[en] This study aims to develop a method for supporting radiotherapy planning for dynamic tumor-tracking irradiation for lung tumors. We propose a finite element based computation model that allows estimating displacement of lung tumor with respiration by using dynamic boundary condition on neighboring anatomical structures. We also propose texture-based Digitally Reconstructed Radiograph (DRR) generation methods for fast visualization of simulation results. Several experiments and trials of simulation established that the proposed methods can estimate the movement of the lung tumor with adequate precision. The developed system is expected to be useful for radiotherapy planning for real-time tumor-tracking irradiation. (author)