Radiotherapy device and radiotherapy method

A radiotherapy method in which a therapeutic radiation is irradiated onto an irradiation target, the method comprising:

• during therapy planning, producing a plurality of three-dimensional CT images of the irradiation target including a landmark present in the irradiation target, taken with elapse of time, analyzing a relationship between a displacement from a position of the landmark in a reference image, which is one three-dimensional CT image among the plurality of three-dimensional CT image which are obtained, to a position of the landmark in another three-dimensional CT image, and deformation of each site in the other CT image, and storing the relationship which is obtained;
• and
• during therapy in which a radiation is irradiated onto the irradiation target, detecting the position of the landmark, estimating deformation of each site by referring to the relationship and according to the detected position of the landmark, and obtaining a three-dimensional CT image.