ACE collaborators recently published their first results with some impressive findings.

Current particle beam therapy commonly uses protons to destroy tumour cells inside a patient. When a beam of heavy, charged particles enters a human body, it initially inflicts very little damage to the tissue. Only in the last millimetre of the journey, when it switches from a gradual slow-down to an abrupt stop at a specific depth (depending on its initial energy) is significant damage done. 'It's similar to a car gently rolling to a stop, and then suddenly putting on the brakes,' explained Michael Doser at CERN, one of the ACE collaborators. The experiment tested the idea of using antiprotons as an alternative treatment, by directly comparing the effectiveness of cell irradiation using protons and antiprotons. To simulate a cross-section of tissue inside a body, tubes were filled with live hamster cells suspended in gelatine. Researchers sent a beam of protons or antiprotons with a range of 2cm in water into one end of the tube, and evaluated the fraction of surviving cells after irradiation vs. the depth in the target.

When a beam of protons and a beam of antiprotons that cause identical damage at the entrance to the target were compared, the results showed that the damage to cells inflicted at the end of the beam path was four times higher using the beam of antiprotons. Michael Holzscheiter, spokesperson of the ACE experiment, explained the significance of this finding: '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, 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. 'CERN is the only place in the world where an antiproton beam of sufficiently low energy and high quality is available. Without access to the antiproton decelerator facility, these experiments would simply not have been possible,' says Niels Bassler, co-spokesperson of ACE. 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.

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.

'Antiprotons at first seem unlikely candidates for research in cancer therapy. However, our results indicate that these antimatter particles cover the best of both worlds from proton and carbon ion therapy, and could potentially lead to a more effective cancer radiation therapy', says Holzscheiter.

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. Special attention will be given to the study of possible late effects as a consequence of the irradiation. This is an important issue since antiprotons generate a highly complex radiation field and secondary particles with a large variety of ranges in tissue are produced.

ACE is a fantastic example of how research in particle physics can bring innovative solutions with potential medical benefits. However, the validation process for any new medical treatment is lengthy. If all goes well, the first clinical application would still be a decade into the future.

For background information on the start of this experiment, see Bulletin No. 48/2003.

Did you know?

Particle beam cancer therapy was started in 1946 by R.R. Wilson's seminal paper on 'Radiological Use of Fast Protons'. 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 compared to the standard X-ray therapy. And as the particles stop at a defined depth, no energy is deposited beyond the target. 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.