Radiation quality assessment in a clinical helium-ion beam using a SOI microdosimeter and a Timepix3 detector

Yasmin Hamad^1*Sandra Barna²Giulio Magrin³Hugo Palmans³Ferisya Kusuma Sari⁴Tim Gehrke⁴Andrea Mairani⁵

• ¹German Cancer Research Consortium (DKTK), Partner Site Tübingen, German Cancer Research Center (DKFZ),, Heidelberg, Germany
• ²Medizinische Universitat Wien, Vienna, Austria
• ³MedAustron, Wiener Neustadt, Austria
• ⁴Deutsches Krebsforschungszentrum, Heidelberg, Germany
• ⁵UniversitatsKlinikum Heidelberg Heidelberger Ionenstrahl-Therapiezentrum, Heidelberg, Germany

Accurate characterization of radiation quality is essential for assessing radiobiological effects in radiotherapy, yet remains a major challenge. As modern treatment strategies increasingly consider not only absorbed dose but also linear energy transfer (LET) as a radiation quality specifier, the need for reliable tools to measure radiation quality in clinical settings is growing. While physical dose is routinely measured during quality assurance procedures, the experimental assessment of radiation quality is still limited. Microdosimetry offers a promising approach to address this gap. In this study, radiation quality is assessed through a microdosimetric approach using two advanced active solid-state detectors with distinct geometries: a 3D-mushroom Silicon-On-Insulator (SOI) microdosimeter developed at the University of Wollongong and a hybrid silicon-pixel Timepix3 detector. Although both being silicon-based, their differing geometries, sensor thicknesses, and detection principles lead to notable variations in their energy deposition spectra. The response functions of the two detection systems exposed to an initially monoenergetic 149.02 MeV/u helium pristine peak are compared in terms of spectral distributions and their expectation values. Experimental data are complemented with Monte Carlo simulations performed using the FLUKA code to validate and interpret the measurements. The advantages and limitations of both detection systems are discussed in the context of efforts to standardize radiation quality measurements. Such standardization could facilitate the integration of LET-based dosimetry into treatment planning systems, thereby improving the precision of radiobiological damage assessments. Ultimately, accounting for detector-specific response differences is crucial for establishing protocols for experimental verification of radiation quality in radiotherapy, as no standard device or accredited experimental methodology has been unequivocally identified yet.