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This study proposes methods for the support of radiotherapy planning for dynamic tumor-tracking irradiation for lung tumors. It aims to simulate the deformation of the lung caused by respiration and to visualize the result as DRRs (Digitally Reconstructed Radiographs) in real time. Our lung- deformation model treats the lung as an elastic object and analyzes the deformation based on linear FEM (the Finite Element Method). The simulation models the lung using CT volume data and generates a model with boundary conditions with freely adjustable regions, displacements, and phases. The doctor planning the radiotherapy can reproduce the movement of the lung tumor by freely adjusting the regions, displacements, and phases of the boundary conditions while comparing the position of the lung tumor in an X-ray photograph. For highspeed display we propose a method for rapid-generation DRRs by slice-based volume rendering. The result of several functional evaluations and trials of simulation established that the proposed method can describe the movement of the lung tumor with adequate precision. The developed system is expected to be useful for radiotherapy planning for real-time tumor-pursuing irradiation.