Ion beams against cancer

In ion beam therapy, atomic nuclei are accelerated to high energies in a particle accelerator, for example a synchrotron, and precisely directed at a tumor. There, they damage cancer cells and destroy tumor tissue. This technique is now used at several specialized medical facilities around the world, including MedAustron in Wiener Neustadt, Austria.

A new and promising approach, which is currently being investigated, involves so-called “mixed beams”, which consist of two different ion species: Carbon nuclei destroy the tumor cells, while helium nuclei simultaneously help monitor the patient anatomy during irradiation. To make this possible, however, the precise ratio of carbon to helium in the beam must be known. But how can an ion beam be measured while it is being used for treatment? TU Wien and MedAustron have now found an elegant solution – an important step on the path toward cancer treatment using mixed ion beams.

Irradiating as little as possible

Despite the high precision of ion beam therapy, uncertainties remain. Organ motion, for example, can shift the location at which the beam deposits most of its energy – also called the “range” of the beam - by a few millimeters. To ensure that the entire tumor is treated despite these uncertainties, a small safety margin around the tumor is irradiated as well. The smaller this margin can be made, the more precise the therapy becomes and the better sensitive organs nearby can be spared.

To minimize side effects, it would be therefore ideal to be able to monitor the range of the beam as precisely as possible during treatment. This is where the concept of mixed beams comes in: Carbon and helium ions are combined in a single beam. All electrons are removed, leaving carbon nuclei with exactly three times the electric charge of helium nuclei and almost exactly three times their mass.

“Because their charge-to-mass ratios are almost identical, the two ion species also follow almost exactly the same trajectory in the particle accelerator, allowing us to accelerate them simultaneously to high energies and direct them toward the patient,” explains Elisabeth Renner.

In tissue, however, a crucial difference emerges: Helium ions can travel much farther through the body – approximately three times as far. A mixed beam can therefore be sent through the body in such a way that the carbon ions are used to destroy the tumor tissue, while the helium ions pass through the body.

The emerging helium ions can then be measured. From the residual energy of the helium ions one can then reconstruct how far the carbon ions have actually penetrated into the tissue and whether the irradiation is proceeding as planned. This could allow to reduce the safety margins and hence spare even more healthy tissue, particularly in the case of tumors located very close to sensitive organs.

“For this method to work, however, we need to know and control very precisely how large the carbon and helium fractions in the beam are,” says Elisabeth Renner. And that is a major challenge: Either the beam is measured, or it is used for irradiation. Doing both at the same time is difficult.

How to measure a beam without stopping it

The research team at TU Wien and MedAustron has now found an elegant solution. Strictly speaking, a carbon nucleus does not have exactly three times the mass of a helium nucleus, but about 0.065% less. This tiny difference means that carbon and helium are forced onto slightly different trajectories inside the particle accelerator.

“In the accelerator, we continuously monitor whether the center of the particle beam remains on its intended trajectory,” explains Elisabeth Renner. “If it deviates, the accelerator’s radio frequency is adjusted ever so slightly. Because helium and carbon nuclei behave slightly differently due to their tiny mass difference, the required frequency correction depends on the mixing ratio.”

By precisely monitoring the frequency needed to stabilize the beam, the carbon-to-helium ratio can therefore be determined.

This was calculated at TU Wien and tested in collaboration with MedAustron – and the results show excellent agreement. “We were able to demonstrate that this method allows us to determine the helium and carbon fractions in the beam without affecting or interrupting the beam itself. This means that during experiments, and possibly also during treatments in the future, we can check whether the helium fraction in the beam corresponds to the desired value. Only if that is the case should the beam be cleared for irradiation.”

The results have now been published and are already attracting international attention: On October 8 and 9, 2026, researchers from a range of international particle accelerator facilities came to TU Wien for the “1st Workshop on Mixed Ion Beam Delivery” to discuss current developments and challenges in the delivery and use of mixed ion beams.

Info box: An atomic nucleus is not a bullet

When an atomic nucleus races through biological tissue, it should not be pictured like a bullet passing through a wooden board. In the case of a bullet, the faster it travels, the greater its penetrating power. Kinetic energy is transferred to the material all along its path.

“When ions pass through a material, the situation is somewhat more complicated,” explains Elisabeth Renner from TU Wien’s Atominstitut. “How strongly an ion interacts with the material depends on its energy. A fast ion can pass through tissue relatively undisturbed. As it slows down, however, its interaction with the surrounding material becomes stronger, until it finally deposits a great deal of energy over a relatively short distance.”

It is precisely this property that makes ion beams such an effective tool for cancer treatment: If their energy is chosen correctly, they pass through healthy tissue while causing comparatively little damage and losing only a small amount of energy. Only once they reach the tumor do they enter the region of maximum interaction with the tissue, depositing most of their energy there. This selectively damages and destroys the tumor cells, while sparing healthy tissue