Multi-burst imaging: markerless tracking using x-ray imaging was developed for clinical use in the carbon-ion PBS gantry room of NIRS (Chiba, Japan). All imaging components were attached on the rotating gantry, and a tracking software was calculating the position of the tumor in real time. To minimize the time lag between respiratory motion and image acquisition, the time interval between exposure to x-rays and the readout process was kept at a minimum (6.5 ms before the internal clock every two steps in addition to the 4ms of x-ray exposure time). The tracking precision was better than 0.5 mm on a moving respiration-simulating chest phantom.