Radiation therapy system and method of operating radiation therapy apparatus

A radiation therapy system, comprising:

• a radiation source;
• a rotation mechanism that supports the radiation source and rotates the radiation source around an isocenter;
• a couch that places a therapy target site of a patient at the isocenter;
• a head swing mechanism that is disposed between the radiation source and the rotation mechanism and that swings an irradiation axis of a radiation emitted from the radiation source by swinging the radiation source;
• and
• a controller that controls the radiation source, the rotation mechanism, and the head swing mechanism, wherein
• the controller holds the head swing mechanism in a state where the irradiation axis of the radiation of the radiation source is shifted from the isocenter in a predetermined direction by a predetermined amount, and rotates the radiation source by the rotation mechanism while emitting the radiation from the radiation source while maintaining the state of the head swing mechanism,
• a direction in which the controller shifts the irradiation axis from the isocenter includes a component in a rotation axis direction of the rotation mechanism, and
• the controller shifts the irradiation axis from the isocenter in a positive direction in the rotation axis direction, rotates the radiation source by the rotation mechanism in this state, and then shifts the irradiation axis from the isocenter in a negative direction in the rotation axis direction, and rotates the radiation source by the rotation mechanism in this state.