Radiation therapy apparatus

A radiation therapy apparatus includes a gantry having a central aperture that defines a cavity, in which a patient is positionable. The apparatus also includes a positioning device, with which the patient is positionable in the central aperture of the gantry, and a radiation head for generating therapeutic radiation. The apparatus includes an extension mechanism for the radiation head. The extension mechanism may be operated, such that in a first operating mode, the radiation head is disposable inside the gantry, such that the gantry has, on a side facing the positioning device, a front surface that constitutes a front limit of the radiation therapy apparatus toward the positioning device. In the first operating mode, the radiation head is behind the front surface viewed from the positioning device. In a second operating mode, the radiation head, viewed from the positioning device, may be moved out in front of the front surface.