Monitoring of ion beam energy by tracking of secondary ions: First measurements in a patient-like phantom

Due to the finite range of ions in matter and the presence of the Bragg-peak, ion beams provide highly localized dose distributions. In radiation therapy with ion beams, unpredictable changes within the patient can deteriorate the quality of the dose distribution in the target. Therefore it is desired to monitor the beam within the patient in a non-invasive way. In this contribution the information carried by secondary ions, which are emerging from a patient during carbon ion beam irradiation, is studied. In a framework of investigations on the feasibility of beam monitoring exploiting secondary ions, the aim of this study was to show the possibility of distinction of two different primary carbon ion beam energies solely from the analysis of the secondary ion directions measured. The situation was complicated by realistic tissue inhomogeneities simulated by using a patient-like phantom. Experiments were performed at the Heidelberg Ion Beam Therapy Center in Germany using narrow carbon ion beams. The emerging secondary ions were registered by the Timepix detector. To determine the direction of the particles, a multi-layered detector (3D voxel detector) was employed. Clear differences between the application of the two beams with different energies (213 and 250 MeV!u) could be observed in the distributions of the measured secondary ions, despite the inherent tissue inhomogeneities. This result was achieved in both brain (homogeneous) and skull base regions (containing inhomogeneities). Differences between the energies could be observed with the detector placed on different positions around the head. The performed first experiments towards 12 C beam monitoring in a patient-like geometry exploiting tracking of secondary ions with a 3D voxel detector show, that the information about the therapeutic carbon ion beam which is carried by secondary ions is promising and worthwhile to be investigated further.