PURPOSE: To enable a lesion position in a patient's body to be precisely grasped, by generating at least an ultrasonic image at the lesion position of a patient, indicating a location where an image generating means is used, and comparing the lesion position in the image with that where a radiotherapy is planned. CONSTITUTION: This radioactive treatment apparatus of a linear accelerator 401 comprises a gantry 402, a turning table 403, and a collimator 405 which adjusts the shape of a radioactive beam. On a treatment table 404, a supersonic image generating means 420 comprising an ultrasonic probe 422 is placed, and this generates the two-dimensional ultrasonic image of a body part including the lesion position 303 of a patient 302 on the treatment table 404. Moreover, a multiplicity of supersonic images are generated by turning the ultrasonic probe 422. Each of the images draws the outline of the outer surface of the lesion position. By comparing an outline of at least one outer surface of the lesion position on the diagnostic image to such outline on the ultrasonic image, it is decided whether the location of the lesion position in the radiotherapy plan is properly adjusted or not.
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