New Beam Measurement Approach Could Support More Precise Ion Therapy

However, organ motion can shift where the beam deposits most of its energy by a few millimeters. Clinicians therefore irradiate a small safety margin around the tumor, which can expose nearby sensitive organs. Researchers have now developed a noninvasive method to measure mixed ion beams during irradiation.

TU Wien and MedAustron (Wiener Neustadt, Austria) have identified a way to determine the ratio of carbon and helium ions in a mixed beam without stopping or disturbing it. Such beams combine carbon nuclei, which destroy tumor tissue, with helium nuclei that help monitor patient anatomy during treatment. To clear the beam for irradiation, however, its carbon-to-helium ratio must first be known precisely.

The two ion species can be accelerated together because their charge-to-mass ratios are nearly identical after all electrons have been removed. This similarity allows them to follow almost the same trajectory in a synchrotron. Their behavior differs once they enter tissue: helium ions travel approximately three times farther than carbon ions and can emerge from the body for measurement.

Measuring the residual energy of these emerging helium ions allows the penetration depth of the carbon ions to be reconstructed, helping determine whether irradiation is proceeding as planned. This monitoring capability could also support smaller safety margins around tumors near sensitive organs, provided that the beam composition matches the desired ratio.

To determine that composition, the new measurement method exploits a small physical difference between the otherwise closely matched ion species. A carbon nucleus has about 0.065% less than three times the mass of a helium nucleus, causing their trajectories to differ slightly. As a result, the radio frequency correction needed to stabilize the beam depends on the carbon-to-helium ratio, providing a way to measure its composition without interrupting irradiation.

The method was calculated at TU Wien and tested with MedAustron, with the results showing excellent agreement. The work was published in Physical Review Accelerators and Beams on September 25, 2026. Researchers from international particle accelerator facilities also met at TU Wien on October 8 and 9, 2026, for the 1st Workshop on Mixed Ion Beam Delivery.

“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,” said Elisabeth Renner of TU Wien.

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