Mixed ion beams for treatment monitoring
 

A recent proposal for online range verification in ion beam therapy is the irradiation of patients with mixed helium (4He2+) and carbon ion (12C6+) beams. In this approach, both ion species are accelerated simultaneously and extracted to the patient. Because helium ions have an approximately three times greater range in matter than carbon ions at the same extraction energy per nucleon, the carbon beam can be used for treatment, while the helium beam is available for range verification and ion imaging in a detector placed behind the patient. 

Together with MedAustron, we are developing new methods at TU Wien for the delivery and application of such mixed ion beams. The generation of mixed helium and carbon ion beams in existing clinical synchrotron facilities without major technical upgrades is not straightforward and required, among other things, the development of a new injection method in which helium and carbon ion beams are injected separately into the synchrotron to be mixed during this process. The acceleration in the synchrotron also presents particular challenges, since even the slight relative difference of 0.065% in the mass-to-charge ratio of the two ion species results in measurable effects. During resonant slow extraction, the different 6D phase space distributions of the two ion species require the development of suitable excitation methods to maintain an approximately constant helium-to-carbon ratio throughout the entire extraction. Another focus of the project is the development of suitable beam diagnostics, both in the accelerator and the experimental room, to enable precise characterization of the properties of the mixed beam.

The first major milestone of our project was reached in summer 2024, when a mixed ion beam was extracted into the MedAustron research room for the first time. Today, the helium fraction in the beam can be adjusted flexibly and reproducibly between 0 and 100%, while the beam can be extracted with an approximately constant He:C ratio over the most relevant mixing ratios. Since late 2025, we have been using the mixed beam in initial range verification experiments. In this project, we work closely with our group’s ion imaging team, the medical physics teams at MedAustron and the Medical University of Vienna, as well as with colleagues at the Marietta Blau Institute, CNAO, and GSI.

Resonant slow extraction 

Resonant slow extraction is a beam manipulation technique which allows to extract the charged particle beam from a synchrotron in a controlled way over millions of turns, corresponding to some seconds, with quasi-constant intensity. This allows, for example, fixed-target experiments in high-energy physics or patients in ion beam therapy to be continuously irradiated over several seconds. In ion beam therapy, this  also enables the small pencil beam to be scanned transversely across the tumour cross-section. 

The choice of extraction technique has a major impact on the resulting beam quality and stability and is therefore of central importance in both high-energy physics and ion beam therapy. In addition to established methods, alternative beam excitation techniques are therefore being investigated by an active international research community in order to further improve beam quality and stability and to enable new beam properties, and thus new applications.

A current research focus of our team in this area is multi-energy extraction. This method allows the beam energy to be varied in a controlled manner during an extraction lasting several seconds. As a result, multiple tumor depths can be irradiated within a single extraction, significantly reducing both treatment time and energy consumption per patient. In addition, we are also working on the continuous  and pulsed extraction of so-called ultra-high dose rate (UHDR) beams, that is, beams with particularly high extraction rates. Such beams provide the foundation for FLASH radiotherapy, an innovative new approach in ion beam therapy.

Feasibility and design studies for novel ion beam modalities and research infrastructure

Increasing demands from scientific users regarding the properties of ion beams are an important driver of our work in accelerator physics. Through feasibility and design studies, we investigate how new requirements, particularly from other research groups at MedAustron, can be met using existing facilities and infrastructure, and, where necessary, develop concepts for targeted extensions of the research infrastructure. This includes studies on the delivery of new ion species such as oxygen ions, as well as the design of beamlines for new experimental stations in the low- and high-energy range.