Technical status of the real-time beam tracking system

For intra-fractionally moving tumours such as lung cancer, treatments with scanned particles [1] without motion compensation can cause unacceptable local underor over-dosage due to interplay effects of sequential dose delivery and target motion [2]. In order to treat moving tumours with scanned particle beams, a beam tracking system, 3DOMC system, was developed [3] and upgraded [4,5]. The beam tracking system employs the raster scanner for lateral beam tracking and a range shifter to adapt the Bragg peak position longitudinally. The upgraded system components and the achieved speed were reported previously [5]. In 2008 experiments to assess the system accuracy, an interlock feature, an implementation to overcome delay times, and real-time dose compensation [6] have been performed. Here we report studies related to the accuracy of the system in detail.