Carbon and Helium Ion Beams Team Up to Track Tumors in Real Time

Particle accelerators are no longer the exclusive domain of high-energy physics. In a growing number of specialized clinics around the world, atomic nuclei are accelerated to enormous energies and steered with millimeter precision toward deep-seated tumors, where they destroy cancer cells while sparing the surrounding healthy tissue. This technique, known as ion beam therapy, is already in clinical operation at facilities including MedAustron in Wiener Neustadt, Austria. Now, a collaboration between TU Wien and MedAustron has achieved a milestone that could make this already-precise form of radiotherapy even sharper: a way to measure the composition of a mixed ion beam while that very beam is being delivered to a patient.

The advance centers on a concept that has been generating excitement in the particle therapy community: so-called mixed beams. Instead of accelerating a single ion species, the idea is to combine two different nuclei in one beam. Carbon nuclei do the destructive work, shattering the DNA of tumor cells at the target site. Helium nuclei, meanwhile, act as messengers, passing through the body and carrying information about what lies along the beam path. The helium ions effectively serve as continuous probes of the patient’s anatomy during irradiation, allowing clinicians to verify that the treatment is proceeding exactly as planned.

The physics behind this dual role is elegant. In the accelerator, every ion is stripped of its electrons, leaving bare nuclei. A carbon nucleus carries exactly three times the electric charge of a helium nucleus and almost exactly three times its mass. Because their charge-to-mass ratios are nearly identical, the two species follow almost exactly the same trajectory through the synchrotron, meaning they can be accelerated simultaneously to high energies and directed toward the patient together. Once inside the body, however, a crucial difference emerges: helium ions can travel roughly three times farther through tissue than carbon ions of the same energy per nucleon.

This difference is what makes the mixed beam so powerful. The beam can be tuned so that the carbon ions come to rest inside the tumor, depositing their destructive energy precisely at the target, while the lighter helium ions punch through the body and emerge on the far side. By measuring the residual energy of these exiting helium ions, physicists can reconstruct how much material the beam traversed and therefore how deeply the carbon ions actually penetrated. If the carbon range deviates from the plan, clinicians can know immediately rather than discovering a problem after the treatment is complete.

The stakes are considerable. Even with the high precision of ion beam therapy, uncertainties remain. Organ motion, for example, can shift the location where the beam deposits most of its energy, known as the beam’s range, by a few millimeters. To guarantee that the entire tumor receives a lethal dose despite these uncertainties, a small safety margin of healthy tissue around the tumor is irradiated as well. The smaller this margin can be made, the more precise the therapy becomes and the better nearby sensitive organs, such as the spinal cord or the brainstem, can be protected. Real-time range verification through helium monitoring could shrink those margins substantially, which is particularly valuable for tumors located very close to critical structures.

There was, however, a fundamental obstacle standing in the way. For the mixed beam concept to work, the ratio of carbon to helium in the beam must be known and controlled with great precision. Yet measuring a beam normally requires intercepting it, and intercepting it means it cannot simultaneously be used for treatment. Either the beam is measured, or it is used for irradiation. Doing both at the same time seemed, until now, to be a nearly impossible demand.

The TU Wien and MedAustron team found an ingenious way around this dilemma, and it hinges on a subtlety of nuclear mass that most textbooks gloss over. A carbon nucleus does not have exactly three times the mass of a helium nucleus; it is in fact about 0.065 percent lighter than that ratio would suggest. This tiny discrepancy means that carbon and helium nuclei are forced onto slightly different trajectories inside the particle accelerator. The difference is far too small to separate the beams for practical purposes, but it is large enough to leave a measurable fingerprint on the accelerator’s operation.

That fingerprint appears in the radio frequency system that keeps the beam on track. In the synchrotron, operators continuously monitor whether the center of the particle beam remains on its intended trajectory. If it deviates, the accelerator’s radio frequency is adjusted ever so slightly to pull it back. Because helium and carbon nuclei respond slightly differently to these corrections, owing to their tiny mass difference, the frequency correction required to stabilize the beam depends directly on the mixing ratio of the two species. By precisely tracking the frequency needed to hold the beam steady, the carbon-to-helium ratio can be determined continuously, without ever touching the beam itself.

The concept was calculated and developed at TU Wien and then tested in collaboration with MedAustron, with results showing excellent agreement between theory and experiment. The team demonstrated that the method allows the helium and carbon fractions in the beam to be determined without affecting or interrupting the beam. This means that during experiments, and possibly during actual treatments in the future, clinicians can verify that the helium fraction matches the desired value before the beam is cleared for irradiation. The work has been published in the journal Physical Review Accelerators and Beams and is already drawing international attention. On October 8 and 9, 2026, researchers from a range of particle accelerator facilities gathered at TU Wien for the first Workshop on Mixed Ion Beam Delivery to discuss the current state of the field and the challenges that remain.

Underlying all of this is a property of ion physics that makes these beams such effective cancer weapons in the first place. An atomic nucleus racing through biological tissue should not be pictured like a bullet passing through a wooden board, where kinetic energy is shed steadily along the path. How strongly an ion interacts with material depends on its energy. A fast ion can pass through tissue relatively undisturbed, losing only a small amount of energy. As it slows down, however, its interaction with the surrounding material grows stronger and stronger, until it finally deposits a large fraction of its energy over a very short distance, a phenomenon known as the Bragg peak. If the ion’s energy is chosen correctly, the beam passes through healthy tissue causing comparatively little damage, and only at the tumor does it enter the region of maximum energy deposition, selectively destroying cancer cells while sparing everything in front of and behind the target. By adding helium-based range verification to this already-precise delivery mechanism, the TU Wien and MedAustron collaboration has taken a significant step toward ion beam therapy that not only hits its target but continuously confirms, in real time, that it is doing so.

Subject of Research: Noninvasive quantification of mixed carbon-helium ion beams in synchrotrons for precision cancer radiotherapy

Article Title: Ion beams against cancer

Article References: Ion beams against cancer. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: ion beam therapy, carbon ions, helium ions, mixed beams, synchrotron, cancer treatment, radiotherapy, beam range verification, TU Wien, MedAustron, particle accelerator, Bragg peak