Particle physics research

Research team reaches milestone in proton irradiation

Fast proton irradiation is a more effective and less invasive cancer treatment than X-rays. However, modern proton therapy requires large particle accelerators, for which experts are investigating alternative accelerator concepts, such as laser systems for accelerate protons. Such systems are being deployed in preclinical studies to pave the way for optimal radiotherapy. A research team led by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has now successfully tested irradiation with laser protons on animals for the first time, as the group reports in the journal Natural Physics.

Radiation therapy is one of the main methods of treating cancer. It typically harnesses strong, focused X-ray light. Protons – the nuclei of hydrogen atoms – accelerated to high energies and bundled into small, precisely targetable bunches are an alternative. They can penetrate deep into tissue where they deposit most of their energy into the tumor, destroying the cancer while leaving surrounding tissue largely intact. This makes the method both more effective and less invasive than radiotherapy. “The method is particularly suitable for irradiating tumors at the base of the skull, in the brain and in the central nervous system”, explains Dr. Elke Beyreuther, researcher at HZDR. “It is also used in pediatric cancer patients to reduce possible long-term effects.”

However, the method is significantly more complex than X-ray therapy as it requires elaborate accelerator facilities to generate the fast protons and transport them to the patient. That is why there are only a few proton therapy centers in Germany, including one at the University Hospital Dresden. Currently, experts are working to regularly improve the method and adapt it to patients. Laser-based proton accelerators could make a decisive contribution here.

Custom laser flashes

“The approach is based on a high-powered laser to generate strong, extremely short pulses of light, which are fired at a thin sheet of plastic or metal,” explains HZDR physicist Dr Florian Kroll. The intensity of these flashes knocks bands of electrons out of the sheet, creating a powerful electric field that can bundle protons into pulses and accelerate them to high energies. Surprisingly, the scale of this process is tiny: the acceleration trajectory is only a few micrometers long.

“We’ve been working on the project for 15 years, but so far the protons haven’t captured enough energy for irradiation,” Beyreuther reports. “Also, the pulse intensity was too variable, so we couldn’t make sure we were delivering the right dose.” But in recent years, scientists have finally achieved crucial improvements, in particular thanks to a better understanding of the interaction between laser flashes and the sheet. “Above all, the precise shape of the laser flashes is particularly important,” says Kroll. “We can now tailor them to create proton pulses that have enough energy and are also stable enough.”

New research requirements

Finally, the parameters had been optimized to the point that the HZDR team was able to launch a series of crucial experiments: the very first controlled irradiation of tumors in mice with laser-accelerated protons. The experiments were carried out in cooperation with experts from the University Hospital Dresden at the OncoRay – National Center for Radiation Research in Oncology and compared with comparative experiments in the conventional proton therapy facility. “We found that our laser-driven proton source can generate biologically valuable data,” Kroll reports. “This paves the way for further studies that will allow us to test and optimize our method.”

Another special feature of laser-accelerated proton pulses is their enormous intensity. While in conventional proton therapy the radiation dose is delivered within minutes, the laser-based process could happen in a millionth of a second. “There are indications that such rapid administration of the dose helps to spare healthy surrounding tissue even better than before,” says Elke Beyreuther. “We want to follow these indications with our experimental setup and conduct preclinical studies to determine when and how this rapid irradiation method should be used to gain advantage in the treatment of cancer.”

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