Absorbed dose to water measurements in the SOBP of a clinical carbon-ion beam using water calorimetry

Up til now, the dosimetry in terms of absorbed dose to water in carbon-ion beams has not reached the same level of accuracy as in high-energy photon beams. This is strongly influenced by the standard uncertainty of the beam quality correction factor kQ. Due to a lack of experimental data, kQ is mainly based on theoretical calculations showing a three times higher uncertainty than in photon beams. In order to reduce this standard uncertainty, experimental kQ factors have been determined for the entrance channel of a monoenergetic carbon-ion beam. Within this thesis, kQ factors are now presented for the spread-out Bragg peak (SOBP) of a carbon-ion beam for the first time. Therefore, the absorbed dose to water in a previously well-characterized irradiation field was determined by means of water calorimetry. To be able to perform these measurements in scanned carbon-ion beams, the SOBP was passively modulated using a static 2D range modulator. Based on the water calorimetric measurements, ionization chambers could be calibrated directly, determining chamber-specific kQ factors with a relative standard uncertainty of 0.7 %. This significant reduction in uncertainty compared to calculated values gives the chance to drastically decrease the overall uncertainty in IC-based dosimetry for clinical carbon-ion beams