Time-of-Flight positron emission tomography for proton therapy

• Applicant: Privatdozent Dr. Christian Bäumer
• Subject Area: Medical Physics, Biomedical Technology
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 525659135

The objective of the application is to improve the computed tomography (CT) images used for the dose calculation in proton therapy (PT). The project employs scans, which are acquired with time-of-flight PET (ToF-PET) devices. Recent advances in PET scanner hardware and reconstruction techniques provide so-called gamma-ray attenuation images (γCTs) from the positron emitters distributed within a patient. This is facilitated with the joint reconstruction of activity and attenuation images, especially with the Maximum Likelihood estimation of Attenuation and Activity (MLAA). A recent paper of the applicant demonstrated the potential of the γCTs from ToF-PET scans and MLAA reconstruction for proton therapy. In principle, this novel method can be applied to PET scans with radiotracer and with β+ activity, which is induced by therapeutic particle fields. The latter scenario means a paradigm change of PET-based in-vivo field verifications, which have been described in previous publications. The application utilizes state-of-the-art MLAA reconstruction software, which is itself not a research topic, to the following current research topics in proton therapy: improved proton stopping-power estimation for an independent dose calculation, metal-artefact mitigation, and adaptive PT. One clinical use case to be investigated concerns the improvement of the estimation of the stopping power in the virtual patient model. γCTs provide an alternative method. It can be combined with a second, independent dose calculation, which is under development as alternative to patient-specific quality assurance measurements. The stopping-power estimation has clearly been improved by dual-energy CT in the last few years, but is still a challenge in the presence of metals with high atomic number, e.g. implants, and their associated image artefacts. Owing to the highly penetrating photon energy of 511 keV, γCTs are expected to contribute to the artefact mitigation. Furthermore, we seek to explore how γCTs can be utilized for adaptive PT. For instance, γCTs could indicate bulk anatomical changes such as deformations from swellings and changes of the organ filling. In this context, the project evaluates the concept of triggered verification CTs by on-line imaging. This means that γCTs from proton-induced radioactivation would initiate x-ray imaging, if γCTs exhibit an indication for an anatomical change. Generally, γCTs pose a novel medical imaging method. Thus, their image quality focusing on spatial resolution will be evaluated together with the impact on calculations of the dose distribution. The proposed study has experimental tasks and tasks, which will be performed in-silico. In the experiments, PET scans of phantoms, which need to be designed and built, will be conducted.

• DFG Programme: Research Grants