Tumours are treated using proton beams at the Paul Scherrer Institute (PSI). Because this irradiation is particularly gentle on the healthy tissue, among the patients who have been treated were 370 children and adolescents younger than 18. Construction of the systems for cancer treatments, which are unique in Switzerland, was only possible because research into proton accelerators has been carried out at the PSI for 40 years.

David Meer has jointly developed the proton beam system Gantry 2 at PSI for the treatment of cancer patients.

The patient is led into the treatment room, and lies down on the treatment table. After ensuring that she is positioned and held in the right place, the table rotates to the treatment position of the therapy station Gantry 2, the latest generation of proton beam systems. With the table held in place, the Gantry 2 starts to rotate around the patient. When it reaches the right position, the radiology assistant in the adjoining room starts the irradiation with the beam of protons, positive-charged particles that are found in the nucleus of every atom. The patient suffers from a brain tumour at the base of the skull, which cannot be removed surgically. Therefore the beam therapy is used to try and stop the growth of the tumour. The proton beams scan the tumour, point by point, precise to the millimetre. To do this, magnets direct the beam like in an old cathode ray tube television. The precisely aimed beam does not damage the surrounding healthy tissue.

Gantry 2 is installed in the Center for Proton Therapy (CPT) on the grounds of the PSI in Villigen. “This installation is unique”, says high-energy physicist David Meer, who has been involved in its development since 2004. There is nothing better to be found any where in the world, he says. This is because the proton beam can scan the diseased tissue very quickly and accurately. Cancer patients have been treated using Gantry 2 since November 2013. Indeed, even its precursor was world-leading: When Gantry 1 started operation in 1996, this procedure, which the experts refer to as the spot-scanning technique, was used for the first time to treat cancer patients. “It was a stroke of genius – even from today's perspective”, says David Meer. Since then over 900 patients have been treated using Gantry 1, which is still in operation next to Gantry 2.

A daring project proves its worth

The protons for the beam therapy have been produced since 2007 by an accelerator that was designed at the Michigan State University in America, and manufactured by the German firm Accel (that is now a part of Varian Medical Systems). In this endeavour one was able to benefit from the experts and the decades of experience of PSI. This is because the first large proton accelerator started operation forty years ago in Villigen, and scientists from all over the world are still using it today, although not for medical purposes but for research experiments. “Without this knowledge and experience we would not have been able to build the systems for cancer treatment at PSI”, David Meer says. In fact the construction of the accelerator, at a cost of 100m CHF, was not without controversy at that time. “It was daring, because there was no precedent anywhere in the world”, an article in the newspaper Neue Zürcher Zeitung stated at the time when the system started operating in the year 1974. But years of work were invested in developing the project with great care.

To view the long-serving, imposing accelerator, one has to ascend a metal scaffold to get to a gallery in the experimental hall. Eight large magnets, each of them weighing 250 tons, are laid out in a circle and shrouded by a concrete cover. Between these are arrayed four “cavities”, in which the protons are accel-erated by applying an alternating voltage. The magnets keep the particles on course. The ring is about 15 metres in diameter, and in this the protons are accelerated with such great force that they race along at nearly 80 % of the speed of light. They then hurtle against special targets made of carbon, and produce muons – particles similar to electrons but with a much greater mass. Or they are used to knock out neutrons, the uncharged components of atomic nucleii, from lead targets.

Measuring stations are arrayed around the large ring accelerator, where the researchers use the newly created particles to perform experiments, some of which are only possible at the PSI. Although initially, 40 years ago, the system was mainly used for basic research in particle physics, in the course of time research experiments were added in materials science, structural biology and even archaeology. Thus researchers today are studying extremely thin layers of new types of materials, with the aid of muons; or they use neutrons to illuminate the inside of prehistoric artefacts. Thanks to continuous upgrades, the PSI has even been a world-record holder since 1994: “Our proton beam is the most powerful in the world”, says Prof. Leonid Rivkin, head of the large research installations department at PSI and Professor for particle accelerator physics at EPFL.

Healing for patients with eye tumours

As far back as thirty years ago, the PSI was already using the proton beams not only for research but also for the treatment of cancer patients. It started with the irradiation of eye tumours. Since then over 6,300 patients with this type of tumour have been successfully treated at the PSI. One specially good result: Even after five years the local tumour control rate for the eye patients is at 98 % . When Gantry 1 started operating, it became possible to treat even deep-set tumours, especially in the head region.

There is an CT scanner and an integrated x-ray system in the treat ment room for imaging.

Because the proton accelerator that was commissioned in 1974 was not ideally suited for medical work, a new accelerator was developed. “Like Gantry 1 and 2, this proton accelerator also took on a pioneer role”, says David Meer. Thanks to superconducting magnets, which produce specially strong fields, the new accelerator commissioned in 2007 was now just three metres in diameter. By participating in the development of the system, PSI was taking a risk, but it turned out to be a risk worth taking, the expert asserts. The manufacturing firm has already managed to deliver three more accelerators, and is currently building several more.

The PSI was also pioneering in its selection of patients: Since 2004 the Center has not only treated young and adult patients from throughout Europe, but also infants who are afflicted with a tumour in the head or torso. While conventional x-rays pene-trate right through the body, the protons come to a stop after passing a certain distance through the body. The tissue behind them is subject to little or no radiation, and is therefore protected from damage. “This is extremely important particularly for children”, says David Meer. The growth of the tumour can, it is true, be blocked by conventional radiation therapy, but then it may also damage the healthy tissue. “Perhaps we may not irradiate better, but we do it more gently”, the expert concludes.

Children and adolescents make up about 50 %

Although there are not yet enough long-term data at hand, the specialists are confident that radiation therapy using protons enhances the quality of life of the children who receive the treatment. To make sure that the little patients lie quite still during the radiation therapy, they are given an anaesthetic. In charge of this are the anaesthetic team of the Zurich Children's Hospital. In the past ten years 220 children have already been treated in this way, most of them under seven years of age. Children and adolescents younger than 18 now form about half of the cancer patients treated at the PSI.

It is hoped that Gantry 2 will allow even more cancer patients to have the benefit of this modern technology in future. For the doctors hope to treat not only rare tumours in the head and spinal regions, but also lung cancer, for example. The difficulty here is that the tumour moves while the person is breathing. To precisely determine the position of the diseased tissue despite this movement, a computer tomograph has been installed in the treatment room next to Gantry 2, which can provide time-resolved images of the moving target region. Furthermore an x-ray system is installed in Gantry 2. “This is highly innovative”, says David Meer, “because it allows one to make x-ray images in the direction of the beam, and it provides information about the displacement of the tumour during the radiation therapy.”

The first treatment of moving tumours in Gantry 2 is to be carried out at the end of 2015. Additionally, another system, Gantry 3, is being constructed. This is being built in a research collaboration with the manufacturer Varian Medical Systems, and co-financed by the lottery fund of the Canton of Zurich, and is to be operated in collaboration with Zurich University Hospital starting in 2016.