2D range modulator for high-precision water calorimetry in scanned carbon-ion beams.

Ionization chamber-based dosimetry for carbon-ion beams still shows a significantly higher standard uncertainty than high-energy photon dosimetry. This is mainly caused by the high standard uncertainty of the correction factor for beam quality k_Q,Q0 . Due to a lack of experimental data, the given values for k_Q,Q0 are based on theoretical calculations. To reduce this standard uncertainty, k_Q,Q0 factors for different irradiation conditions and ICs are determined experimentally by means of water calorimetry. In this article, we describe the process of creating a suitable irradiation technique and the characterization of the dose distribution for water calorimetry in a spread-out Bragg peak (SOBP) for scanned carbon-ion beams using a 2D range modulator. Here, the aim is to achieve a field homogeneity with a standard deviation of the measured dose values of below 1 % and a scanning time of under two minutes for a reasonable field size and applied dose.