Patent Number: 
Section: claims

1. A radiation therapy apparatus comprising:a gantry comprising a central aperture that defines a cavity, in which a patient is positionable for irradiation;a positioning device, with which the patient is positionable in the central aperture of the gantry;a radiation head for generating therapeutic radiation that is directable at the patient; andan extension mechanism for the radiation head,wherein the extension mechanism is configured such that, in a first operating mode of the radiation therapy apparatus, the radiation head is disposed inside the gantry such that the gantry has, on a side facing the positioning device, a front surface that is a front limit of the radiation therapy apparatus toward the positioning device, and that in the first operating mode, the radiation head is behind the front surface viewed from the positioning device, andwherein the extension mechanism is configured such that, in a second operating mode of the radiation therapy apparatus, the radiation head, viewed from the positioning device, is movable to be in front of the front surface that in the first operating state is the front limit. 2. The radiation therapy apparatus as claimed in claim 1, wherein the gantry has, on a side facing away from the positioning device, a back surface that, in the first operating mode, is a rear limit of the gantry, andwherein the extension mechanism is configured, such that in a third operating mode of the radiation therapy apparatus, the radiation head is movable, using the extension mechanism, to be behind the back surface viewed from the positioning device. 3. The radiation therapy apparatus as claimed in claim 2, wherein the gantry is tiltable about a horizontal axis. 4. The radiation therapy apparatus as claimed in claim 2, wherein the central aperture of the gantry has a longitudinal axis, andwherein the extension mechanism enables a translatory movement of the radiation head along a direction having a component along the longitudinal axis. 5. The radiation therapy apparatus as claimed in claim 2, wherein the extension mechanism is extendable out along an isocentric circular segment. 6. The radiation therapy apparatus as claimed in claim 1, wherein the gantry is tiltable about a horizontal axis. 7. The radiation therapy apparatus as claimed in claim 6, wherein the central aperture of the gantry has a longitudinal axis, andwherein the extension mechanism enables a translatory movement of the radiation head along a direction having a component along the longitudinal axis. 8. The radiation therapy apparatus as claimed in claim 1, wherein the central aperture of the gantry has a longitudinal axis, andwherein the extension mechanism enables a translatory movement of the radiation head along a direction having a component along the longitudinal axis. 9. The radiation therapy apparatus as claimed in claim 8, wherein the extension mechanism is configured such that, in addition to the translatory movement, a tilting movement of the radiation head that tilts the radiation head relative to the extension mechanism is executable. 10. The radiation therapy apparatus as claimed in claim 8, wherein the extension mechanism provides only a translatory movement of the radiation head in the direction of the longitudinal axis. 11. The radiation therapy apparatus as claimed in claim 10, wherein the extension mechanism is configured such that, in addition to the translatory movement, another movement of the radiation head is executable, the other movement having a component in a direction perpendicular to the translatory movement. 12. The radiation therapy apparatus as claimed in claim 10, wherein the extension mechanism is configured such that, in addition to the translatory movement, a tilting movement of the radiation head that tilts the radiation head relative to the extension mechanism is executable. 13. The radiation therapy apparatus as claimed in claim 10, wherein the extension mechanism is extendable out along an isocentric circular segment. 14. The radiation therapy apparatus as claimed in claim 8, wherein the extension mechanism is configured such that, in addition to the translatory movement, another movement of the radiation head is executable, the other movement having a component in a direction perpendicular to the translatory movement. 15. The radiation therapy apparatus as claimed in claim 14, wherein the extension mechanism is configured such that, in addition to the translatory movement, a tilting movement of the radiation head that tilts the radiation head relative to the extension mechanism is executable. 16. The radiation therapy apparatus as claimed in claim 14, wherein the extension mechanism is configured such that the radiation head is alignable to an isocentric emission of a beam of the therapeutic radiation. 17. The radiation therapy apparatus as claimed in claim 6, wherein the extension mechanism is extendable out along an isocentric circular segment. 18. The radiation therapy apparatus as claimed in claim 8, wherein the extension mechanism is configured such that the radiation head is alignable to an isocentric emission of a beam of the therapeutic radiation. 19. The radiation therapy apparatus as claimed in claim 1, wherein the extension mechanism is extendable out along an isocentric circular segment. 20. The radiation therapy apparatus as claimed in claim 1, wherein the extension mechanism is configured such that the radiation head is alignable to an isocentric emission of a beam of the therapeutic radiation.