For the dosimetry of (heavy) charged particles, three issues are of importance in the context of radiotherapy: first, the exact knowledge of the dose distribution caused by the possible high dose gradients and higher dose conformity, second, the biological effect summarized in the relative biological effectiveness (RBE), and furthermore, the distribution of the linear energy transfer (LET). These three points also have influence on the dose planning and the dosimetry for ions in a clinical setting.
The LET and the local energy deposition on a microscopic scale depend on the structure of the ionization density around the track of the charged particle in the medium which is determined by the particle species and energy. It is the microscopic energy distribution which is important for the characterisation of both the biological effects and the detector response.
We work with alanine and lithium formate detectors in collaboration with National Physical Laboratory (NPL), Teddington, UK and the "Radiumhospitalet" in Oslo, Norway.
Together with the University Hospital in Umeå, Sweden and the German Cancer Research Center in Heidelberg (DKFZ), we are investigating recombination effects in liquid ionization chambers. Ion beams show a strong variation of the LET as a function of their energy (or depth), and therefore not only detectors with a high spatial resolution are needed, but also a detector which with a LET response which at least on a phenomenological level can be understood and modelled. The use of a liquid as medium in ionisation chambers allows for small sensitive volumes due to its high density compared to air-filled chamber, providing therefore a higher signal. Furthermore, the response of the liquid is LET dependent and allows a direct LET determination based on the voltage dependence of the response.
We perform research on fundamental dosimetric questions with the intention to improve and further enhance dosimetry protocols used for clinical particle therapy. This includes contributions on stopping power ratios, definition of the beam quality for heavy ions and the water equivalence of materials. Among others, we collaborate intensively on these issues with NPL and DKFZ. Our activity already had impact on clinical dosimetry protocols and currently we are actively contributing to the newly arising German DIN norm for clinical ion dosimetry.
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