Out-of-field dose measurements in radiotherapy for ALLEGRO

These experiments compare the out-of-field dose delivered during different treatment techniques (photons, active and passive delivered protons and carbon ions) in standardized conditions. A water phantom is used as tissueequivalent material and is irradiated with a rectangular field. The dose is measured in lateral direction in the plane of the field as shown in Fig. 1. The composition of the out-of-field radiation changes with the treatment technique and thus also the biological effect. In photon therapy the out-of-field dose consists of photons and photoneutrons, whereas the main contributors to the out-of-field dose in ion therapy are nuclear fragments. Moreover not only the particle species but also the particle energy changes. Therefore it is essential to identify the particle and its energy contributing to the physical dose to determine the biological effect. To achieve this goal different detectors must be used. The measurements in photon therapy are done with diamond detectors, ion chambers and bubble detectors. The diamond detector and ion chamber mainly measure the photon dose while the bubble detector is a passive detector specialized for neutron dosimetry. The measurements of the photon contribution to the outof-field dose in photon therapy are complete. However, only preliminary experiments have been performed with the bubble detectors. First they were exposed to an AmBe source for calibration and for testing the stability of the detector response. The tests with the bubble detectors were successful so that the neutron measurements in photon therapy can soon be completed. All irradiations were done at several institutes under patient conditions. The waterphantom was irradiated with a scanned carbon ion field at GSI. Photon irradiations were performed with a linac generating bremsstrahlung at the Klinikum Goethe Universitaet (KGU) in Frankfurt. The scanned proton irradation will be done at the Paul-ScherrerInstitut in Switzerland, also under patient conditions. In the medical treatment room Cave M at GSI the waterphantom was irradiated with a 5×5 cm field of 250 MeV/u C ions. To measure the out-of-field dose a diamond detector was used. ∗ [email protected] The results of this measurement are compared to the outof-field dose measured with a diamond detector during a photon irradation in Fig. 2. The dose is measured in the depth of maximum dose in the plane of the field and normalized to the isocentre. The out-of-field dose of carbon is significantly lower than for photons. This difference is assumed to increase when neutrons are also considered; the dose coming from neutrons will be analyzed in future experiments. Future measurements will also give results from treatments with both active and passive proton beams.