The research area Medical Radiation Physics is currently mainly active in two research projects at the MedAustron Research and Therapy Centre:

• Tomography by means of ions
• Microdosimetry

At MedAustron, protons can be accelerated to an energy of 800 MeV and carbon nuclei to up to 400 MeV/nucleon in a synchrotron. These particle beams are accessible in an experimental room dedicated to research. Since December 2016, regular clinical operation has been taking place at MedAustron, initially with protons at energies up to a maximum of 250 MeV, and since 2019 also with carbon ions.

Ion computed tomography

The radiation planning necessary for ion beam therapy is based on the exact knowledge of the tissue and its composition which the ion has to cross on its way to the tumour and is slowed down or loses energy in the process. Currently, the stopping power required for particle therapy planning is determined from a computed tomography using X-ray radiation. This information is sufficient for conventional radiotherapy, which is also based on photons. However, the ions used in particle therapy follow completely different physical laws. An estimate of the stopping power or the energy transfer for charged particles based on photons leads to errors in the range of the ions in the tissue, which must be compensated for by larger safety margins in the radiation planning. This ultimately leads to an undesired higher radiation dose in healthy tissue in the vicinity of the irradiated tumour.

The stopping power mentioned can also be estimated directly with the help of the ion beam (for example protons, carbon ions or helium ions). For this purpose, the beam is directed through the body and its position (tracking) and energy (calometry) are measured as it exits the body, as shown in the figure below / right.

Ions with a defined energy (E) pass through an object and lose part of their energy (ΔE) there. In a special detector, these are stopped and their residual energy (E-ΔE) is measured.

The measured position and energy of the ion beam is used to reconstruct a three-dimensional image of the energy transfer of the proton beam in the body with the help of suitable algorithms. First measurements with a tomography system have already been performed at MedAustron and a three-dimensional image of a small cube with steps (step phantom) could be reconstructed from the measured data. The experimental setup at MedAustron and the reconstructed image are shown in the figures below.

Microdosimetry

Proton and ion beam therapy has established itself as a method of cancer treatment in more and more facilities worldwide and has also been available in Austria at MedAustron since 2016.

Determining the dose to the irradiated tissue in ion beam therapy is an essential part of any treatment. Currently, the dose delivered during the irradiation process is usually expressed as deposited energy per unit mass. This is a macroscopic quantity that does not provide information about the interactions at the level of cellular structures, where energy deposition is heterogeneous, such as the particle tracks of ion beams. Therefore, research is being conducted at MedAustron to characterise energy deposition at the microscopic level using microdosimeters. These are radiation detectors in which the dimensions of the sensitive volume are of the order of a few micrometres to simulate cell nuclei. Silicon and diamond microdosimeters as well as gas-filled tissue-equivalent proportional counters are regularly used and are being studied in an ongoing joint research project at MedAustron. In a joint project of the Medical Radiation Physics group, HEPHY and MedAustron, these studies are now to be extended to include detectors based on silicon carbide.