In heavy ion radiation therapy for cancer, tracking the region of interest (ROI) is important in order to monitor the position of the treatment target, which varies with respect to respiration, during therapy. We previously proposed a method to compute the optical flow, which is a 2-D velocity field, and the locus of the ROI in the internal organs using the optical flow and its reliability. In the present work, this locus is assumed to be prior knowledge and an on-line tracking method of the ROI is proposed. In this method, the ROI position is predicted using the estimated locus, and the template matching technique usingthe respiratory information is applied. The locus is then updated using the template matching result obtained via the Kalman filter. The effectiveness and reliability of this method are confirmed through real image experiments
